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The adult brain lacks sensitivity to changes in the sensory environment found in the juvenile brain. The transplantation of embryonic interneurons has been shown to restore juvenile plasticity to the adult host visual cortex. We show that the transplanted PV interneurons mature, disperse and synaptically integrate into the host brain. However, it is unclear how the integration of transplanted cells re-sensitizes the host brain to visual deprivation. It is not known whether transplanted interneurons directly mediate the renewed cortical plasticity or whether these cells act indirectly by modifying the host interneuron circuitry. Here we find that the transplant-induced reorganization of host circuits is specifically mediated by a developmental signaling pathway Neuregulin (NRG1)/ErbB4 in host parvalbumin (PV) interneurons. It has been shown that during development, NRG1/ErbB4 regulates the excitatory inputs onto PV interneurons. NRG1/ErbB4-mediated loss of excitatory synapses on PV interneurons is necessary for deprivation-induced cortical plasticity in development. Here we show that in transplant recipients, brief visual deprivation reduces the visual activity of host PV interneurons but has negligible effects on the responses of transplanted PV interneurons. Exogenous NRG1 both prevents the deprivation-induced reduction in the visual responses of host PV interneurons and blocks the transplant-induced reorganization of the host circuit. Furthermore, transplantation fails to induce plasticity in recipients that have ErbB4 receptors knocked out specifically in PV interneurons. Altogether, our results indicate that transplanted embryonic interneurons reactivate cortical plasticity by rejuvenating a developmentally restricted signaling pathway in host PV interneurons.

CRANIAL OSTEOLOGY OF JEHOLORNIS Premaxilla The premaxillae are complete in STM 3-8 (Fig. 2A–F). They are edentulous, as reported in previous publications (Zhou & Zhang, 2002, 2003; Lefèvre et al., 2014), and their external surfaces are marked by several nutrient foramina. No pits are present to receive the dentary teeth, which is different from the condition present in Ichthyornis Marsh, 1873 (Field et al., 2018). The tip of the corpus forms a ventral projection, suggesting that the tip of the beak might have been slightly hooked. Caudal to the rostral ‘hook’, the ventral margin of the premaxillary corpus is straight. The relative level of the ventral projection of this rostral ‘hook’ varies among previously reported specimens, being absent in Jeholornis YFGP-yb2 and exaggerated relative to STM 3-8 in Kompsornis AGB- 6997 (Lefèvre et al., 2014; Wang et al., 2020a). This is interpreted here as attributable to variation in preservation, but further studies are needed to exclude the possibility of intraspecific variation confidently. The premaxillary corpora are fused, whereas the frontal processes are separated (Hu et al., 2022). The frontal (nasal) process of the premaxilla is relatively short and therefore does not contact the frontal but instead articulates distally with the dorsal surface of the nasal, as in other non-ornithothoracine stem birds (e.g. Archaeopteryx and Sapeornis; Rauhut, 2014; Kundrát et al., 2018; Hu et al., 2020a) apart from Confuciusornis Hou et al., 1995 (Chiappe et al., 1999; Elżanowski et al., 2018; Wang et al., 2019a). This is evidenced by the extension level of the articular facet present in the frontal process of the premaxilla. The maxillary process of the premaxilla is short and articulates medially with the premaxillary process of the maxilla (Fig. 2A, B). It appears to be step-like, with the dorsal margin extending farther caudally than the ventral margin. The palatal process is crushed mediolaterally but could still be distinguished from the maxillary process in the 3D reconstruction. Maxilla The maxillae in Jeholornis STM 3-8 are well preserved, being almost in articulation with the premaxillae and the lacrimals (Fig. 2A, B, J–L). The premaxillary process bears a lateral depression that receives the dorsal part of the maxillary process of premaxilla. The jugal process is slender, being half of the dorsoventral height of the premaxillary process and more than twice its length. A shallow groove is present along the