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Ikaite and pseudomorphs thereafter (“glendonites”) are a potentially powerful tool for palaeoclimatic studies, as a low-temperature proxy. However, much uncertainty still surrounds the drivers of ikaite formation, in particular prerequisite thermal and chemical conditions. Furthermore, the ikaite toglendonite transformation is not fully understood, and it was unclear which calcite phases in glendonites were ikaite-derived and which were later diagenetic calcites. This leads to difficulties in choosing which phase to analyse in order to reconstruct the original ikaite growth environmental conditions. Petrographicexamination of air-transformed ikaite from the Isatkoak Lagoon in Utqiaġvik, Alaska, confirms that both ’Type I’ and ’Type II’ calcite phases seen in glendonites are directly derived from ikaite breakdown and not from secondary sources. Clumped isotope temperature reconstructions for transformed ikaites from Utqiaġvik, and comparison to Recent glendonites from the White Sea, Russia, confirm that clumped isotope thermometry may be used to reconstruct ikaite growth temperatures, whilst stable isotopes and minor elemental analysis reveal that a range of geochemical conditions characterise ikaite growth sites

Eubaculites carinatus (Morton, 1834) Figure 10A–J, N–S Eubaculites carinatus (Morton, 1834) figs. 29–35 Baculites carinatus Morton, 1834: 44, pl. 13, fig. 1. Baculites lyelli d’Orbigny, 1847: pl. 1, figs. 3–7. Baculites tippaensis Conrad, 1858: 334, pl. 3, fig. 27. Baculites spillmani Conrad, 1858: 335, pl. 35, fig. 24. Baculites sheromingensis Crick, 1924: 139, pl. 9, figs. 1–3. Eubaculites lyelli (d’Orbigny, 1847). Kennedy, 1987: 195, pl. 27, figs. 5–8; pl. 32, figs. 13, 14 (with full synonymy). Eubaculites carinatus (Morton, 1834). Klinger and Kennedy, 1993: 218, text-figs. 7a–e, 21–30, 31a–g, 32– 35, 36e, f, 37, 38, 42a, 52g, h. Eubaculites carinatus (Morton, 1834). Kennedy and Cobban, 2000: 180, pl. 2, figs. 1–23, 27, 28; text figs. 3, 4 (with additional synonymy). Eu baculites carinatus (Morton, 1834). Kennedy et al., 2001: 168, fig. 4a, e. Eubaculites carinatus (Morton, 1834). Landman et al., 2004a: 35, fig. 15P, Q. Eubaculites carinatus (Morton, 1834). Landman et al., 2004b: 55, figs. 27–29, 30A, B, 33U–Y. Eubaculites carinatus (Morton, 1834). Landman et al., 2007a: 64, figs. 29–35. TYPE: The holotype, by monotypy, is ANSP 72866, the original of Morton (1834: pl. 13, fig. 1), from the Maastrichtian Prairie Bluff Chalk of Alabama. MATERIAL: Approximately 40 specimens in the AMNH collections in the top 1 m of the Corsicana Formation at AMNH locs. 3620 and 3621, and as reworked material in the upper portion of the first unit (mudstone-clast-bearing unit) of the overlying K-Pg event deposit at the base of the Kincaid Formation, AMNH loc. 3620. DESCRIPTION: Nearly all the specimens are crushed but bear weak to strong, slightly crescentic nodate swellings on the flanks. The largest specimen is AMNH 108203, which retains part of the body chamber. The whorl height at the broken end of the body chamber is 30.5 mm, suggesting that it is a macroconch. The only fully three-dimensional specimen is AMNH 111960 from the K-Pg event deposit. It is a fragment 58.0 mm long with a whorl height of 30.0 mm at its adoral end. It is completely septate suggesting that it is probably a macroconch. The dorsum is very broadly rounded, and the dorsolateral margin is fairly sharply rounded. The inner flanks are broadly rounded with maximum width at one-third whorl height. The outer flanks converge to a tabulate venter bordered by a shallow longitudinal groove on each side. The flanks bear two prominent nodate swellings and the venter is covered with transverse ribs that project slightly forward, producing a serrated appearance. REMARKS: Several specimens from the Corsicana Formation are associated with oysters and represent postmortem encrustations (fig. 5A, B). The whorl height of the largest specimen in our collection (30.5 mm) exceeds that of the largest specimen of this species at the ageequivalent Tinton Formation in Monmouth County, New Jersey (25.4 mm). However, the specimens in the Corsicana Formation do not approach the size of the largest specimens of this species from Argentina and Zululand, which are approximately 90 mm in whorl height (Klinger and Kennedy, 2001: 62). Our collection is too small to confirm the hypothesis of dimorphism in Eubaculites carinatus, as suggested by bimodal size distributions of specimens from South Africa and South American localities by Klinger and Kennedy (2001). OCCURRENCE: Top 1 m of the Corsicana Formation at AMNH locs. 3620 and 3621, and basal Kincaid Formation at AMNH loc. 3620. We also found specimens of Eubaculites carinatus 17.68–18.29 mbs and 20.12–20.73 mbs in the Mullinax-1 core, correlated to planktonic foraminiferal zone CF3. The species has also been recorded throughout an approximately 11 m succession of the Corsicana Formation at Brazos by Kennedy et al. (2001). This species is widely distributed and, according to Henderson et al. (1992: 153), “is a useful indicator of middle to late Maastrichtian age and represents the last widely distributed heteromorph taxon to appear in the stratigraphic record.” On the eastern Gulf Coastal Plain, it is reported from the Owl Creek Formation in Mississippi, Missouri, and Tennessee (Kennedy and Cobban, 2000) and the Prairie Bluff Chalk in Alabama and Mississippi (Cobban and Kennedy, 1995). On the Atlantic Coastal Plain, it occurs in the Tinton and New Egypt formations, and as reworked material in the Hornerstown Formation in Monmouth and Gloucester counties, New Jersey (Kennedy and Cobban, 1996; Landman et al., 2004b; 2007a) and in the Severn Formation in Prince Georges and Anne Arundel counties, Maryland (Kennedy et al., 1997; Landman et al., 2004). It has also been reported from southeastern and southwestern France, northern Spain, Austria, the Netherlands, Zululand (South Africa), Mozambique, Madagascar, South India, Western Australia, Chile, Argentina, and California (Klinger and Kennedy, 1993; Klinger et al., 2001)., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on pages 25-27, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Morton, S. G. 1834. Synopsis of the organic remains of the Cretaceous group of the United States. Illustrated by 19 plates, to which is added an appendix containing a tabular view of the Tertiary fossils hitherto discovered in North America. Philadelphia: Key and Biddle.","Conrad, T. A. 1858. Observations on a group of Cretaceous fossil shells found in Tippah County, Mississippi, with descriptions of 56 new species. Journal of the Academy of Natural Sciences of Philadelphia (Series 2) 3: 275 - 298.","Crick, G. C. 1924. On Upper Cretaceous Cephalopoda from Portugese East Africa. Transactions of the Geological Society of South Africa 26: 130 - 140.","Kennedy, W. J. 1987. The ammonite fauna of the type Maastrichtian with a revision of Ammonites colligatus Binkhorst, 1861. Bulletin de l'Institut Royal des Sciences Naturelles de Belgique 56: 151 - 267.","Klinger, H. C., and W. J. Kennedy. 1993. Cretaceous faunas from Zululand and Natal, South Africa. The heteromorph ammonite genus Eubaculites Spath, 1926. Annals of the South African Museum 102: 185 - 264.","Kennedy, W. J., and W. A. Cobban. 2000. Maastrichtian (Late Cretaceous) ammonites from the Owl Creek Formation in northeastern Mississippi, U. S. A. Acta Geologica Polonica 50: 175 - 190.","Kennedy, W. J., A. S. Gale, and T. A. Hansen. 2001. The last Maastrichtian ammonites from the Brazos River sections in Falls County, Texas. Cretaceous Research 22: 163 - 171.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 1, Maryland and North Carolina. American Museum Novitates 3454: 1 - 64.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 b. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 2, Northeastern Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 287: 1 - 107.","Landman, N. H., R. O. Johnson, M. P. Garb, L. E. Edwards, and F. T. Kyte. 2007 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 3, Manasquan River Basin, Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 303: 1 - 122.","Henderson, R. A., W. J. Kennedy, and K. J. Mcnamara. 1992. Maastrichtian heteromorph ammonites from the Carnarvon Basin, Western Australia. Alcheringa: an Australasian Journal of Palaeontology 16: 133 - 170.","Cobban, W. A., and Kennedy, W. J. 1995. Maastrichtian ammonites chiefly from the Prairie Bluff Chalk in Alabama and Mississippi. Memoir, the Paleontological Society 44: 1 - 40.","Kennedy, W. J., and W. A. Cobban. 1996. Maastrichtian ammonites from the Hornerstown Formation in New Jersey. Journal of Paleontology 70: 798 - 804.","Kennedy, W. J., W. A. Cobban, and N. H. Landman. 1997. Maastrichtian ammonites from the Severn Formation of Maryland. American Museum Novitates 3210: 1 - 30."]}

