Histology‐Based Morphology of the Neurocentral Synchondrosis in Alligator Mississippiensis (Archosauria, Crocodylia)

Abstract

Morphology of the neurocentral synchondroses-thin cartilaginous layers between centra and neural arches-are documented in the extant crocodilian, Alligator mississippiensis (Archosauria, Crocodylia). Examination of dry skeletons demonstrates that neurocentral suture closure occurs in very late postnatal ontogeny (after reaching sexual maturity and/or body size ca. 40% from the upper range). Before sexual maturity (body length (BL) ! ca. 1.80 m), completely fused centra and neural arches are restricted to the caudal vertebral series. In contrast, the presacral vertebrae often remain unfused throughout postnatal ontogeny, retaining open sutures in very mature individuals (BL ! 2.80 m). These unfused centra and neural arches are structurally supported by the relatively large surface area of the neurocentral junctions, which results from primarily horizontal (mediolateral) increases with strong positive allometry. Cleared and stained specimens show that the cartilaginous neurocentral synchondrosis starts to form after approximately 40 embryonic days. Histological examination of the neurocentral junction in dorsal and anterior caudal vertebrae of six individuals (BL ¼ 0.28-3.12 m) shows : (1) neurocentral fusion is the result of endochondral ossification of the neurocentral synchondrosis, (2) the neurocentral synchondrosis exhibits bipolar organization of three types of cartilaginous cells, and (3) complex neurocentral sutures (i.e., curved, zigzagged, and/or interdigitated boundaries) come from clumping of bone cells of the neural arches and centra into the neurocentral synchondrosis. The last two morphological features can be advantageous for delaying neurocentral fusion, which seems to be unique in crocodilians and possibly their close relatives, including nonavian dinosaurs and other Mesozoic archosaurs. Anat Rec, 295:18-31, 2012. V V C 2011 Wiley Periodicals, Inc.

Comparative histology of some craniofacial sutures and skull‐base synchondroses in non‐avian dinosaurs and their extant phylogenetic bracket

“…F). The different synchondroseal zones are easily identifiable in Alligator 3, 4 and 5, and are similar to those reported by Ikejiri () in the vertebral column of this species (Fig. D).…”

Novel insight into the origin of the growth dynamics of sauropod dinosaurs

“…sampled specimens of Mussaurus and Massospondylus ) indicates that the complete closure of neurocentral sutures in the vertebral column was achieved late during ontogeny, and well after the attainment of sexual maturity. This pattern agrees with that reported in extant pseudosuchian archosaurs [38, 48], and with Carballido & Sander’s [49] proposal that sexual maturity in sauropods is attained before the complete closure of the neurocentral sutures. General interpretations pertaining to ontogenetic growth should be supported by a large sample of representatives of different ontogenetic stages of each species.…”

The Ontogenetic Osteohistology of Tenontosaurus tilletti

“…Bone surface texture does not correlate with age or size in living Alligator [30], so this method is potentially problematic for fossil archosaurs in the absence of absolute age data. The correlation between age and the progression of neurocentral fusion has been established in living crocodylians [31], [32], with a similar sequence suggested for phytosaurs and aetosaurs [24]. However, neurocentral fusion sequences for dinosaurs are poorly understood [24], though ceratopsians [33], the sauropodomorph Camarasaurus [34], and living birds [35] are known to differ from living crocodylians.…”