On the homology of crocodylian post‐dentary bones and their macroevolution throughout Pseudosuchia

Abstract

The lower jaw of early tetrapods is composed of several intramembranous ossifications. However, a tendency toward the independent reduction of the number of bones has been observed in the mandible of mammals, lepidosaurs, turtles, crocodiles, and birds. Regarding archosaurs, the coronoid and prearticular bones are interpreted to be lost during the evolution of stem‐birds and stem‐crocodiles, respectively, but the homology of the post‐dentary bones retained in living pseudosuchians remains unclear. Here, we combine paleontological and embryological evidence to explore in detail the homology of the crocodylian post‐dentary bones. We study the mandible embryogenesis on a sample of 71 embryos of Caiman and compare this pattern with the mandibular transformations observed across pseudosuchian evolution. In the pre‐hatching ontogeny of caimans, at least five intramembranous ossification centers are formed along the margins of the internal mandibular fenestra (perifenestral centers) and, subsequently, merge to form the coronoid (three intramembranous centers), angular (one intramembranous center), and articular (one intramembranous and one chondral center). In the fossil record, an independent prearticular is lost around the base of Mesoeucrocodylia (optimized as reappearing in Thalattosuchia if they are placed within Neosuchia), and the coronoid is apomorphically lost in notosuchians. The integration of embryological and paleontological data indicates that most perifenestral centers are involved in the origin of the prearticular of non‐mesoeucrocodylian pseudosuchians. These centers are rearranged during the evolution to contribute to different post‐dentary bones in mesoeucrocodylians bolstering the idea that the coronoid and the articular of Crocodylia are not completely homologous to those of other diapsids.

Cranial osteology and paleoneurology of Tarjadia ruthae: An erpetosuchid pseudosuchian from the Triassic Chañares Formation (late Ladinian‐?early Carnian) of Argentina

“…The medial surface of the coronoid has a rugose eminence (= coronoid eminence sensu Tsai & Holliday, 2011) -BSPG 1934 VIII 514). By contrast, the coronoid is absent as an independent ossification in aetosaurs and pterosaurs (Bona et al, 2022). However, the morphology of the anteroposteriorly short coronoid of Tarjadia ruthae and its location resembles that of "rauisuchians" and living crocodiles, and in particular, its concave posterior margin is more similar to that of crocodiles.…”

“…A large foramen intermandibularis caudalis (= Meckelian/inframeckelian foramen) is identified between this process, the anteroventral margin of the prearticular, and the dorsomedial margin of the angular for the passage of nerves and vessels (Figure 7b,f). This foramen is also present in the doswelliid Sphodrosaurus pennsylvanicus (Ezcurra & Sues, 2021), phytosaurs (e.g., Parasuchus hislopi : ISIR 42; Nicrosaurus kapfii : NHMUK PV 38036, SMNS 5730), Ornithosuchus woodwardi (Bona et al, 2022), Longosuchus meaedi (TMM 31185‐84B), ornithischian dinosaurs (e.g., Heterodontosaurus tucki : Norman et al, 2011; Lesothosaurus diagnosticus : Porro et al, 2015), and several crocodylomorphs (Bona et al, 2022). However, the foramen intermandibularis caudalis is also formed by the angular in Sphodrosaurus pennsylvanicus , phytosaurs and crocodylomorphs (Bona et al, 2022; Iordansky & Gans, 1973).…”

“…The coronoid bone is also registered in several loricatans (i.e., Decuriasuchus quartacolonia : de França et al, 2013; Prestosuchus chiniquensis : UFRGS‐PV‐0629‐T; Saurosuchus galilei : Alcober, 2000; Sphenosuchus acutus and Orthosuchus strombergi : Walker, 1990; living crocodiles: MACN‐He 1420), ornithosuchids ( Riojasuchus tenuisceps : PVL 3827 and Ornithosuchus woodwardi : NHMUK PV R3142), phytosaurs (e.g., Parasuchus hislopi : ISIR 42), dinosaurs (e.g., Heterodontosaurus tucki : Norman et al, 2011; Eoraptor lunensis : Sereno et al, 2013), proterochampsians ( Chanaresuchus bonapartei : Romer, 1972; Proterochampsa barrionuevoi : Dilkes & Arcucci, 2012; Sphodrosaurus pennsylvanicus : Ezcurra & Sues, 2021), Euparkeria capensis (Sookias et al, 2020), erythrosuchids ( Garjainia prima : Ezcurra et al, 2019; Erythrosuchus africanus : Gower, 2003), and proterosuchids ( Proterosuchus fergusi : SNSB‐BSPG 1934 VIII 514). By contrast, the coronoid is absent as an independent ossification in aetosaurs and pterosaurs (Bona et al, 2022). However, the morphology of the anteroposteriorly short coronoid of Tarjadia ruthae and its location resembles that of “rauisuchians” and living crocodiles, and in particular, its concave posterior margin is more similar to that of crocodiles.…”

“…The prearticular resembles the general morphology of several other pseudosuchians, such as ornithosuchids ( Riojasuchus tenuisceps : PVL 3827; Ornithosuchus woodwardi : NHMUK PV R3142) and loricatans ( Batrachotomus kupferzellensis : SMNS 52790; Postosuchus kirkpatricki : TTUP 09000; Prestosuchus chiniquensis : UFRGS PV‐0629‐T; Rauisuchus tiradentes : SNSB‐BSPG AS 68), but differing from aetosaurs ( Paratypothorax andressorum : SMNS 19003; Neoaetosauroides engaeus : PVL 3525; Stagonolepis olenkae : ZPAL Ab III/578/34; Stagonolepis robertsoni : NHMUK PV R4787), in which the posterior end of the prearticular has a tongue‐like projection. The prearticular forms the medial wall of the hemimandible ventral to the internal mandibular fenestra, as occurs in most Triassic archosauriforms, but contrasting with the dorsoventrally reduced prearticular, which allows the angular to participate on the medial surface of the mandible of Proterochampsa spp., Sphodrosaurus pennsylvanicus , and phytosaurs (Bona et al, 2022; Ezcurra & Sues, 2021). The foramen intermandibularis medius cannot be identified on any of the specimens of Tarjadia ruthae , but the foramen intermandibularis caudalis is easily recognizable in the anteroventral margin of the prearticular of CRILAR‐Pv 478 and 495, being anteriorly delimited by the splenial and ventrally by the angular (see above).…”

Ontogenetic and interelemental study of appendicular bones of Caiman latirostris Daudin, 1802 sheds light on osteohistological variability in crocodylians

“…Crocodiles in general and alligators in particular have been well studied in terms of their ecology, physiology, anatomy, and macroevolution (e.g., Allen et al, 2009; Bona & Desojo, 2011; Bona et al, 2022; Fernandez Blanco et al, 2018, 2020; Larriera et al, 2004; Simoncini et al, 2016). Notably, among these studies are the detailed macroanatomical skeletal analysis performed on different species of crocodiles (e.g., Iordansky, 1973; Romer, 1923, 1956).…”