medial surface of the jugal process, indicating extensive contact area with the palatine, similar to Archaeopteryx (Mayr et al., 2007). The medial surface of the maxilla is rarely visible among Mesozoic bird specimens, precluding further comparisons. Although the palatal process is mostly crushed, it seems that it was sheet-like and well developed, and therefore most probably contacted the vomer, similar to the Late Cretaceous enantiornithine Gobipteryx Elżanowski, 1974 and the ornithuromorph Ichthyornis (Chiappe et al., 2001; Field et al., 2018), the only other Mesozoic birds in which the morphology of the palatal process of the maxilla is known so far. A dorsoventrally elongate oval fenestra is present between the jugal process and the ascending process in Jeholornis, being enclosed caudally by a thin, bony bar. We identify this tentatively as the maxillary fenestra (Witmer, 1997). However, it is unclear whether it is homologous with the maxillary fenestra or promaxillary fenestra of non-avian theropods and Archaeopteryx (Witmer, 1997; Barsbold & Osmólska, 1999; Xu & Wu, 2001; Mayr et al., 2007; Rauhut, 2014; Rauhut et al., 2018) or with the accessory fenestra present in the enantiornithine bird Pengornis Zhou, Clarke & Zhang, 2008 (O’Connor & Chiappe, 2011). Two alveoli are present in the maxilla. Two teeth are preserved in the left maxilla, and another two similar-sized teeth are dislocated beside the right maxilla. The maxillary teeth are straight and subconical, with blunt crowns and an expanded root. Lacrimal Both lacrimals are well preserved in articulation with the maxillae in Jeholornis STM 3-8 (Fig. 2A, B, G–I). The rostrodorsal ramus is remarkably short, approximately one-quarter the length of the long caudodorsal ramus. This differs from most other Early Cretaceous birds and non-avian theropods (e.g. Archaeopteryx and Sinornithosaurus Xu, Wang & Wu, 1999; Xu & Wu, 2001; Rauhut, 2014; Kundrát et al., 2018; Rauhut et al., 2018), in which the rostrodorsal ramus is long and the caudodorsal ramus short. The ventral ramus is caudally recurved in Jeholornis, such that the caudal margin formed by the ventral and caudodorsal processes is concave, forming the rostral/ rostrodorsal margin of the orbit. This is similar to the morphology in more crownward birds (e.g. the Late Cretaceous ornithurine bird Ichthyornis), although the rostrodorsal ramus is even more strongly reduced in Ichthyornis and does not contact the maxilla, unlike in Jeholornis (Field et al., 2018). The lacrimal morphology of Jeholornis also contrasts with the morphology of most other Early Cretaceous birds and non-avian theropods, in which the ventral ramus is almost perpendicular to the ventral margin of the skull or is inclined cranially (Wang et al., 2021). The lacrimal of the confuciusornithiforms appears to be slender and reduced, also presenting a short or totally absent rostrodorsal ramus and slightly caudally recurved ventral ramus (Elżanowski et al., 2018; Wang & Zhou, 2018; Wang et al., 2019a), potentially similar to Jeholornis. However, owing to the potential uncertainty from 2D preservation of currently published skulls of confuciusornithiforms, 3D data are needed to confirm this in future analyses. Disarticulation prevents detailed reconstruction of articulations between the lacrimal, the nasal and the preorbital ossification in Jeholornis STM 3-8. The ventral ramus of the lacrimal is interpreted as contacting the jugal process of the maxilla and, potentially, might have contacted the rostral tip of the jugal, whereas the lacrimal contacts the jugal in other theropods (Xu & Wu, 2001; Rauhut, 2014; Kundrát et al., 2018; Rauhut et al., 2018). The caudal margin of the lacrimal, which forms the cranial margin of the orbit, is remarkably excavated, and the excavation extends across both the ventral and caudodorsal processes. A lacrimal foramen lies within the centre of the excavation, entering medially into the main body of the lacrimal at around its mid-height, at the junction of the ventral ramus and the caudodorsal ramus. The size and central position of this foramen resemble the condition in Ichthyornis (Field et al., 2018), although the lacrimal of Ichthyornis differs in lacking the rostrodorsal ramus and thus the contact with the maxilla. In contrast, this foramen is much