The gastralial apparatus of Plateosaurus engelhardti: morphological description and soft-tissue reconstructio

Abstract

The fragmentary nature of the fossil record of the gastralial apparatus of sauropodomorph dinosaurs has considerable impact on our understanding its functional morphology and evolution in this group. With the aim of increasing our knowledge on the functional morphology of the gastralial apparatus of sauropodomorph dinosaurs, remains of the gastralial apparatus of Plateosaurus engelhardti are described. Soft-tissues and their function are reconstructed applying the Extant Phylogenetic Bracket, in this instance comprising crocodiles and birds. The gastralial apparatus of Plateosaurus consists of at least 18 gastralial rows. With exception of the 1 st row, which has a chevron-shaped medial and two lateral gastralia, the gastralial rows consist of two medial and two lateral gastralia. The lateral gastralia are 2.2-2.8 times longer than the associated medial gastralia. The proximal ends of the medial gastralia overlap in a latticelike arrangement, forming a mid-ventral imbricating articulation. Characteristic for Plateosaurus is the sigmoid form of the medial and lateral gastralia, the increasing angle of the gastralial rows from proximal to distal, as well as the increasing robustness of the gastralia rows from proximal to distal. According to the Extant Phylogenetic Bracket, the gastralial apparatus of Plateosaurus is embedded into M. rectus abdominis, which attaches to the caudal aspect of the sternum and indirectly to the distal end of the pubis. As in extant archosaurs, m. rectus abdominis is closely related to the other muscles of the hypaxial abdominal muscle group. The muscles of the hypaxial abdominal muscle group are active during ventilation and locomotion, which suggests that the functions of the gastralial apparatus of Plateosaurus may be more complex than previously suggested.

A new Heterodontosaurus specimen elucidates the unique ventilatory macroevolution of ornithischian dinosaurs

“…The first two pairs of gastralia have slightly thickened medial facets that are absent from all subsequent pairs ( Figure 1C ), with the overall thickness of gastralia diminishing posteriorly. The gastralia are autapomorphic among non-avian dinosaurs in lacking a lateral segment ( Claessens, 2004 ; Fechner and Gößling, 2014 ; Barrett et al, 2019 ), which is retained in even the diminished gastral basket of early branching avialans ( O’Connor et al, 2015 ).…”

Osteology, relationships and functional morphology ofWeigeltisaurus jaekeli(Diapsida, Weigeltisauridae) based on a complete skeleton from the Upper Permian Kupferschiefer of Germany

“…It suggests a very slender trunk region with a strongly convex venter. We consider it likely that the medial elements of the baskets were unbranched, overlapping rods as in Proterosuchus alexanderi (NMQR 1484; Cruickshank, 1972 ), Postosuchus alisonae (e.g., Weinbaum, 2013 ), and some dinosaurs (e.g., Sternberg, 1933 ; Fechner & Gößling, 2014 ). As preserved in SMNK-PAL 2882, the gastralia were internal to the ventral bases of the patagial ossifications.…”

Evolutionary changes in pubic orientation in dinosaurs are more strongly correlated with the ventilation system than with herbivory

“…Thus, we only interpreted it as strong evidence of the absence of cuirassal ventilation when even the best‐preserved specimens of extensively known groups completely lacked gastralia, as is the case in ornithischians (Carrier & Farmer ; Rasskin‐Gutman & Buscalioni ; Claessens ). Gastralia also seem to be absent in most derived eusauropods (Claessens ; Tschopp & Mateus ; Fechner & Gößling ). Where they have been reported (mostly in non‐neosauropod sauropods and diplodocids; Tschopp & Mateus ), gastralia have a very different shape to those used for ventilation in theropods and basal sauropodomorphs (Claessens ; Fechner & Gößling ) indicating that they might have lost their ventilatory function during the evolution of sauropods.…”

“…Gastralia also seem to be absent in most derived eusauropods (Claessens ; Tschopp & Mateus ; Fechner & Gößling ). Where they have been reported (mostly in non‐neosauropod sauropods and diplodocids; Tschopp & Mateus ), gastralia have a very different shape to those used for ventilation in theropods and basal sauropodomorphs (Claessens ; Fechner & Gößling ) indicating that they might have lost their ventilatory function during the evolution of sauropods.…”

“…Two pelvic muscles are attached to the ischium: (1) from the ischium to the anterior margin of the pubis (M. ischiopubis); and (2) to the posterior‐most gastralia (M. ischiotruncus; Claessens ). Given that gastralia also occur in dinosaurs, pterosaurs, and a wide range of Triassic archosauriforms (Claessens ; Tschopp & Mateus ; Fechner & Gößling ), their presence is most parsimoniously interpreted as a plesiomorphic condition of Archosauria. Thus, the anatomical and muscular connections of the gastralia in dinosaurs were also probably the same as those in extant crocodiles (Claessens ), with the possible exception of the ischiopubis muscle; this muscle mostly acts on the pubis, which is kinetic in crocodiles, but not in other archosaurs.…”