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When describing and illustrating vertebrae, it is important to consistently depict their orientation, so we can objectively assess and compare the slope of the neural arch, neural canal, or articular surfaces. However, differing vertebral shapes across taxa and across regions of the spinal column make it difficult to maintain consistency, or even define what we mean by the directions “cranial” and “caudal”. Consequently, characters such as “Neural arch slopes cranially 30° relative to the vertical” are disputable rather than objective measurements. Cranial and caudal are defined as directed along the horizontal axis, but several different definitions of “horizontal” are possible:1. Long axis of centrum is horizontal. This is appealing for elongate vertebrae such as sauropod cervicals, but is not always well defined, and is difficult to determine for craniocaudally short vertebrae such as most caudals.2. Articular surfaces of centrum are vertical. Difficult to determine when dealing with facets that are concave or (worse) convex; and ambiguous for “keystoned” vertebrae in which the facets are not parallel.3. Neural canal is horizontal. Anatomically informative, but difficult to determine in vertebrae that have not been fully prepared or CT-scanned, and impossible to see in lateral view. Ambiguous for vertebrae where the dorsal and ventral margins of the canal are not straight or not parallel.4. Similarity in articulation (“horizontal” is defined as a line joining the same point on two similarly oriented copies of the same vertebra when optimally articulated). This is less intuitive than definitions 1–3, but takes the entire vertebra into account.We advocate explicitly stating a definition and using it consistently. In most cases, definition 3 (“Neural canal is horizontal”) best reflects anatomical and developmental realities, and it is therefore preferred. Low-tech methods can be used to determine neural canal orientation with adequate precision for most purposes.

In the 1970s, Jim Jensen excavated multiple gigantic sauropod dinosaurs from Dry Mesa Quarry (DMQ), Colorado. In 1985, he formally named Supersaurus , Ultrasaurus (later Ultrasauros ), and Dystylosaurus based on these specimens. Later, Brian Curtice and coauthors referred the holotype vertebrae of Ultrasauros and Dystylosaurus to Supersaurus , and the referred scapulocoracoid of Ultrasauros to Brachiosaurus . In 2016, we determined that a large cervical vertebra referred to Supersaurus in fact belongs to Barosaurus . Either Supersaurus is synonymous with Barosaurus , or it is distinct but some Barosaurus material has been incorrectly referred. The holotype of Dystylosaurus , an anterior dorsal vertebra, cannot belong to Barosaurus due to its unsplit neural spine, but no shared apomorphies support its referral to Supersaurus and the convenient referral of all large diplodocid material from DMQ to Supersaurus is no longer supportable in light of the Barosaurus cervical. Nomenclatural issues pertaining to Supersaurus must be resolved by reference to its holotype scapulocoracoid. Jensen assigned two scapulocoracoids to Supersaurus , but his vague descriptions, and pervasive confusion around published specimen numbers, make it uncertain which of the two is the type. The two elements have subtle differences and may not belong to the same animal. This is unfortunate, since Supersaurus is the most complete, phylogenetically informative, and nomenclaturally stable of the “Big Three” Dry Mesa sauropods — or at least it was until now. Finally, while the scapulocoracoid referred to Ultrasauros is probably from a titanosauriform, its coracoid does not closely resemble that of the holotype of Brachiosaurus , nor its scapulae those of Giraffatitan . In summary, the DMQ material includes at least three giant sauropods: a titanosauriform that may not be Brachiosaurus , and two diplodocids: Barosaurus and Supersaurus – but the diagnosis of the latter is muddied both by possible confusion with Barosaurus , and by definite confusion regarding the holotype.

Bony ridges occur on the walls of the neural canal in caudal vertebrae of numerous sauropod dinosaurs. These neural canal ridges (NCRs) are anteroposteriorly elongated but do not extend to the ends of the canal. To date, we have observed NCRs in caudal vertebrae of Alamosaurus, Apatosaurus, Astrophocaudia,Brontomerus, Camarasaurus, and Diplodocus. Numerous similar structures occur in extant vertebrates: (1) Neurocentral joints are ventral to NCRs in sauropod caudal vertebrae, and NCRs occur in unfused juvenile arches. Hypothesis rejected. (2) Attachment scars from ligamentum flavum occur at the ends of the dorsal roof of the canal, not the midpoint of the lateral edges, and this mammalian ligament was probably absent in dinosaurs. Hypothesis rejected. (3) Smooth ridges separate the spinal cord from the dorsal spinal vein and paramedullary airways in some crocodilians and birds, respectively. However, these septa persist to the ends of the canal, giving it an 8-shape, unlike the discrete NCRs of dinosaurs. Hypothesis rejected. (4) Bony attachments for denticulate ligaments occur in some non-mammalian vertebrates. The dura mater around the spinal cord fuses to the periosteum of the neural canal in non-mammals, so the denticulate ligaments that support the spinal cord can leave ossified attachment scars. These spinal cord supports have been identified in teleosts, salamanders, and a juvenile lizard, and they are the best match for the morphology of the NCRs in sauropod vertebrae. Functions of NCRs remain obscure. Denticulate ligaments are largest in regions of the vertebral column that experience strong lateral flexion. The hypothesis that NCRs supported the spinal cord of sauropods during lateral tail-whipping is attractive, but inconsistent with our recent discovery of NCRs in a hadrosaur caudal. NCRs are a new osteological correlate of the peripheral nervous system in dinosaurs, and highlight the need for more study in this area.