Discoscaphites mullinaxorum, new species Figure 14 Discoscaphites iris Conrad, 1858). Kennedy and Cobban, 2000: pl. 3, fig. 21. DIAGNOSIS: Small, closely coiled shell, with no gap between the phragmocone and body chamber; body chamber covered with thin, sharp, slightly flexuous lirae and four rows of tiny tubercles (umbilicolateral, flank, and two rows of ventrolateral tubercles); umbilicolateral and lateral tubercles are radially elongated. ETYMOLOGY: This species is named after Ronnie and Jackie Mullinax, who have generously granted permission to scores of geologists and paleontologists to explore and collect fossils on their ranch and ensured the preservation of key outcrops for further study. Without their help, the K-Pg sections along the Brazos River would not be as well known worldwide as they are today. TYPES: The holotype is AMNH 112086, a crushed microconch, from AMNH loc. 3620, from the top of the Corsicana Formation, Darting Minnow Creek, Falls County, Texas. The paratypes are AMNH 108188, 111958, and 112024, from the same locality. MATERIAL: A total of 10 specimens, all of which are crushed microconchs, from the top 1.5 m of the Corsicana Formation, AMNH loc. 3620 (Darting Minnow Creek), Falls County, Texas. MICROCONCH DESCRIPTION: LMAX averages 22.4 mm and ranges from 17.5 to 29.1 mm. The shell is closely coiled with a large umbilicus. The body chamber occupies approximately one-half whorl and terminates in a constricted aperture (fig. 14P). The whorl height gradually increases in passing from the phragmocone to the body chamber. The umbilical shoulder of the body chamber follows the curvature of the venter. The ornament on the phragmocone consists of broad indistinct ribs. The body chamber is covered with thin, sharp, slightly flexuous lirae, especially on the adoral one-half. The lirae become less flexuous, coarser, and more closely spaced near the aperture (fig. 14Q, R). Four rows of tubercles are present on the adoral one-half of the body chamber (fig. 14A, B, G, H). In the holotype, the umbilicolateral tubercles are bullate and increase in size toward the aperture. They are perched on the umbilical shoulder (fig. 14G, H). The flank tubercles are tiny and radially elongate; they are generally not associated with the lirae. Because of crushing, the outer ventrolateral tubercles are not exposed in the holotype, but all four rows of tubercles are visible in AMNH 111958 (fig. 14A, B). The number of inner ventrolateral tubercles exceeds the number of flank tubercles. REMARKS: This species differs from Discoscaphites iris in its smaller size, its flatter flanks, and more delicate ornament. In particular, the tubercles in D. mullinaxorum are thin and radially elongate, whereas they are conical and pointy in D. iris. Although our collection consists only of microconchs, Kennedy and Cobban (2000: pl. 3, fig. 21) illustrated a small macroconch of this species, which they described as D. iris, from the upper part of the Owl Creek Formation in northeastern Mississippi. This macroconch is approximately 1.3× the average size of the microconchs in our collection. Discoscaphites mullinaxorum also resembles D. minardi Landman et al., 2004a, with its subdued ornament. However, the tubercles on the flanks of the body chamber in D. mullinaxorum are thin and radially elongate whereas they are rare or absent in D. minardi. In its subdued ornament, D. mullinaxorum also resembles D. conradi, but there are only four rows of tubercles in D. mullinaxorum compared to as many as six rows in D. conradi, sometimes even including a midventral row. OCCURRENCE: This species is rare in the upper part of the Corsicana Formation along the Brazos River and its tributaries in Falls County, Texas. Elsewhere on the Gulf Coastal Plain, it occurs in the Owl Creek Formation in Mississippi (Kennedy and Cobban, 2000).

Discoscaphites mullinaxorum, new species Figure 14 Discoscaphites iris Conrad, 1858). Kennedy and Cobban, 2000: pl. 3, fig. 21. DIAGNOSIS: Small, closely coiled shell, with no gap between the phragmocone and body chamber; body chamber covered with thin, sharp, slightly flexuous lirae and four rows of tiny tubercles (umbilicolateral, flank, and two rows of ventrolateral tubercles); umbilicolateral and lateral tubercles are radially elongated. ETYMOLOGY: This species is named after Ronnie and Jackie Mullinax, who have generously granted permission to scores of geologists and paleontologists to explore and collect fossils on their ranch and ensured the preservation of key outcrops for further study. Without their help, the K-Pg sections along the Brazos River would not be as well known worldwide as they are today. TYPES: The holotype is AMNH 112086, a crushed microconch, from AMNH loc. 3620, from the top of the Corsicana Formation, Darting Minnow Creek, Falls County, Texas. The paratypes are AMNH 108188, 111958, and 112024, from the same locality. MATERIAL: A total of 10 specimens, all of which are crushed microconchs, from the top 1.5 m of the Corsicana Formation, AMNH loc. 3620 (Darting Minnow Creek), Falls County, Texas. MICROCONCH DESCRIPTION: LMAX averages 22.4 mm and ranges from 17.5 to 29.1 mm. The shell is closely coiled with a large umbilicus. The body chamber occupies approximately one-half whorl and terminates in a constricted aperture (fig. 14P). The whorl height gradually increases in passing from the phragmocone to the body chamber. The umbilical shoulder of the body chamber follows the curvature of the venter. The ornament on the phragmocone consists of broad indistinct ribs. The body chamber is covered with thin, sharp, slightly flexuous lirae, especially on the adoral one-half. The lirae become less flexuous, coarser, and more closely spaced near the aperture (fig. 14Q, R). Four rows of tubercles are present on the adoral one-half of the body chamber (fig. 14A, B, G, H). In the holotype, the umbilicolateral tubercles are bullate and increase in size toward the aperture. They are perched on the umbilical shoulder (fig. 14G, H). The flank tubercles are tiny and radially elongate; they are generally not associated with the lirae. Because of crushing, the outer ventrolateral tubercles are not exposed in the holotype, but all four rows of tubercles are visible in AMNH 111958 (fig. 14A, B). The number of inner ventrolateral tubercles exceeds the number of flank tubercles. REMARKS: This species differs from Discoscaphites iris in its smaller size, its flatter flanks, and more delicate ornament. In particular, the tubercles in D. mullinaxorum are thin and radially elongate, whereas they are conical and pointy in D. iris. Although our collection consists only of