Dinosauria

DINOSAURIA: Saurischia & Ornithischia

Approximately 315.5 Ma, diapsids (Diapsida) split off from sauropsids, and 307 Ma, Sauria split off from them, which in turn split 265.1 Ma into the ancestors of lizards (Lacertilia) and archosaurs (Archosauria) (2014EzcurraMD_ButlerRJ). The split of turtles (Testudines) from archosaurs occurred at least 255 Ma (2012ChiariY_DelsucF). The oldest archosaur of the avian lineage, Asilisaurus kongwe, which is a sister taxon to Dinosauria, dates back to the interval 247.2-242.0 Ma, designated the Anisian stage of the Middle Triassic (2010NesbittSJ_TsujiLA; 2023CohenKM_CarN). The branch extending to dinosaurs originates from archosaurs. Their oldest remains were found in a rock formed 233.23±0.73 Ma (2018LangerMC_DaRosaÁAS). Among dinosaurs, there are Saurischia and Ornithischia varieties.

Dinosaurs.
Fragment of the exposition of the Darwin Museum (Moscow), photo by the author.

The study of soft tissues of the musculoskeletal system of such early tetrapods is possible only indirectly, focusing on bone markers. In particular, the non-ossified inner wall of the acetabulum is an osteological correlate of the presence of internal ligaments in the hip joint (2015TsaiHP_HollidayCM). A.N. Kuznetsov, A.G. Sennikov (2000) distinguish three types of perforated acetabulums: those with small and large holes, as well as through marginal notches. According to the authors' observations, the non-perforated (closed) acetabulum is the original structure and is found only in primitive forms. Thus, the morphological series of progressive transformation from early thecodonts to birds begins with a non-perforated acetabulum. Subsequently, a small acetabular opening appears (the first type). It gradually increases in size (type II) and reaches its full development (type III), called "open acetabulum". The opening in the acetabulum is found in the following ancient species: Lagosuchus talampayensis (236-234 Ma), Stauricosaurus pricei (233.23 Ma), Herrerasaurus ischigualastensis (237-201 Ma), Terrestrisuchus gracilis (209-201 Ma), Orthosuchus strombergi (209-191 Ma), Tyrannosaurus rex (83.5-66.0 Ma), Ankylosaurus sp. (68-66 Ma) (2000KuznetsovAN_SennikovAG).

Skeleton of a tabrosaurus baby Tabrosaurus bataar (Upper Cretaceous, Mongolia, copy); exhibit of the Darwin Museum (Moscow), photo by the author.

It was established that fossil archosauromorphs had three ligaments in the hip joint: iliofemoral, ischiofemoral, and pubofemoral. The pubofemoral and ischiofemoral ligaments were connected distally, forming the ligamentum capitis femoris (LCF), attached to the fovea capitis. These two ventral ligaments formed an intracapsular double origin of the LCF in the case of a completely perforated acetabulum. In the imperforate acetabulum, the pubofemoral and ischiofemoral ligaments originated at the outer pubic and ischial margins of the acetabulum, respectively. In mature skeletons, the LCF formed a flat or concave fossa on the femoral head. In immature skeletons, the LCF articulated with the cartilage that formed the epiphysis, as in modern juvenile birds and crocodilians. For this reason, the femoral head fossa is not found in fossil bones. Among dinosaurs, distinctly concave or flat fovea capitis have rarely been observed in coelurosaurian theropods. In the saurischian lineage, the femoral head fossa varied from shallow in sauropodomorphs to flat in tyrannosaurids. Basal dinosauromorphs had an indistinct femoral head fossa, suggesting a significant thickness of hyaline cartilage. The evolution of the hip joint in basal dinosaurs is characterized by independent modifications of the two ventral capsular ligaments in the LCF. The pubofemoral and ischiofemoral ligaments in diapsids and sauropodomorphs had a common femoral attachment at the fossa of the femoral head. It was located entirely on the epiphyseal hyaline cartilage, so it did not leave a fossa on the subchondral surface. It is assumed that the LCF was relatively shorter in sauropodomorphs, more basal than mussaurids (2018TsaiHP_HollidayCM). In the illustrations to their work, the cited researchers provided examples of the presence of LCF features: a fossa on the femoral head and a hole in the floor of the acetabulum. Based on the above, we can confidently state that LCF was present in the following extinct species: Allosaurus fragilis (157-145 Ma), Anzu wyliei (67.2-66 Ma), Apatosaurus sp. (161-145 Ma), Asilisaurus kongwe (247-242 Ma), Coelophysis (237-183 Ma), Coelurus fragilis (157-145 Ma), Deinonychus antirrhopus (125-101 Ma), Diplodocus carnegii (157-145 Ma), Falcarius utahensis (139-134.6 Ma), Herrerasaurus ischigualastensis (237-201 Ma), Liliensternus liliensterni (228-201.3 Ma), Mussaurus (228-209 Ma), Piatnitzkysaurus ﬂoresi (180-168 Ma), Plateosaurus engelhardti (228-209 Ma), Tyrannosaurus rex (83.5-66.0 Ma).