Birds are unique among extant tetrapods in having pneumatic diverticula of the lungs and air sacs that pass among the viscera and muscles, under the skin, and into the skeleton. In addition, some birds also have supramedullary diverticula inside the neural canal, adjacent to the spinal cord. The anatomy of the supramedullary diverticula has only been documented in a handful of birds to date. Using CT scans, we surveyed the morphology of the supramedullary diverticula in a phylogenetically broad sample of extant birds. Although supramedullary diverticula are present in some small-bodied taxa such as hummingbirds and pigeons, the diverticula are typically larger and more complex in large forms such as ostriches and pelicans. Absence of these diverticula in loons and grebes is consistent with the lack of skeletal pneumatization in diving birds. Other absences, for example in the kestrel, are less explicable. When present, the supramedullary diverticula are best developed in presacral vertebrae, but only occasionally present in the synsacrum, and rarely present in caudal vertebrae. In most taxa we have surveyed, the diverticula are not continuous craniocaudally, but exist as separate segments extending fore and aft from each intervertebral joint, as documented by Müller for the pigeon. Occasionally supramedullary diverticula communicate with interosseous diverticula through foramina in the wall or roof of the neural canal. These foramina are osteological correlates of supramedullary diverticula and allow the presence of these diverticula to be inferred in dry skeletal material and fossil organisms.

AMNH FARB 291 is a unique sauropod dinosaur specimen, excavated in 1898 from the Bone Cabin Quarry of Wyoming (Upper Jurassic; Morrison Formation). It consists of a sequence of five contiguous dorsal vertebrae, all with biconvex centra: a morphology not previously observed in any sauropod, and which is very rare even in other amniotes. The specimen was initially catalogued as Brontosaurus, one of only half a dozen sauropods then known. But it lacks diplodocid features such as bifid neural spines, short centra and dual centroprezygapophyseal laminae. Instead, it resembles Brachiosaurus in its elongated centra, forward-shifted neural arches and lobe-like transverse processes. Centrum lengths (from anteriormost to posteriormost) are 207, 205, 232, 222 and 207 mm, about 60% those of the Brachiosaurus holotype. This specimen could represent: 1. normal development: the condyle would have ossified separately but the individual died before this could happen; 2. a developmental anomaly; 3. a Brachiosaurus with a rare variation not previously seen due to paucity of specimens; 4. a bizarre taphonomic event; 5. a unique taxon in which this is a normal, inherited trait. Vertebrae typically ossify as rings of bone around the notochord, which sometimes persists as a tunnel through the centrum. There is a developmental continuum from biconcave centra with a persistent notochordal tube (as in most fishes), through biconcave centra with the notochord persisting as intervertebral cartilage balls, to the normal amniote condition. However, all known baby sauropod presacrals have already ossified opisthocoelous or amphiplatyan centra, so the present specimen remains mysterious.

Barosaurus is a diplodocid sauropod from the Upper Jurassic Morrison Formation of the western United States, and is known for its very long neck. It is related to the sympatric Diplodocus, and often thought of as more or less identical except with a longer neck. The holotype YPM 429 includes three and a half posterior cervical vertebrae, somewhat distorted and damaged, which are nevertheless very distinctive and quite different from those of Diplodocus. The cervicals of the better known and more complete referred Barosaurus specimen AMNH 6341 show the same characteristic features as the holotype, though not to the same extent: transversely broad but anteroposteriorly short zygapophyseal facets; prezygapophyses carried on broad, squared-off rami; zygapophyses shifted forward relative to the centrum; diapophyses, parapophyses and neural spines shifted backwards; and broad diapophyseal “wings”. These features form a single functional complex, enabling great lateral flexibility, but restricting vertical flexibility. This may indicate that Barosaurus used a different feeding style from other sauropods perhaps sweeping out long arcs at ground level. The Morrison Formation contains at least nine diplodocid species in six to eight genera whose relationships are not yet fully understood, but Barosaurus remains distinct from its relatives.