microconchs, Kennedy and Cobban (2000: pl. 3, fig. 21) illustrated a small macroconch of this species, which they described as D. iris, from the upper part of the Owl Creek Formation in northeastern Mississippi. This macroconch is approximately 1.3�� the average size of the microconchs in our collection. Discoscaphites mullinaxorum also resembles D. minardi Landman et al., 2004a, with its subdued ornament. However, the tubercles on the flanks of the body chamber in D. mullinaxorum are thin and radially elongate whereas they are rare or absent in D. minardi. In its subdued ornament, D. mullinaxorum also resembles D. conradi, but there are only four rows of tubercles in D. mullinaxorum compared to as many as six rows in D. conradi, sometimes even including a midventral row. OCCURRENCE: This species is rare in the upper part of the Corsicana Formation along the Brazos River and its tributaries in Falls County, Texas. Elsewhere on the Gulf Coastal Plain, it occurs in the Owl Creek Formation in Mississippi (Kennedy and Cobban, 2000)., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on pages 35-37, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Conrad, T. A. 1858. Observations on a group of Cretaceous fossil shells found in Tippah County, Mississippi, with descriptions of 56 new species. Journal of the Academy of Natural Sciences of Philadelphia (Series 2) 3: 275 - 298.","Kennedy, W. J., and W. A. Cobban. 2000. Maastrichtian (Late Cretaceous) ammonites from the Owl Creek Formation in northeastern Mississippi, U. S. A. Acta Geologica Polonica 50: 175 - 190.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 1, Maryland and North Carolina. American Museum Novitates 3454: 1 - 64."]}

Discoscaphites iris (Conrad, 1858) Figure 12A–N Scaphites iris Conrad, 1858: 335, pl. 35, fig. 23. Scaphites iris Conrad. Whitfield, 1892: 265, pl. 44, figs. 4–7. Discoscaphites iris (Conrad). Stephenson, 1955: 134, pl. 23, figs. 23–30. Discoscaphites iris (Conrad, 1858). Kennedy and Cobban, 2000: 183, fig. 5; pl. 3, figs. 3–35. Discoscaphites iris (Conrad, 1858). Landman et al., 2004a: 39, figs. 15A, B, G–O, 17A–G, 18R. Discoscaphites iris (Conrad, 1858). Landman et al., 2004b: 71, figs. 34E–W (non A–D = Discoscaphites sphaeroidalis Kennedy and Cobban, 2000), 35, 36A–H, K–Q, S–Z, l–p, 37A–l, 38, 39A–P, 41A–D. Discoscaphites iris (Conrad, 1858). Landman et al., 2007a: 82, figs. 40-46, 47A–C. Discoscaphites iris. Keller et al., 2011: 85, fig. 3E. Discoscaphites iris (Conrad, 1858). Machalski et al., 2009: 375, fig. 2. Discoscaphites iris. Sessa et al., 2015: 15563, fig. 1A–C. Discoscaphites iris. Larina et al., 2016: 132, fig. 2B;145, fig. 14.1-26 Discoscaphites iris. Witts et al., 2018: 150, fig. 2C; 156, fig. 7A–U. Discoscaphites iris. Ferguson et al., 2019: 321. TYPE: The holotype is the original illustrated in Conrad, 1858 (335, pl. 35, fig. 23), labeled ANSP 50989, from the bluffs of Owl Creek, Tippah County, Mississippi. See Landman et al. (2004b) for a more complete description of this specimen. MATERIAL: A total of 27 specimens, mostly consisting of the body chamber or parts of the phragmocone and body chamber, plus numerous fragments, in the AMNH and UNM collections. The specimens are nearly equally divided between 13 microconchs and 14 macroconchs. All the specimens are crushed but retain their original aragonitic shell. The specimens are primarily derived from the top 1.5 m of the Corsicana Formation at AMNH locs. 3620, 3621, and 3968, but two (AMNH 111961 and AMNH 112037) also occur in the first (mudstoneclast-bearing) and second unit (ejecta-spherule-rich coarse sandstone) of the K-Pg event deposit at AMNH loc. 3620. D. iris is the most common scaphitid at Brazos, and as such, many of the abundant juvenile scaphitids present in these sections likely belong to this species, but because of their small size, they cannot be identified to the species level. MACROCONCH DESCRIPTION: Although the specimens are crushed, it is possible to measure the maximum length (LMAX). They range from 37.2 to 61.6 mm with most specimens falling between 50 and 55 mm (fig. 13). AMNH 112082 and 108182 are examples of small and large specimens, respectively. The ratio of the size of the largest specimen to that of the smallest is 1.7. Specimens are tightly coiled with a small umbilicus. The body chamber occupies approximately one-half whorl. In passing from the phragmocone to the shaft, the whorl height increases slightly, and then decreases again at the aperture. As in other scaphitid macroconchs, the umbilical shoulder of the body chamber is straight and occasionally shows a slight bulge. The aperture is constricted and the angle of the aperture averages 30°. The spire is covered by prorsiradiate ribs. They are broad and straight in AMNH 111959 and thin and slightly sinuous in AMNH 108178. Intercalation and branching occur at onethird and two-thirds whorl height. The ribs become broader and more widely spaced toward the adoral part of the spire. Two rows of ventrolateral tubercles are visible on the adapical part of the spire, although the outer row is difficult to discern because of crushing. An additional two rows of tubercles appear on the flanks soon thereafter. The ornament on the body chamber consists of four rows of tubercles—umbilicolateral, flank, and inner and outer ventrolateral tubercles. The tubercles occur on broad, low convex ribs that become more prominent on the hook. All the tubercles end in sharp points. The most prominent tubercles are the two umbilicolateral tubercles on the midshaft just below the umbilical margin (e.g., AMNH 198178). The flank tubercles are slightly smaller than the umbilicolateral tubercles and occur midway between the umbilicolateral and inner ventrolateral tubercles. MICROCONCH DESCRIPTION: Microconchs are, on average, smaller than macroconchs. Microconchs range in LMAX from 25.1 to 42.6 mm with most specimens falling between 30 and 35 mm (fig. 13). AMNH 111963 (fig. 10J) and 108186 (fig. 10N) are examples of small and large specimens, respectively. The ratio of the size of the largest specimen to that of the smallest is 1.7. In passing from the spire into the body chamber, the whorl height increases slightly. As a result, the umbilical seam follows the curvature of the venter. The body chamber is slightly uncoiled and occupies approximately one-half whorl. The ornamentation on the phragmocone consists of thin, slightly flexuous prorsiradiate ribs and two rows of ventrolateral tubercles. The prorsiradiate ribs on the body chamber are more poorly defined. They bear four rows of tubercles, of which the umbilicolateral tubercles are the most prominent. They are perched on the umbilical shoulder and attain their greatest height just adoral of midshaft. REMARKS: In comparison to specimens of Discoscaphites iris from New Jersey and the eastern Gulf Coastal Plain, the specimens from the Brazos River localities are larger (fig. 13). For example, LMAX of the largest macroconch from the Brazos River locality is 61.6 mm whereas it is 54.2 mm from New Jersey (Landman et al., 2007a). In contrast, the specimen from Libya is still larger, with an estimated diameter of 80 mm (Machalski et al., 2009). At least one specimen associated with oysters represents postmortem encrustations (fig. 5C, D). OCCURRENCE: This species is known from the upper part of the Corsicana Formation and the base of the Kincaid Formation along the Brazos River and its tributaries in Falls County, Texas. It has also been reported in the Mullinax-1 and Mullinax-3 cores from the Corsicana Formation in the same area (Keller et al., 2011). Elsewhere on the Gulf Coastal Plain, it occurs in the Owl Creek Formation in Mississippi, Tennessee, and Missouri (Stephenson, 1955; Sohl, 1960, 1964; Kennedy and Cobban, 2000). On the Atlantic