smaller and penetrates lateromedially in enantiornithine IVPP V12707, which also lacks any excavation on the lacrimal on the rostral orbit margin of the lacrimal (Wang et al. 2021). This contrasts with the craniocaudal extension of the lacrimal foramen in Jeholornis. Nasal The left nasal is well preserved (Fig. 3A, B). The nasal corpus is mediolaterally broad, similar to Sapeornis (Hu et al., 2019, 2020a) but unlike the more elongated condition present in Archaeopteryx (Mayr et al., 2007; Rauhut, 2014; Kundrát et al., 2018), confuciusornithiforms (Elżanowski et al., 2018; Wang et al., 2019a) and enantiornithines (O’Connor & Chiappe, 2011). Both the premaxillary and the maxillary processes are delicate and sharply tapered rostrally. The premaxillary process is slightly longer than the maxillary process, and the deflections of both processes in the left nasal are taphonomic, resulting from crushing between the right nasal and left lacrimal. The premaxillary process does not extend to the base of the frontal process of the premaxilla, therefore leaving the premaxilla to form most of the rostrodorsal margin of the external naris. This is different from the condition in Archaeopteryx, in which the premaxillary process is substantially longer than the maxillary process and forms part of the dorsal–rostrodorsal margin of the external naris (Rauhut, 2014; Kundrát et al., 2018). The maxillary process of the premaxilla of Jeholornis is also relatively short and does not extend to the base of the ascending process of the maxilla, therefore not contacting the premaxilla. This leaves the maxilla to form the caudoventral margin of the external naris, similar to Archaeopteryx (Rauhut, 2014). Preorbital ossification Specimen STM 3-8 preserves a mysterious pair of sheet-like elements previously referred to as ‘preorbital ossifications’ (Fig. 3C, D; Hu et al., 2022), which might represent prefrontals based on their overall shape and location. This is supported by their location almost parallel to the craniodorsal process of the lacrimal, which rules out identification as the ectethmoid, especially considering that other rostral elements are mostly preserved in situ. If this element is the prefrontal, it differs from the prefrontals of all other pennaraptorans so far, which are strongly reduced or absent, being typically smaller than the nasal (e.g. in Archaeopteryx and Sinornithosaurus; Xu & Wu, 2001; Rauhut et al., 2018). This could suggest that an unfused, expanded prefrontal might be a derived feature of Jeholornis and challenges the hypothesis based on the embryonic observations that the prefrontal fused to form the caudodorsal ramus of the lacrimal in all birds (Smith-Paredes et al., 2018). If correctly identified, this suggests that a broad prefrontal co-exists with a lacrimal with a well-developed caudodorsal process in Jeholornis. However, owing to the lack of available comparisons of any similar ossifications among non-avian dinosaurs and birds, we cannot exclude the possibility that this bone represents some other element that is rarely preserved or developed in Mesozoic birds. For example, the preorbital ossification described here could be a palpebral, although we consider this to be much less likely owing to its preserved location, close to the midline of the skull. Jugal Only the left jugal is preserved in STM 3-8 (Fig. 3G, H). The maxillary process is as slender as the jugal process of the maxilla, similar to the jugal of Archaeopteryx (Elżanowski & Wellnhofer, 1996; Kundrát et al., 2018; Rauhut et al., 2018) but in contrast to the relatively more robust condition in Sapeornis (Hu et al., 2020a). The rostral quarter of the maxillary process is slightly constricted and bears a depression in the distal end of the dorsal margin, defining the articulation with the maxilla. The articulation between the jugal and the maxilla is much shorter than in Sapeornis, in which the maxilla extends caudally almost to the base of the postorbital bar (Hu et al., 2020a). An oval concavity is present centrally on the lateral surface of the maxillary process. A similar depression is also present in Archaeopteryx, although in a more rostral position (Mayr et al., 2007; Rauhut, 2014), but is absent in most other Mesozoic birds (e.g. Sapeornis, Ichthyornis and enantiornithines; Wang & Hu, 