View of the hip joint of Diplodocus carnegi Hatcher, Upper Jurassic (161.5-145.0 Ma); exposition of the Orlov Paleontological Museum (Moscow); photo by the author.

Signs of the presence of LCF are noted on the femur and in early dinosauromorphs - contemporaries of dinosaurs and other archosaurs of the late Triassic period (237-201.3±0.2 Ma): Lagerpeton chanarensis, Dromomeron romeri, Dromomeron gregorii. These creatures, unlike dinosaurs, had a groove for attaching the LCF in the form of a barely noticeable gap between the anteromedial and posteromedial tubercles on the head of the femur (2009NesbittSJ_RoweT).

The brief overview of LCF in extinct species indicates that this anatomical element was present in the hip joints of the earliest tetrapods. LCF continued to develop among archosaurs and dinosaurs. While in early tetrapod forms following Tiktaalik roseae, the LCF was located in the upper section of the hip joint, it "migrated" to the lower section after Eryops megacephalus. In dinosaurs, the LCF became significantly more complex, forming a structure of multiple ligaments that functioned together within a perforated acetabulum.

References

Ezcurra MD, Scheyer TM, Butler RJ. The origin and early evolution of Sauria: reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence. PLOS ONE. 2014;9(2)e89165. [journals.plos.org]

Chiari Y, Cahais V, Galtier N, Delsuc F. Phylogenomic analyses support the position of turtles as the sister group of birds and crocodiles (Archosauria). Bmc Biology. 2012;10(1)1-15. [link.springer.com]

Nesbitt SJ, Sidor CA, Irmis RB, Angielczyk KD, Smith RM, Tsuji LA. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature. 2010;464(7285)95-8. [nature.com , cienciaescolar.wordpress.com]

Cohen KM, Harper DAT, Gibbard PL, Car N. The International Commission on Stratigraphy (ICS) International Chronostratigraphic Chart. September 2023. [stratigraphy.org]

Langer MC, Ramezani J, Da Rosa ÁAS. U-Pb age constraints on dinosaur rise from south Brazil. Gondwana Research. 2018;57:133-40. [sciencedirect.com]

Tsai HP, Holliday CM. Articular soft tissue anatomy of the archosaur hip joint: structural homology and functional implications. Journal of Morphology. 2015;276(6)601-30. [researchgate.net, onlinelibrary.wiley.com]

Kuznetsov AN, Sennikov AG. On the function of a perforated acetabulum in archosaurs and birds. Paleontological Journal. 2000;34(4)439-48. [researchgate.net]

Tsai HP, Middleton KM, Hutchinson JR, Holliday CM. Hip joint articular soft tissues of non-dinosaurian Dinosauromorpha and early Dinosauria: evolutionary and biomechanical implications for Saurischia. Journal of Vertebrate Paleontology. 2018;38(1)e1427593. [tandfonline.com , researchonline.rvc.ac.uk]

Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH, Rowe T. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate paleontology. 2009;29(2)498-516. [tandfonline.com]

Keywords

ligamentum capitis femoris, ligamentum teres, ligament of head of femur, doctrine, animals, reptile, dinosauria

The original text in Russian is available at the link: Dinosauria

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500–1 Mya

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