Coastal Plain, it occurs in the upper part of the Tinton Formation and as reworked material at the base of the Hornerstown Formation, central Monmouth County; the upper part of the New Egypt Formation and as reworked material at the base of the Hornerstown Formation in northeastern and southwestern Monmouth County (Landman et al., 2007a); and the upper part of the Severn Formation in Kent and Anne Arundel counties, Maryland (Landman et al. 2004a). It is the name bearer of the Discoscaphites iris Zone in the Gulf and Atlantic Coastal Plains, where it represents the upper part of the upper Maastrichtian, corresponding to the upper part of calcareous nannofossil Subzone CC26b (Landman et al., 2004a; 2004b; 2007a; Larina et al., 2016). This species has also been reported from the upper Maastrichtian of northwest Libya (Machalski et al., 2009)., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on pages 29-33, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Conrad, T. A. 1858. Observations on a group of Cretaceous fossil shells found in Tippah County, Mississippi, with descriptions of 56 new species. Journal of the Academy of Natural Sciences of Philadelphia (Series 2) 3: 275 - 298.","Whitfield, R. P. 1892. Gasteropoda and Cephalopoda of the Raritan clays and Greensand marls of New Jersey. United States Geological Survey Monograph 18: 1 - 402.","Stephenson, L. W. 1955. Owl Creek (Upper Cretaceous) fossils from Crowley's Ridge, southeastern Missouri. United States Geological Survey Professional Paper 274: 97 - 140","Kennedy, W. J., and W. A. Cobban. 2000. Maastrichtian (Late Cretaceous) ammonites from the Owl Creek Formation in northeastern Mississippi, U. S. A. Acta Geologica Polonica 50: 175 - 190.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 1, Maryland and North Carolina. American Museum Novitates 3454: 1 - 64.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 b. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 2, Northeastern Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 287: 1 - 107.","Landman, N. H., R. O. Johnson, M. P. Garb, L. E. Edwards, and F. T. Kyte. 2007 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 3, Manasquan River Basin, Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 303: 1 - 122.","Machalski, M., J. W. M. Jagt, N. H. Landman, and J. Uberna. 2009. First record of the North American scaphitid ammonite Discoscaphites iris from the upper Maastrichtian of Libya. Neues Jahrbuch fur Geologie und Palaontologie Abhandlungen 254: 373 - 378.","Sessa, J. A., et al. 2015. Ammonite habitat revealed via isotopic composition and comparisons with cooccurring benthic and planktonic organisms. Proceedings of the National Academy of Sciences of the United States of America 112: 15562 - 15567.","Larina, E., et al. 2016. Upper Maastrichtian ammonite biostratigraphy of the Gulf Coastal Plain (Mississippi Embayment, southern USA). Cretaceous Research 60: 128 - 151.","Irizarry, K. M., M. P. Garb, J. D. Witts, A. Danilova, and N. H. Landman. 2018. Faunal analysis of the Cretaceous-Paleogene (K-Pg) mass extinction boundary, Brazos River, Texas. Munstersche Forschungen zur Geologie und Palaontologie 110: 57 - 58","Ferguson, K., K. G. Macleod, N. H. Landman, and J. A. Sessa. 2019. Evaluating growth and ecology in baculitid and scaphitid ammonites using stable isotope sclerochronology. Palaios 34: 317 - 329.","Sohl, N. F. 1960. Archaeogastropoda, Mesogastropoda and stratigraphy of the Ripley, Owl Creek, and Prairie Bluff Formations. United States Geological Survey Professional Paper 331 - A: 1 - 151.","Sohl, N. F. 1964. Neogastropoda, Opisthobranchia, and Basommatophora from the Ripley, Owl Creek, and Prairie Bluff Formations. United States Geological Survey Professional Papers 331 - B: 153 - 344."]}

Genus Eubaculites Spath, 1926 TYPE SPECIES: Baculites vagina Forbes var. ootacodensis Stoliczka, 1866: 199, pl. 90, figs. 14,?15, by original designation by Spath, 1926: 80., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on page 25, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Spath, L. F. 1926. New ammonites from the English chalk. Geological Magazine 63: 77 - 83.","Stoliczka, F. 1866. Ammonitidae, with revision of the Nautilidae. The fossil Cephalopoda of the Cretaceous rocks of southern India. Memoirs of the Geological Survey of India. Palaoentologica Indica (Series 3) 1: 155 - 216."]}

Discoscaphites sp. Figures 15, 16 MATERIAL: All sites we studied at the Brazos River localities yield many small juveniles of Discoscaphites (fig. 15). Because of their small size, it is impossible to identify them to a species level. DESCRIPTION: Juvenile specimens range in LMAX from 1.7 to 10.4 mm and retain part or all of their body chamber (fig. 15; table 1) Although the specimens preserve traces of the outer shell wall, the ornamentation is subdued or absent. Several specimens show exquisite details of the microornamentation of the embryonic shell (ammonitella; fig. 16). REMARKS: The presence of complete to nearly complete juveniles suggests that the animals lived at the site in which they are buried. All the specimens appear as isolated occurrences except for AMNH 63319, which occurs in a concentration of shell hash along with other juveniles and fish debris. OCCURRENCE: All specimens are from the upper portion of the Corsicana Formation just below the K-Pg boundary at AMNH locs. 3620 and 3621, Darting Minnow and Cottonmouth creeks, respectively. Lower jaws of Discoscaphites Figure 11E–J MATERIAL: Four isolated specimens (AMNH 108284, 108179, 108282, 63317) and one preserved in situ (AMNH 63312) from the top 1.25 m of the Corsicana Formation at AMNH loc. 3620 and AMNH loc. 3621, Darting Minnow and Cottonmouth creeks, Falls County, Texas. The specimens consist of the outer calcareous valve (aptychus), although three of them also retain traces of the inner black layer DESCRIPTION: Each valve is roughly triangular in outline. The ratio of valve length to valve width ranges from 1.4 to 1.9 in our sample. The symphysal edge is straight and forms a flange that increases in height posteriorly. The anterior margin is nearly straight, the lateral margin is broadly rounded, and the posterior margin is sharply rounded. The ventral surface of each valve is covered with small folds that parallel the posterior margin. Most of the jaws are preserved with only one valve exposed. However, in AMNH 63317 the two valves are folded folio style. As a result, the ventral surface of one valve and part of the dorsal surface of the other valve are exposed. AMNH 66312 occurs inside a very poorly preserved specimen of Discoscaphites iris surrounded by small oysters. The specimen of D. iris is 32.1 mm in maximum length, so that the ratio of LMAX to the length of the jaw is 2.3. REMARKS: These specimens are similar to the lower jaws of scaphites illustrated from Maastrichtian deposits of South Dakota (Landman and Waage, 1993: figs. 37–41, 167E–I) and northern Europe (Birkelund, 1982: pl. 2, figs. 6, 7; Birkelund, 1993: pl. 17, figs. 2–4; Machalski, 2005b: fig. 26)., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on pages 37-39, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Landman, N. H., and K. M. Waage. 1993. Scaphitid ammonites of the Upper Cretaceous (Maastrichtian) Fox Hills Formation in South Dakota and Wyoming. Bulletin of the American Museum of Natural History 215: 1 - 257.","Birkelund, T. 1982. Maastrichtian ammonites from Hemmoor, Niederelbe (NW-Germany). Geologisches Jahrbuch 61: 13 - 33.","Birkelund, T. 1993. Ammonites from the Maastrichtian White Chalk of Denmark. Bulletin of the Geological Society of Denmark 40: 33 - 81.","Machalski, M. 2005 b. Late Maastrichtian and earliest Danian scaphitid ammonites from central Europe: Taxonomy, evolution, and extinction. Acta Palaeontologica Polonica 50: 653 - 696."]}