2017; Field et al., 2018; Hu et al., 2020a). The quadratojugal process of the jugal of Jeholornis lacks the notch present in Sapeornis and many non-avian theropods, which is also absent in known enantiornithines but possibly present in Ichthyornis (Rauhut, 2003; Xu et al., 2015; Wang & Hu, 2017; Field et al., 2018; Hu et al., 2020b). Because of this, the quadratojugal of Jeholornis articulates with the dorsolateral surface of the quadratojugal process of the jugal, differing from the wedge-like articulation seen in Sapeornis and other Mesozoic theropods (e.g. Linheraptor Xu et al., 2015; Xu et al., 2015; Hu et al., 2020a). The postorbital process of the jugal of Jeholornis is triangular with a broad base and is dorsally oriented. This contrasts with the caudodorsal orientation seen in Archaeopteryx and Sapeornis (Mayr et al., 2007; Rauhut, 2014; Kundrát et al., 2018; Hu et al., 2020a). A shallow impression on the rostrolateral surface of the postorbital process defines the articulation with the postorbital, indicating the presence of a complete postorbital bar in Jeholornis. Quadratojugal The left quadratojugal is complete, but slightly disarticulated from the jugal (Fig. 3E, F). The jugal process is twice as long as the squamosal process and is more slender; both are bluntly tapered. The ventromedial surface of the jugal process contacts the jugal, in contrast to the inserting articulation with the caudal notch of the jugal in Sapeornis and most non-avian theropods (Xu et al., 2015; Hu et al., 2020a). The squamosal process is reduced and does not contact the squamosal dorsally, similar to the condition in other Mesozoic birds, including Archaeopteryx, Sapeornis and various others (e.g. RapaxaƲis pani Morschhauser et al., 2009 and Cruralispennia multidonta Wange et al., 2017; Mayr et al., 2007; O’Connor et al., 2011; Rauhut, 2014; Wang et al., 2017b; Hu et al., 2020a). Postorbital The left postorbital is completely preserved and the right is broken in STM 3-8 (Fig. 3I–K). The postorbital is triradiate and more robust than that of Archaeopteryx (Kundrát et al., 2018; Rauhut et al., 2018; Hu et al., 2020a). The jugal process is long and tapers ventrally, extending most of the skull height ventrally, and therefore forming most of the postorbital bar. In contrast, one specimen of Archaeopteryx preserves a slightly longer jugal process (Rauhut et al., 2018), whereas others preserve a jugal process almost equal in length to the other processes (Kundrát et al., 2018). The elongate jugal process of Jeholornis more closely resembles the condition in some enantiornithines (e.g. Longusunguis Wang et al., 2014 and enantiornithines LP4450 and IVPP V12707). However, it is much more robust than that of some other enantiornithines (Sanz et al., 1997; Hu et al., 2020b; Zhou et al., 2008). The squamosal process of the postorbital of Jeholornis is short, less than half the length of the frontal process, and has a sharply tapered end, whereas this process is longer in Sapeornis and Archaeopteryx (Rauhut et al., 2018; Hu et al., 2020a). The dorsal surface of the squamosal process bears a concave facet for articulation with the squamosal. Squamosal Both squamosals are preserved, although only the right is complete in STM 3-8 (Fig. 3L, M). The squamosal is not fused to the braincase, similar to the condition in non-avian theropods, Archaeopteryx and enantiornithines (e.g. LP4450 and IVPP V12707; Elżanowski & Wellnhofer, 1996; Sanz et al., 1997; Rauhut, 2003; Norman et al., 2004; Xu et al., 2015; Rauhut et al., 2018; Wang et al., 2021). The rarity with which squamosals are preserved in other stem birds complicates interpretation of the morphology seen in Jeholornis. However, the concavity in the medial surface of this bone fits the otic process of the quadrate, and thus could be interpreted as the quadrate cotyle of the squamosal, supporting our identification of this element as a squamosal. The triangular, sharply tapered, rostroventrally directed process is identified as the postorbital process, resembling that in enantiornithine IVPP V12707 (Wang et al., 2021), and contrasts with the forked condition in Archaeopteryx (Elżanowski & Wellnhofer, 1996; Kundrát et al., 2018). The ventrally oriented quadratojugal process is short, with a blunt ventral margin, not contacting the