Eutrephoceras sp. Figure 7 MATERIAL: One small phragmocone UNM 15495 from the Middle Sandstone Bed (Yancey, 1996; Hart et al., 2012), Kincaid Formation (Littig Member), from an outcrop just west of a culvert upstream of the waterfall section at AMNH loc. 3620. DESCRIPTION: The specimen is a wholly septate steinkern 31.6 mm in diameter with a whorl height of 26.5 mm at the adoral end. The septa are closely spaced with 13 septa in the last whorl. The specimen bears traces of another whorl. REMARKS: The holotype of Eutrephoceras dekayi ANSP 19484 is the original of Morton (1834: pl. 8, fig. 4), from the “marls of Monmouth and Burlington counties, New Jersey,” by subsequent designation by Whitfield (1892: 243, pl. 37, figs. 2, 3) and is illustrated by Landman et al. (2004a: 41, figs. 17–21). This species was more fully documented by Landman et al. (2017) who relied on conspecific material from the U.S. Western Interior, as Hyatt (1894) had done previously. They concluded that the species is early Maastrichtian, suggesting that younger forms (e.g., late Maastrichtian) should be carefully scrutinized to determine whether they conform to the description of this species. The present specimen is assigned to Eutrephoceras based on its size, shape, and suture pattern. However, because it is only a fragment of a phragmocone, it cannot be identified to species level. OCCURRENCE: This genus is worldwide and is reported from the Campanian to the Eocene. In Texas, it has been reported from the Upper Cretaceous (Campanian-Maastrichtian) Navarro Group of Hunt, Navarro, and Kaufman counties (Emerson et al., 1994), and the Eocene Cook Mountain Formation of Leon County (Miller, 1947). To our knowledge, UNM 15495 is the only reported specimen of Eutrephoceras from the Brazos River localities., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on pages 19-20, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Yancey, T. E. 1996. Stratigraphy and Depositional Environments of the Cretaceous-Tertiary boundary complex and basal Paleocene section, Brazos River, Texas. Gulf Coast Association of Geological Societies Transactions 46: 433 - 442.","Hart, M. B., et al. 2012. The Cretaceous-Paleogene boundary on the Brazos River, Texas: new stratigraphic sections and revised interpretations. Gulf Coast Association of Geological Societies 1: 69 - 80.","Morton, S. G. 1834. Synopsis of the organic remains of the Cretaceous group of the United States. Illustrated by 19 plates, to which is added an appendix containing a tabular view of the Tertiary fossils hitherto discovered in North America. Philadelphia: Key and Biddle.","Whitfield, R. P. 1892. Gasteropoda and Cephalopoda of the Raritan clays and Greensand marls of New Jersey. United States Geological Survey Monograph 18: 1 - 402.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 1, Maryland and North Carolina. American Museum Novitates 3454: 1 - 64.","Landman, N. H., J. W. Grier, J. K. Cochran, J. C. Grier, J. G. Petersen, and W. H. Towbin. 2017. Nautilid nurseries: hatchlings and juveniles of Eutrephoceras dekayi from the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east-central Montana. Lethaia 51: 48 - 74.","Hyatt, A. 1894. Phylogeny of an acquired characteristic. Proceedings of the American Philosophical Society 32: 349 - 647.","Emerson, B. L., J. H. Emerson, R. A. Akers, and T. J. Akers. 1994. Texas Cretaceous ammonites and nautiloids. Houston: Paleontology Section, Houston Gem and Mineral Society, 439 pp.","Miller, A. K. 1947. Tertiary nautiloids of the Americas. Geological Society of America Memoirs 23: 1 - 234."]}

Discoscaphites sphaeroidalis Kennedy and Cobban, 2000 Figure 12O, P Discoscaphites sp. Stephenson, 1955: 135, pl. 23, figs. 20–22. Discoscaphites sphaeroidalis Kennedy and Cobban, 2000: 185, pl. 1, figs. 1–11, text-fig. 6 Discoscaphites sphaeroidalis Kennedy and Cobban, 2000. Kennedy et al., 2001: 169, fig. 4b, c. Discoscaphites iris (Conrad, 1858). Landman et al., 2004b: 71, fig. 34A–D. Discoscaphites sphaeroidalis Kennedy and Cobban, 2000. Landman et al., 2007a: 92, figs. 47D, 48. TYPE: The holotype is USNM 465615, a macroconch missing part of the hook. It is from the Owl Creek Formation at its type locality, 4 km northeast of Ripley, Tippah County, Mississippi. MATERIAL: The material from Brazos consists of six specimens, three of which are identifiable as adult microconchs (AMNH 108185, 111957, and 116345) and three of which are fragments of adult microconchs (AMNH 108174, 108184, and 133094) from the top of the Corsicana Formation in Darting Minnow and Cottonmouth creeks (AMNH locs. 3620 and 3621), Falls County, Texas. MICROCONCH DESCRIPTION: The three nearly complete specimens range in size from 31.6 to 36.1 mm. The phragmocone extends slightly below the line of maximum length and the body chamber occupies approximately one-half whorl. As shown in AMNH 116345 (fig. 12P), the aperture is constricted with a weak ventral projection. The ornamentation on the phragmocone is well preserved in AMNH 11957 (fig. 12O). The ribs are narrow, long, and uniformly spaced. They bear four rows of tubercles, of which the inner ventrolateral row is the most prominent. The ribs on the body chamber are weaker, but the tubercles are stronger than those on the phragmocone. The tubercles are widely spaced on the shaft and more closely spaced on the hook. Both rows of ventrolateral tubercles extend to the aperture. REMARKS: Because these specimens are crushed, no information is available about the shape of the whorl section, which is important for identifying this species and distinguishing it from Discoscaphites iris. However, the ornamentation on the phragmocone is well preserved and consists of narrow, long ribs, one of the defining features of this species. It is curious that our collection consists only of microconchs whereas other collections of this species contain both dimorphs (Kennedy and Cobban, 2000; Kennedy et al., 2001; Landman et al., 2007a). If macroconchs were present, they would be easily recognizable. Their absence is probably due to the small size of the collection. OCCURRENCE: This species is known from the upper part of the Corsicana Formation along the Brazos River and its tributaries in Falls County, Texas. Kennedy et al. (2001) reported it as occurring throughout the top ~ 8 m of the Corsicana Formation. Elsewhere on the Gulf Coastal Plain, it occurs in the Owl Creek Formation in Mississippi and Missouri (Stephenson, 1955; Kennedy and Cobban, 2000). On the Atlantic Coastal Plain, it occurs in the upper part of the Tinton Formation and as reworked material at the base of the Hornerstown Formation, central Monmouth County, and in the upper part of the New Egypt Formation and as reworked material at the base of the Hornerstown Formation in northeastern Monmouth County (Landman et al., 2007a)., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on pages 34-35, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Kennedy, W. J., and W. A. Cobban. 2000. Maastrichtian (Late Cretaceous) ammonites from the Owl Creek Formation in northeastern Mississippi, U. S. A. Acta Geologica Polonica 50: 175 - 190.","Stephenson, L. W. 1955. Owl Creek (Upper Cretaceous) fossils from Crowley's Ridge, southeastern Missouri. United States Geological Survey Professional Paper 274: 97 - 140","Kennedy, W. J., A. S. Gale, and T. A. Hansen. 2001. The last Maastrichtian ammonites from the Brazos River sections in Falls County, Texas. Cretaceous Research 22: 163 - 171.","Conrad, T. A. 1858. Observations on a group of Cretaceous fossil shells found in Tippah County, Mississippi, with descriptions of 56 new species. Journal of the Academy of Natural Sciences of Philadelphia (Series 2) 3: 275 - 298.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 b. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 2, Northeastern Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 287: 1 - 107.","Landman, N. H., R. O. Johnson, M. P. Garb, L. E. Edwards, and F. T. Kyte. 2007 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 3, Manasquan River Basin, Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 303: 1 - 122."]}