quadratojugal. This suggests that the loss of the quadratojugal–squamosal contact might have evolved independently in Jeholornis and in ornithurines, but remained present in at least some enantiornithines (e.g. IVPP 12707) [although it also could have been regained secondarily as a derived feature (Wang et al., 2021)]. It cannot be determined whether the dorsal portion is complete, hence the shape of the parietal and the paroccipital processes of the squamosal remain uncertain. Quadrate Both quadrates are almost completely preserved (Fig. 4A–C). The shaft of the quadrate extends from the otic process dorsally to the lateral condyle caudoventrally. The orbital process is broad and lateromedially thin, resembling that of Archaeopteryx (Rauhut et al., 2018), Sapeornis (Hu et al., 2020a) and known enantiornithines [e.g. Zhouornis Zhang et al., 2013 (Zhang et al., 2013) and Pterygornis Wang et al., 2014 (Wang et al., 2015)], and differs from the narrow and rostrally projecting condition in Ichthyornis and crown birds, including the Late Cretaceous Asteriornis Field et al., 2020 (Elżanowski & Stidham, 2010; Field et al., 2018, 2020). The otic process is plesiomorphically single headed, as in Sapeornis and enantiornithines (Wang et al., 2015, 2021; Hu et al., 2020a), but differing from the divided otic capitulum and squamosal capitulum in neognaths, including Asteriornis (Field et al., 2020). A dorsoventrally oriented longitudinal ridge is present caudally on the medial surface of the otic process, similar to the condition in Sapeornis and enantiornithines (Zhang et al., 2013; Wang et al., 2015; Hu et al., 2020a), defining the caudal margin of a gentle excavation on the medial surface of the orbital process. The lateral surface is also excavated by a similar dorsoventrally oriented longitudinal ridge, but it cannot be determined whether this is attributable to the lateromedially crushed preservation of the orbital process. No pneumatic foramen is observed, different from the condition in most modern birds and Late Cretaceous ornithurines (e.g. Ichthyornis and Asteriornis; Elżanowski & Stidham, 2010; Field et al., 2018, 2020). However, two potential pneumatic recesses could be identified on the lateral surface of the quadrate in Jeholornis. Both the lateral and medial condyles are of a similar size and project caudally, defining a concave caudal margin for the quadrate. Frontal The frontals are tightly articulated with each other in STM 3-8, but not entirely fused, with the interfrontal suture clearly visible (Fig. 4D, E), similar to the condition observed in other Jeholornis specimens (Lefèvre et al, Published as part of Hu, Han, Wang, Yan, Fabbri, Matteo, O, Jingmai K., Connor, Mcdonald, Paul G., Wroe, Stephen, Yin, Xuwei, Zheng, Xiaoting, Zhou, Zhonghe & Benson, Roger B. J., 2023, Cranial osteology and palaeobiology of the Early Cretaceous bird Jeholornis prima (Aves: Jeholornithiformes), pp. 93-112 in Zoological Journal of the Linnean Society 198 (1) on pages 95-107, DOI: 10.1093/zoolinnean/zlac089, http://zenodo.org/record/7926859, {"references":["Zhou Z, Zhang F. 2002. A long-tailed, seed-eating bird from the Early Cretaceous of China. Nature 418: 405 - 409.","Zhou Z, Zhang F. 2003. Jeholornis compared to Archaeopteryx, with a new understanding of the earliest avian evolution. Naturaeissenschaften 90: 220 - 225.","Lefevre U, Hu D, Escuillie F, Dyke G, Godefroit P. 2014. A new long-tailed basal bird from the Lower Cretaceous of north-eastern China. Biological Journal of the Linnean Society 113: 790 - 804.","Field DJ, Hanson M, Burnham D, Wilson LE, Super K, Ehret D, Ebersole JA, Bhullar BAS. 2018. Complete Ichthyornis skull illuminates mosaic assembly of the avian head. Nature 557: 96 - 100.","Wang X, Huang J, Kundrat M, Cau A, Liu X, Wang Y, Ju S. 2020 a. A new jeholornithiform exhibits the earliest appearance of the fused sternum and pelvis in the evolution of avialan dinosaurs. Journal of Asian Earth Sciences 199: 104401.","Hu H, Wang Y, McDonald PG, Wroe S, O'Connor JK, Bjarnason A, Bevitt JJ, Yin X, Zheng X, Zhou Z, Benson RBJ. 2022. Earliest evidence for fruit consumption and potential seed dispersal by birds. eLife 11: e 74751.","Rauhut OWM. 2014. New observations on the skull of Archaeopteryx. Palaontologische Zeitschrift 88: 211 - 221.","Kundrat M, Nudds J, Kear BP, Lu J, Ahlberg P. 2018. The first specimen of Archaeopteryx from the Upper Jurassic Mornsheim Formation of Germany. Historical Biology 31: 3 - 63.","Hu H, O'Connor JK, McDonald PG, Wroe S. 2020 a. Cranial osteology of the Early Cretaceous Sapeornis chaoyangensis (Aves: Pygostylia). Cretaceous Research 113: 104496.","Chiappe LM, Ji SA, Ji Q, Norell MA. 1999. Anatomy and systematics of the Confuciusornithidae (Theropoda, Aves) from the late Mesozoic of northeastern China. Bulletin of the American Museum of Natural History 242: 1 - 89.","Elzanowski A, Peters DS, Mayr G. 2018. Cranial morphology of the Early Cretaceous bird Confuciusornis. Journal of Vertebrate Paleontology 38: e 1439832.","Wang M, Jingmai K, Zhou Z. 2019 a. A taxonomical revision of the Confuciusornithiformes (Aves: Pygostylia). Vertebrata Palasiatica 57: 1 - 37.","Mayr G, Pohl B, Hartman S, Peters DS. 2007. The tenth skeletal specimen of Archaeopteryx. Zoological Journal of the Linnean Society 149: 97 - 116.","Chiappe LM, Norell M, Clark J. 2001. A new skull of Gobipteryx minuta (Aves: Enantiornithes) from the Cretaceous of the Gobi Desert. American Museum NoVitates 3346: 1 - 15.","Witmer LM. 1997. The evolution of the antorbital cavity of archosaurs: a study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Journal of Vertebrate Paleontology 17: 1 - 76.","Barsbold R, Osmolska H. 1999. The skull of Velociraptor [Theropoda] from the Late Cretaceous of Mongolia. Acta Palaeontologica Polonica 44: 189 - 219.","Xu X, Wu XC. 2001. Cranial morphology of Sinornithosaurus millenii Xu et al. 1999 (Dinosauria: Theropoda: Dromaeosauridae) from the Yixian Formation of Liaoning, China. Canadian Journal of Earth Sciences 38: 1739 - 1752.","Rauhut OWM, Foth C, Tischlinger H. 2018. The oldest Archaeopteryx (Theropoda: Avialiae): a new specimen from the Kimmeridgian / Tithonian boundary of Schamhaupten, Bavaria. PeerJ 6: e 4191.","Zhou Z, Clarke J, Zhang F. 2008. Insight into diversity, body size and morphological evolution from the largest Early Cretaceous enantiornithine bird. Journal of Anatomy 212: 565 - 577.","Wang M, Stidham TA, Li Z, Xu X, Zhou Z. 2021. Cretaceous bird with dinosaur skull sheds light on avian cranial evolution. Nature Communications 12: 3890.","Wang M, Zhou Z. 2018. A new confuciusornithid (Aves: Pygostylia) from the Early Cretaceous increases the morphological disparity of the Confuciusornithidae. Zoological Journal of the Linnean Society 185: 417 - 430.","Hu H, Sansalone G, Wroe S, McDonald PG, O'Connor JK, Li Z, Xu X, Zhou Z. 2019. Evolution of the vomer and its implications for cranial kinesis in Paraves. Proceedings of the National Academy of Sciences of the United States of America 116: 19571 - 19578.","Smith-Paredes D, Nunez-Leon D, Soto-Acuna S, O'Connor J, Botelho JF, Vargas AO. 2018. Dinosaur ossification centres in embryonic birds uncover developmental evolution of the skull. Nature Ecology & EVolution 2: 1966 - 1973.","Elzanowski A, Wellnhofer P. 1996. Cranial morphology of Archaeopteryx: evidence from the seventh skeleton. Journal of Vertebrate Paleontology 16: 81 - 94.","Wang Y, Hu H, O'Connor JK, Wang M, Xu X, Zhou Z, Wang X, Zheng X. 2017 a. A previously undescribed specimen reveals new information on the dentition of Sapeornis chaoyangensis. Cretaceous Research 74: 1 - 10.","Rauhut OW. 2003. Special papers in palaeontology, the interrelationships and eVolution of basal theropod dinosaurs. London: Blackwell Publishing.","Xu X, Pittman M, Sullivan C, Choiniere JN, Tan Q, Clark JM, Norell MA, Wang S. 2015. The taxonomic status of the Late Cretaceous dromaeosaurid Linheraptor exquisitus and its implications for dromaeosaurid systematics. Vertebrata Palasiatica 53: 29 - 62.","Hu H, O'Connor JK, Wang M, Wroe S, McDonald PG. 2020 b. New anatomical information on the bohaiornithid Longusunguis and the presence of a plesiomorphic diapsid skull in Enantiornithes. Journal of Systematic Palaeontology 18: 1481 - 1495.","Wang M, O'Connor JK, Pan Y, Zhou Z. 2017 b. A bizarre Early Cretaceous enantiornithine bird with unique crural feathers and an ornithuromorph plough-shaped pygostyle. Nature Communications 8: 14141.","Norman DB, Sues HD, Witmer LM, Coria RA. 2004. The Dinosauria. Berkeley: University of California Press, 393 - 412.","Zhang Z, Chiappe LM, Han G, Chinsamy A. 