Sphenodiscus pleurisepta (Conrad, 1857) Figure 9B, C Ammonites pleurisepta Conrad, 1857: 159, pl. 15, fig. 1. Sphenodiscus lenticularis (Owen). Kellum, 1962: 68, pl. 4, figs. 3, 4; pl. 5, fig. 1; pl. 6, figs. 1, 2. Sphenodiscus pleurisepta (Conrad, 1857). Cobban and Kennedy, 1993: 58, figs. 1, 3t. Sphenodiscus pleurisepta (Conrad, 1857). Cobban and Kennedy, 1995: 12, fig. 8.5 (with full synonymy). Sphenodiscus pleurisepta (Conrad, 1857). Kennedy et al., 1996: 11, figs. 4A, 5–12. Sphenodiscus pleurisepta (Conrad, 1857). Kennedy et al., 1997: 9, figs. 9J, 11–14. Sphenodiscus pleurisepta. Larson et al., 1997: 91. Sphenodiscus pleurisepta (Conrad, 1857). Landman and Cobban, 2003: 17, figs. 12–15. TYPE: The holotype is USNM 9888, said to be from “Jacun, 3 miles below Laredo,” but is probably from Maverick County, Texas, in the Escondido Formation of the Rio Grande Region (Stephenson, 1941, 1955). MATERIAL: One nearly complete specimen when found, now fragmentary (AMNH 116378), from the Corsicana Formation 1.25 m below the K-Pg boundary at AMNH loc. 3620, Darting Minnow Creek, Falls County, Texas, and one fragmentary specimen (UNM 15489) from the basal unit (mudstone-clast-bearing conglomerate) of the K-Pg event deposit also at AMNH loc. 3620. DESCRIPTION: AMNH 116378 is a specimen with the body chamber preserved, ~ 12 cm in diameter. The body chamber bears a hole 12.1 mm in diameter. UNM 15489 is a fragment of the umbilical portion of a body chamber. Both show the characteristic rows of nodes on the smooth flanks that distinguish this species from S. lobatus. REMARKS: Like Spenodiscus lobatus, S. pleurisepta is rare at Brazos, although the presence of a complete specimen suggests that it may not have experienced postmortem floating. OCCURRENCE: The two specimens of Sphenodiscus pleurisepta come from the top 1.25 m of the Corsicana Formation and basal ~ 20 cm of the Kincaid Formation, Falls County, Texas. This species has also been reported from the Corsicana Formation in Navarro County, Texas (Kennedy and Cobban, 1993a; see also Stephenson, 1941, 1955). It occurs in the Escondido Formation in Texas (Böse, 1928) and northern Mexico. It also occurs in the Maastrichtian Cerro del Pueblo Formation of the Difunta Group at Rincón Colorado, Coahuila (Ifrim et al., 2004; Ifrim et al., 2005). On the eastern Gulf Coastal Plain, it occurs in the upper part of the Owl Creek Formation in Missouri, Mississippi, and Tennessee and in the Prairie Bluff Chalk in Alabama and Mississippi (Cobban and Kennedy, 1995). On the Atlantic Coastal Plain, it occurs in the top of the New Egypt Formation and as reworked material at the base of the Hornerstown Formation in Monmouth County, New Jersey (Weller, 1907; Reeside, 1962; Gallagher, 1993; Landman et al., 2004a, 2004b) and in the Severn Formation in Prince Georges and Kent Counties, Maryland (Kennedy et al., 1997). In the Western Interior, this species occurs in the Hoploscaphites birklundae Zone of the Pierre Shale in Meade and Pennington counties, South Dakota (Kennedy et al., 1996) and in the upper part of the Pierre Shale and Fox Hills Formation in Weld County, Colorado (Landman and Cobban, 2003). Despite possible postmortem drift of Sphenodiscus lobatus specimens, these new data suggest that both species of sphenodiscid cooccur in the Corsicana Formation of the Brazos River area immediately below the K-Pg boundary. Poorly preserved examples of both S. lobatus and S. pleurisepta are also found in the basal unit of the K-Pg event deposit itself. As noted above, this is different from the Maastrichtian deposits of the U.S. Western Interior, with S. pleurisepta present in the Hoploscaphites birklundae Zone, being replaced by S. lobatus in the overlying Hoploscaphites nicolletii Zone. A similar situation was noted by Ifrim et al. (2005), and Ifrim and Stinnesbeck (2010) in their studies of Maastrichtian ammonite faunas from Mexico. These authors suggested that the two sphenodiscid species may have inhabited slightly different environments, explaining why they are rarely found together in the same outcrop or bed. In the U.S. Gulf Coastal Plain, both species range to the top of the Maastrichtian (Landman et al., 2015), and appear to cooccur at three localities in the Discoscaphites iris Range Zone (fig. 6): in the Owl Creek Formation in Mississippi (AMNH loc. 3460) and Missouri (AMNH loc. 3458) (Larina et al., 2016) and the Corsicana Formation at Brazos., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on pages 23-24, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Conrad, T. A. 1857. Descriptions of Cretaceous and Tertiary fossils. In W. H. Emory (editor), Report on the United States and Mexican Boundary Survey. United States 34 th Congress, 1 st Session, Senate Ex Document 108 and House Ex Document 135 1 (2): 141 - 174.","Kellum, L. B. 1962. Upper Cretaceous mollusca from Niobrara County, Wyoming. Michigan Academy of Science and Arts Letters 47: 37 - 70.","Cobban, W. A., and Kennedy, W. J. 1995. Maastrichtian ammonites chiefly from the Prairie Bluff Chalk in Alabama and Mississippi. Memoir, the Paleontological Society 44: 1 - 40.","Kennedy, W. J., W. A. Cobban, and N. H. Landman. 1997. Maastrichtian ammonites from the Severn Formation of Maryland. American Museum Novitates 3210: 1 - 30.","Larson, N. L., S. D. Jorgensen, R. A. Farrar, and P. L. Larson. 1997. Ammonites and the other cephalopods of the Pierre Seaway. Tucson, AZ: Geoscience Press.","Landman, N. H., and W. A. Cobban. 2003. Ammonites from the upper part of the Pierre Shale and Fox Hills Formation of Colorado. American Museum Novitates 3388: 1 - 45.","Stephenson, L. W. 1941. The larger invertebrate fossils of the Navarro Group of Texas (exclusive of corals and crustaceans and exclusive of the fauna of the Escondido Formation). University of Texas Bulletin 4101: 1 - 641.","Stephenson, L. W. 1955. Owl Creek (Upper Cretaceous) fossils from Crowley's Ridge, southeastern Missouri. United States Geological Survey Professional Paper 274: 97 - 140","Kennedy, W. J., and W. A. Cobban. 1993 a. Maastrichtian ammonites from the Corsicana Formation in northeast Texas. Geological Magazine 130: 57 - 67.","Bose, E. 1928. Cretaceous ammonites from Texas and northern Mexico. Bulletin of the University of Texas, Bureau of Economic Geology and Technology, Austin 2748, 143 - 312.","Ifrim, C., W. Stinnesbeck, and J. G. Lopez-Oliva. 2004. Maastrichtian cephalopods from Cerralvo, northeastern Mexico. Palaeontology 47: 1575 - 1627.","Ifrim, C., W. Stinnesbeck, and A. Schafhauser. 2005. Maastrichtian shallow-water ammonites of northeastern Mexico. Revista Mexicana de Ciencias Geologicas 22: 48 - 64.","Weller, S. 1907. A report on the Cretaceous paleontology of New Jersey. Geological Survey of New Jersey. Paleontology Series 4: 1 - 870.","Reeside, J. B., Jr. 1962. Cretaceous ammonites of New Jersey. In H. G. Richards (editor), The Cretaceous fossils of New Jersey. New Jersey Department of Conservation and Economic Development Bulletin 61, Part 2: 113 - 137.","Gallagher, W. B. 1993. The Cretaceous / Tertiary mass extinction event in the northern Atlantic coastal plain. Mosasaur 5: 75 - 155.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 1, Maryland and North Carolina. American Museum Novitates 3454: 1 - 64.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 b. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 2, Northeastern Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 287: 1 - 107.","Ifrim, C., and W. Stinnesbeck. 2010. Migration pathways of the late Campanian and Maastrichtian shallow facies ammonite Sphenodiscus in North America. Palaeogeography, Palaeoclimatology, Palaeoecology 292: 96 - 102.","Landman, N. H., S. Goolaerts, J. W. M. Jagt, E. A. Jagt-Yazykova, and M. Machalski. 2015. Ammonites on the brink of extinction: diversity, abundance, and ecology of the order Ammonoidea at the Cretaceous / Paleogene (K / Pg) Boundary. In C. Klug, D. Korn, K. De Baets, I. Kruta, and R. H. Mapes (editors), Ammonoid paleobiology: from macroevolution to paleogeography: 497 - 553. Dordrecht: Springer.","Larina, E., et al. 2016. Upper Maastrichtian ammonite biostratigraphy of the Gulf Coastal Plain (Mississippi Embayment, southern USA). Cretaceous Research 60: 128 - 151."]}