2013. A large bird from the Early Cretaceous of China: new information on the skull of enantiornithines. Journal of Vertebrate Paleontology 33: 1176 - 1189.","Wang M, Hu H, Li Z. 2015. A new small enantiornithine bird from the Jehol Biota, with implications for early evolution of avian skull morphology. Journal of Systematic Palaeontology 14: 481 - 497.","Field DJ, Benito J, Chen A, Jagt JWM, Ksepka DT. 2020. Late Cretaceous neornithine from Europe illuminates the origins of crown birds. Nature 579: 397 - 401.","Elzanowski A, Stidham TA. 2010. Morphology of the quadrate in the Eocene anseriform Presbyornis and extant galloanserine birds. Journal of Morphology 271: 305 - 323.","Clark JM, Norell MA, Rowe T. 2002. Cranial anatomy of Citipati osmolskae (Theropoda, Oviraptorosauria), and a reinterpretation of the holotype of OViraptor philoceratops. American Museum NoVitates 2002: 1 - 24.","Yin YL, Pei R, Zhou CF. 2018. Cranial morphology of SinoVenator changii (Theropoda: Troodontidae) on the new material from the Yixian Formation of western Liaoning, China. PeerJ 6: e 4977.","Fabbri M, Koch NM, Pritchard AC, Hanson M, Hoffman E, Bever GS, Balanoff AM, Morris ZS, Field DJ, Camacho J, Rowe TB, Norell MA, Smith RM, Abzhanov A, Bhullar BAS. 2017. The skull roof tracks the brain during the evolution and development of reptiles including birds. Nature Ecology & EVolution 1: 1543 - 1550.","Beyrand V, Voeten DFAE, Bures S, Fernandez V, Janacek J, Jirak D, Rauhut O, Tafforeau P. 2019. Multiphase progenetic development shaped the brain of flying archosaurs. Scientific Reports 9: 10807.","Watanabe A, Gignac PM, Balanoff AM, Green TL, Kley NJ, Norell MA. 2019. Are endocasts good proxies for brain size and shape in archosaurs throughout ontogeny? Journal of Anatomy 234: 291 - 305.","Alonso PD, Milner AC, Ketcham RA, Cookson MJ, Rowe TB. 2004. The avian nature of the brain and inner ear of Archaeopteryx. Nature 430: 666 - 669.","Balanoff AM, Bever GS, Rowe TB, Norell MA. 2013. Evolutionary origins of the avian brain. Nature 501: 93 - 96.","Tsuihiji T, Barsbold R, Watabe M, Tsogtbaatar K, Chinzorig T, Fujiyama Y, Suzuki S. 2014. An exquisitely preserved troodontid theropod with new information on the palatal structure from the Upper Cretaceous of Mongolia. Naturaeissenschaften 101: 131 - 142.","O'Connor JK, Sun C, Xu X, Wang X, Zhou Z. 2012. A new species of Jeholornis with complete caudal integument. Historical Biology 24: 29 - 41.","Elzanowski A. 1991. New observations of the skull of Hesperornis with reconstructions of the bony palate and otic region. Postilla 207: 1 - 20.","Xu L, Buffetaut E, O'Connor J, Zhang X, Jia S, Zhang J, Chang H, Tong H. 2021. A new, remarkably preserved, enantiornithine bird from the Upper Cretaceous Qiupa Formation of Henan (central China) and convergent evolution between enantiornithines and modern birds. Geological Magazine 158: 2087 - 2094.","O'Connor JK, Zhang Y, Chiappe LM, Meng Q, Quanguo L, Di L. 2013 b. A new enantiornithine from the Yixian formation with the first recognized avian enamel specialization. Journal of Vertebrate Paleontology 33: 1 - 12.","Dumont M, Tafforeau P, Bertin T, Bhullar BA, Field D, Schulp A, Strilisky B, Thivichon-Prince B, Viriot L, Louchart A. 2016. Synchrotron imaging of dentition provides insights into the biology of Hesperornis and Ichthyornis, the ' last' toothed birds. BMC EVolutionary Biology 16: 178.","O'Connor JK, Stidham TA, Harris JD, Lamanna MC, Bailleul AM, Hu H, Wang M, You H. 2022. Avian skulls represent a diverse ornithuromorph fauna from the Lower Cretaceous Xiagou Formation, Gansu Province, China. Journal of Systematics and EVolution 60: 1172 - 1198.","Zheng X, O'Connor J, Wang X, Wang M, Zhang X, Zhou Z. 2014. On the absence of sternal elements in Anchiornis (Paraves) and Sapeornis (Aves) and the complex early evolution of the avian sternum. Proceedings of the National Academy of Sciences of the United States of America 111: 13900 - 13905.","Zelenitsky DK, Therrien F, Ridgely RC, McGee AR, Witmer LM. 2011. Evolution of olfaction in non-avian theropod dinosaurs and birds. Proceedings of the Royal Society B: Biological Sciences 278: 3625 - 3634.","Choiniere JN, Neenan JM, Schmitz L, Ford DP, Chapelle KEJ, Balanoff AM, Sipla JS, Georgi JA, Walsh SA, Norell MA, Xu X, Clark JM, Benson RBJ. 2021. Evolution of vision and hearing modalities in theropod dinosaurs. Science 372: 610 - 613."]}

Hu, Han, Wang, Yan, Fabbri, Matteo, O, Jingmai K., Connor, Mcdonald, Paul G., Wroe, Stephen, Yin, Xuwei, Zheng, Xiaoting, Zhou, Zhonghe, Benson, Roger B. J. (2023): Cranial osteology and palaeobiology of the Early Cretaceous bird Jeholornis prima (Aves: Jeholornithiformes). Zoological Journal of the Linnean Society 198 (1): 93-112, DOI: 10.1093/zoolinnean/zlac089, URL: https://academic.oup.com/zoolinnean/article/198/1/93/6768673

EMENDED DIAGNOSIS OF J. PRIMA Based on the morphological study of this specimen, we provide the following revised diagnosis for J. prima. A large stem bird with the following combination of features: premaxilla edentulous with short maxillary process; two teeth with blunt crowns in maxilla and three relatively smaller teeth in dentary (new); paired, sheet-like preorbital ossifications present near the nasals (new, autapomorphy); C-shaped lacrimal with short rostrodorsal ramus and lacrimal foramen (new); unreduced postorbital forming a complete postorbital bar with jugal (new); pterygoid rami of vomer much longer than the fused rostral portion, expanded in the middle and lacking the caudodorsal process (new); palatine with broad pterygoid wing and jugal process (new); narrow and restricted mandibular fenestra between prearticular and surangular (new); 27 caudal vertebrae in total, with the transition point occurring after the fifth vertebra; lateral trabecula of sternum absent; caudalmost pair of sternal ribs expanded; first phalanx of the third manual digit twice as long as the second phalanx; ratio of forelimb (humerus plus ulna plus carpometacarpus) to hindlimb (femur plus tibiotarsus plus tarsometatarsus) of ~1.2:1; dorsal margin of the ilium nearly straight and craniodorsal–caudoventrally oriented (modified from Zhou & Zhang, 2002; O’Connor et al., 2012; Zheng et al., 2020)., Published as part of Hu, Han, Wang, Yan, Fabbri, Matteo, O, Jingmai K., Connor, Mcdonald, Paul G., Wroe, Stephen, Yin, Xuwei, Zheng, Xiaoting, Zhou, Zhonghe & Benson, Roger B. J., 2023, Cranial osteology and palaeobiology of the Early Cretaceous bird Jeholornis prima (Aves: Jeholornithiformes), pp. 93-112 in Zoological Journal of the Linnean Society 198 (1) on page 107, DOI: 10.1093/zoolinnean/zlac089, http://zenodo.org/record/7926859, {"references":["Zhou Z, Zhang F. 2002. A long-tailed, seed-eating bird from the Early Cretaceous of China. Nature 418: 405 - 409.","O'Connor JK, Sun C, Xu X, Wang X, Zhou Z. 2012. A new species of Jeholornis with complete caudal integument. Historical Biology 24: 29 - 41.","Zheng X, Sullivan C, O'Connor JK, Wang X, Wang Y, Zhang X, Zhou Z. 2020. Structure and possible ventilatory function of unusual, expanded sternal ribs in the Early Cretaceous bird Jeholornis. Cretaceous Research 116: 104597."]}

Although it is commonly considered that in birds, there is a trend towards reduced dentition, teeth persisted in birds for 90 Ma and numerous macroscopic morphologies are observed. However, the extent to which the microstructure of bird teeth differs from other lineages is poorly understood. To explore the microstructural differences of the teeth of birds in comparison to closely related non-avialan dinosaurs, the enamel and dentine-related features were evaluated in four Mesozoic paravian species from the Yanliao and Jehol biotas. Different patterns of dentinal tubular tissues with mineralized extensions of the odontoblast processes were revealed through the examination of histological sectioning under electron microscopy. Secondary modification of the tubular structures, forming reactive sclerotic dentin of Longipteryx, and the mineralization of peritubular dentin of Sapeornis were observed in the mantle dentin region. The new observed features combined with other dentinal-associated ultrastructure suggest that the developmental mechanisms controlling dentin formation are quite plastic, permitting the evolution of unique morphologies associated with specialized feeding behaviours in the toothed birds. Proportionally greater functional stress placed on the stem bird teeth may have induced reactive dentin mineralization, which was observed more often within tubules of these taxa, suggests modifications to the dentin to counteract potential failure.