Genus Sphenodiscus Meek, 1871 TYPE SPECIES: Ammonites lenticularis Owen, 1852: 579 (non Young and Bird, 1828: 269, fig. 5), by original designation, = Ammonites lobata Tuomey, 1856: 168., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on page 21, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Meek, F. B. 1871. Preliminary paleontological report, consisting of lists of fossils, with descriptions of some new types etc. Preliminary Report of the United States Geological Survey of Wyoming and portions of contiguous territories 4: 287 - 318.","Owen, D. D. 1852. Report of a geological survey of Wisconsin, Iowa, and Minnesota and incidentally of a portion of Nebraska Territory. Philadelphia: Lippincott.","Young, G., and J. Bird. 1828. A geological survey of the Yorkshire coast: describing the strata and fossils occurring between the Humber and the Tees, from the German ocean to the plain of York. 2 nd ed. Whitby, England: R. Kirby.","Tuomey, M. 1856. Description of some new fossils from the Cretaceous rocks of the southern States. Proceedings of the Academy of Natural Sciences of Philadelphia 7: 167 - 172."]}

Gaudryceras sp. Figure 8 MATERIAL: One small specimen AMNH 111956 from the Corsicana Formation 1 m below the K-Pg boundary at AMNH loc. 3620 (Darting Minnow Creek). DESCRIPTION: Diameter of the specimen is 15.6 mm. It is entirely septate and mostly covered with shell (fig. 2F), with a tiny bit of suture exposed. It is serpenticonic with UD = 8.9 mm. The rate of whorl expansion is low. The ornament consists of thin lirae that are convex on the umbilical wall and shoulder and straight and prorsiradiate on the middle and outer flanks. They cross the venter with a slight adoral projection. REMARKS: Because of the small size of this specimen, it is impossible to identify it to the species level (R. Hoffmann and Y. Shigeta, personal commun., 2019). However, it is the only specimen of Gaudryceras ever found at Brazos and one of only a handful of lytoceratid ammonite specimens described from the Maastrichtian of North America (Cobban and Kennedy, 1995; Kennedy et al., 2000). OCCURRENCE: Gaudryceras is worldwide in its distribution and is reported from the Maastrichtian of Tunisia, Zululand, and Pondoland, South Africa (Klinger and Kennedy, 1979), Madagascar, south India, Japan, western Australia, California, Chile, and the Antarctic Peninsula. The closest occurrences of this genus to the Brazos River locality are in the lower Campanian of Travis County, Texas (Young, 1963), the lower upper Maastrichtian Corsicana Formation near San Antonio, Bexar County, Texas (Woehr, 2013), the upper Campanian Saratoga Chalk of Arkansas (Kennedy and Cobban, 1993b), and the Maastrichtian Méndez Formation of northeastern Mexico (Ifrim et al., 2004)., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on page 20, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Cobban, W. A., and Kennedy, W. J. 1995. Maastrichtian ammonites chiefly from the Prairie Bluff Chalk in Alabama and Mississippi. Memoir, the Paleontological Society 44: 1 - 40.","Kennedy, W. J., and H. C. Klinger 1979. Cretaceous faunas from Zululand and Natal, South Africa. The ammonite family Gaudryceratidae. Bulletin of the British Museum of Natural History (Geology) 31: 121 - 174.","Young, K. 1963. Upper Cretaceous ammonites from the Gulf Coast of the United States. University of Texas Bulletin 6304. 373 pp.","Woehr, D. A. 2013. Die Fauna der Corsicana-Formation (Maastrichtium) von Texas. Der Steinkern 14: 44 - 67.","Kennedy, W. J., and W. A. Cobban. 1993 b. Ammonites from the Saratoga Chalk (Upper Cretaceous), Arkansas. Journal of Paleontology 67: 404 - 434.","Ifrim, C., W. Stinnesbeck, and J. G. Lopez-Oliva. 2004. Maastrichtian cephalopods from Cerralvo, northeastern Mexico. Palaeontology 47: 1575 - 1627."]}

Sphenodiscus lobatus (Tuomey, 1856) Figure 9A Ammonites lenticularis Owen, 1852: 579, pl. 8, fig. 5. Ammonites lobatus Tuomey, 1856: 168. Sphenodiscus pleurisepta (Tuomey, 1854). Cobban and Kennedy, 1995: 61, fig. 3a, v. Sphenodiscus lobatus (Tuomey, 1856). Cobban and Kennedy, 1995: 12, figs. 6.2, 6.3, 8.4, 8.6– 8.11, 12.18, 12.19, 16.16, 16.17 (with full synonymy). Sphenodiscus lobatus (Tuomey, 1856). Kennedy and Cobban, 1996: 802, fig. 2.4–2.6, 2.13, 2.14, 2.19, 2.21. Sphenodiscus lobatus (Tuomey, 1856). Kennedy et al., 1997: 4, figs. 3–8, 9A–I, 10. Sphenodiscus lobatus (Tuomey, 1856). Landman et al., 2004a: 28, fig. 12. Sphenodiscus lobatus (Tuomey, 1856). Landman et al., 2004b: 51, figs. 23–25. Sphenodiscus lobatus (Tuomey, 1856). Landman et al., 2007a: 58, figs. 26–28. TYPE: The holotype, from Noxubee County, Mississippi, is lost (fide Stephenson, 1941: 434). MATERIAL: Three fragments AMNH 112039, 112070, and 112085, all from the upper 1 m of the Corsicana Formation just below the K-Pg boundary, and one fragment AMNH 135054 from the basal unit (mudstone-clast-bearing conglomerate) of the K-Pg event deposit itself. DESCRIPTION: AMNH 112070 is a fragment of a phragmocone, AMNH 112085 is a fragment of the outer flanks of a body chamber, and AMNH 135054 is a single small chamber with part of the septum. All show the characteristic smooth flanks indicative of this species. REMARKS: In contrast to the preservation of whole scaphites and baculitids, all the specimens of Sphenodiscus lobatus in our collection are fragmentary. This suggests that perhaps the shells floated into the area after death and may have lived closer to shore, which is consistent with isotopic evidence from shell samples of this species from the age-equivalent Owl Creek Formation in Mississippi (Sessa et al., 2015). OCCURRENCE: The few fragments are from the top 1 m of the Corsicana Formation (AMNH loc. 3620 and AMNH loc. 3621) and the basal unit of the overlying K-Pg event deposit at AMNH loc. 3620. Sphenodiscus lobatus has also been reported from the Corsicana Formation in Navarro County, Texas (Kennedy and Cobban, 1993a; see also Stephenson, 1941, 1955). It occurs in the Escondido Formation in Texas (Böse, 1928) and northern Mexico. Elsewhere in Mexico, it occurs in the Maastrichtian Cerro del Pueblo Formation of the Difunta Group at Rincón Colorado, Coahuila (Ifrim et al., 2004, 2005). On the eastern Gulf Coastal Plain, it occurs in the upper part of the Ripley Formation in Mississippi and the Prairie Bluff Chalk in Alabama and Mississippi (Cobban and Kennedy, 1995). On the Atlantic Coastal Plain, it occurs in the top of the Tinton Formation, the upper part of the Navesink Formation, the lower part of the New Egypt Formation, and as reworked material at the base of the Hornerstown Formation in Monmouth County (Weller, 1907; Reeside, 1962; Gallagher, 1993; Landman et al., 2004a, 2007a) and in the upper part of the Navesink Formation in Gloucester County, New Jersey (Gallagher, 1993; Kennedy et al., 1995; Kennedy and Cobban, 1996); in the Providence Sand in the Chattahoochee River area, Alabama and Georgia; in the upper part of the Peedee Formation in North Carolina (Landman et al., 2004a); and in the Severn Formation in Prince Georges County, Maryland (Kennedy et al., 1997). In the Western Interior, this species occurs in the Hoploscaphites nicolletii and H. nebrascensis Zones of the Fox Hills Formation in north-central South Dakota (Landman and Waage, 1993) and in the H. nebrascensis Zone of the Pierre Shale in southeastern South Dakota and northeastern Nebraska (Kennedy et al., 1998)., Published as part of Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, pp. 1-52 in American Museum Novitates 2021 (3964) on page 22, DOI: 10.1206/3964.1, http://zenodo.org/record/4566589, {"references":["Tuomey, M. 1856. Description of some new fossils from the Cretaceous rocks of the southern States. Proceedings of the Academy of Natural Sciences of Philadelphia 7: 167 - 172.","Owen, D. D. 1852. Report of a geological survey of Wisconsin, Iowa, and Minnesota and incidentally of a portion of Nebraska Territory. Philadelphia: Lippincott.","Cobban, W. A., and Kennedy, W. J. 1995. Maastrichtian ammonites chiefly from the Prairie Bluff Chalk in Alabama and Mississippi. Memoir, the Paleontological Society 44: 1 - 40.","Kennedy, W. J., and W. A. Cobban. 1996. Maastrichtian ammonites from the Hornerstown Formation in New Jersey. Journal of Paleontology 70: 798 - 804.","Kennedy, W. J., W. A. Cobban, and N. H. Landman. 1997. Maastrichtian ammonites from the Severn Formation of Maryland. American Museum Novitates 3210: 1 - 30.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 1, Maryland and North Carolina. American Museum Novitates 3454: 1 - 64.","Landman, N. H., R. O. Johnson, and L. E. Edwards. 2004 b. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 2, Northeastern Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 287: 1 - 107.","Landman, N. H., R. O. Johnson, M. P. Garb, L. E. Edwards, and F. T. Kyte. 2007 a. Cephalopods from the Cretaceous / Tertiary boundary interval on the Atlantic coastal plain, with a description of the highest ammonite zones in North America. Part 3, Manasquan River Basin, Monmouth County, New Jersey. Bulletin of the American Museum of Natural History 303: 1 - 122.","Stephenson, L. W. 1941. The larger invertebrate fossils of the Navarro Group of Texas (exclusive of corals and crustaceans and exclusive of the fauna of the Escondido Formation). University of Texas Bulletin 4101: 1 - 641.","Sessa, J. A., et al. 2015. Ammonite habitat revealed via isotopic composition and comparisons with cooccurring benthic and planktonic organisms. Proceedings of the National Academy of Sciences of the United States of America 112: 15562 - 15567.","Kennedy, W. J., and W. A. Cobban. 1993 a. Maastrichtian ammonites from the Corsicana Formation in northeast Texas. Geological Magazine 130: 57 - 67.","Stephenson, L. W. 1955. Owl Creek (Upper Cretaceous) fossils from Crowley's Ridge, southeastern Missouri. United States Geological Survey Professional Paper 274: 97 - 140","Bose, E. 1928. Cretaceous ammonites from Texas and northern Mexico. Bulletin of the University of Texas, Bureau of Economic Geology and Technology, Austin 2748, 143 - 312.","Ifrim, C., W. Stinnesbeck, and J. G. Lopez-Oliva. 2004. Maastrichtian cephalopods from Cerralvo, northeastern Mexico. Palaeontology 47: 1575 - 1627.","Ifrim, C., W. Stinnesbeck, and A. Schafhauser. 2005. Maastrichtian shallow-water ammonites of northeastern Mexico. Revista Mexicana de Ciencias Geologicas 22: 48 - 64.","Weller, S. 1907. A report on the Cretaceous paleontology of New Jersey. Geological Survey of New Jersey. Paleontology Series 4: 1 - 870.","Reeside, J. B., Jr. 1962. Cretaceous ammonites of New Jersey. In H. G. Richards (editor), The Cretaceous fossils of New Jersey. New Jersey Department of Conservation and Economic Development Bulletin 61, Part 2: 113 - 137.","Gallagher, W. B. 1993. The Cretaceous / Tertiary mass extinction event in the northern Atlantic coastal plain. Mosasaur 5: 75 - 155.","Kennedy, W. J., R. O. Johnson, and W. A. Cobban. 1995. Upper Cretaceous ammonite faunas of New Jersey. In J. E. B. Baker (editor), Contributions to the paleontology of New Jersey 12: 24 - 55.","Landman, N. H., and K. M. Waage. 1993. Scaphitid ammonites of the Upper Cretaceous (Maastrichtian) Fox Hills Formation in South Dakota and Wyoming. Bulletin of the American Museum of Natural History 215: 1 - 257.","Kennedy, W. J., N. H. Landman, W. K. Christensen, W. A. Cobban, and J. M. Hancock. 1998. Marine connections in North America during the late Maastrichtian: palaeogeographic and palaeobiogeographic significance of Jeletzkytes nebrascensis Zone cephalopod fauna from the Elk Butte Member of the Pierre Shale, SE South Dakota and NE Nebraska. Cretaceous Research 19: 745 - 775."]}

The early Eocene (c. 56 - 48 million years ago) experienced some of the highest global temperatures in Earth’s history since the Mesozoic, with no polar ice. Reports of contradictory ice-rafted erratics and cold water glendonites in the higher latitudes have been largely dismissed due to ambiguity of the significance of these purported cold-climate indicators. Here we apply clumped isotope paleothermometry to a traditionally qualitative abiotic proxy, glendonite calcite, to generate quantitative temperature estimates for northern mid-latitude bottom waters. Our data show that the glendonites of the Danish Basin formed in waters below 5 °C, at water depths of, The early Eocene was characterized by exceptionally high global temperatures and no polar ice. Here, clumped isotope paleothermometry of glendonite calcite from the Danish Basin shows that these were formed in waters below 5 °C, indicating that regionalised cool episodes punctuated the background warmth of the early Eocene.

Understanding the end-Triassic mass extinction event (201.36 Ma) requires a clear insight into the stratigraphy of boundary sections, which allows for long-distance correlations and correct distinction of the sequence of events. However, even after the ratification of a Global Stratotype Section and Point, global correlations of TJB successions are hampered by the fact that many of the traditionally used fossil groups were severely affected by the crisis. Here, a new correlation of key TJB successions in Europe, U.S.A. and Peru, based on a combination of biotic (palynology and ammonites), geochemical (δ 13 C org ) and radiometric (U/Pb ages) constraints, is presented. This new correlation has an impact on the causality and temporal development during the end-Triassic event. It challenges the hitherto used standard correlation, which has formed the basis for a hypothesis that the extinction was caused by more or less instantaneous release of large quantities of light carbon (methane) to the atmosphere, with catastrophic global warming as a consequence. The new correlation instead advocates a more prolonged scenario with a series of feedback mechanisms, as it indicates that the bulk of the hitherto dated, high-titanium, quartz normalized volcanism of the Central Atlantic Magmatic Province (CAMP) preceded or was contemporaneous to the onset of the mass extinction. In addition, the maximum phase of the mass extinction, which affected both the terrestrial and marine ecosystems, was associated with a major regression and repeated, enhanced earthquake activity in Europe. A subsequent transgression resulted in the formation of hiati or condensed successions in many areas in Europe. Later phases of volcanic activity of the CAMP, producing low titanium, quartz normalized and high-iron, quartz normalized basaltic rocks, continued close to the first occurrence of Jurassic ammonites and the defined TJB. During this time the terrestrial ecosystem had begun to recover, but the marine ecosystem remained disturbed.