Turtles have a highly modified body plan, including a rigid shell that constrains postcranial anatomy. Skull morphology and neck mobility may therefore be key to ecological specialization in turtles. However, the ecological signal of turtle skull morphologies has not been rigorously evaluated, leaving uncertainties about the roles of ecological adaptation and convergence. We evaluate turtle cranial ecomorphology using three‐dimensional geometric morphometrics and phylogenetic comparative methods. Skull shape correlates with allometry, neck retraction capability, and different aquatic feeding ecologies. We find that ecological variables influence skull shape only, whereas a key functional variable (the capacity for neck retraction) influences both shape and size. Ecology and functional predictions from three‐dimensional shape are validated by high success rates for extant species, outperforming previous two‐dimensional approaches. We use this to infer ecological and functional traits of extinct species. Neck retraction evolved among crownward stem‐turtles by the Late Jurassic, signaling functional decoupling of the skull and neck from the shell, possibly linked to a major episode of ecomorphological diversification. We also find strong evidence for convergent ecological adaptations among marine groups. This includes parallel loss of neck retraction, evidence for active hunting, possible grazing, and suction feeding in extinct marine groups. Our large‐scale assessment of dietary and functional adaptation throughout turtle evolution reveals the timing and origin of their distinct ecomorphologies, and highlights the potential for ecology and function to have distinct effects on skull form.

The labyrinth of the vertebrate inner ear is a sensory system that governs the perception of head rotations. Central hypotheses predict that labyrinth shape and size are related to ecological adaptations, but this is under debate and has rarely been tested outside of mammals. We analyze the evolution of labyrinth morphology and its ecological drivers in living and fossil turtles, an understudied group that underwent multiple locomotory transitions during 230 million years of evolution. We show that turtles have unexpectedly large labyrinths that evolved during the origin of aquatic habits. Turtle labyrinths are relatively larger than those of mammals, and comparable to many birds, undermining the hypothesis that labyrinth size correlates directly with agility across vertebrates. We also find that labyrinth shape variation does not correlate with ecology in turtles, undermining the widespread expectation that reptilian labyrinth shapes convey behavioral signal, and demonstrating the importance of understudied groups, like turtles.

Pleurodires are less diverse than cryptodires, together forming the two major lineages of crown turtles. Their fossil record, however, is rich. A particularly large number of fossil pleurodires, many belonging to the Podocnemidoidae, has been recovered from the Upper Cretaceous Bauru Group outcrops of south‐central Brazil. Herein we describe an additional pleurodire from this region, Amabilis uchoensis gen. et sp. nov., based on a partially preserved skull. A. uchoensis is recognized as belonging to the Podocnemidoidae by the small entrance to the antrum postoticum and completely developed cavum pterygoidei, being unique among other non‐podocnemidid podocnemidoids for its medially open fenestra postotica and absent basioccipital–opisthotic contact. Our cladistic analysis places A. uchoensis as a sister taxon to Hamadachelys + other podocnemidoids. We further explore the neuroanatomy of side‐necked turtles with the aid of micro‐computed tomography of specimens of the main pleurodiran lineages. Our data shed light on the different carotid circulation patterns in pleurodires, and we propose new phylogenetic characters to describe the neuroanatomical variation of the group. Optimization of these characters shows two independent acquisitions of a foramen for the palatal branch of the carotid in chelids and podocnemidoids, and a unique loss of the vidian nerve canal in chelids, in turtles in general.

The humerus is central for locomotion in turtles as quadrupedal animals. Osteological variation across testudine clades remains poorly documented. Here, we systematically describe the humerus anatomy for all major extant turtle clades based on 38 species representing the phylogenetic and ecological diversity of crown turtles. Three Late Triassic species of shelled stem turtles (Testudindata) are included to establish the plesiomorphic humerus morphology. Our work is based on 3D models, establishing a publicly available digital database. Previously defined terms for anatomical sides of the humerus (e.g., dorsal, ventral) are often not aligned with the respective body sides in turtles and other quadrupedal animals with sprawling gait. We propose alternative anatomical directional terms to simplify communication: radial and ulnar (the sides articulating with the radius/ulna), capitular (the side bearing the humeral head), and intertubercular (opposite to capitular surface). Turtle humeri show low morphological variation with exceptions concentrated in locomotory specialists. We propose 15 discrete characters to summarize osteological variation for future phylogenetic studies. Disparity analyses comparing non‐shelled and shelled turtles indicate that the presence of the shell constrains humerus variation. Flippered aquatic turtles are released from this constraint and significantly increase overall disparity. Ontogenetic changes of turtle humeri are related to increased ossification and pronunciation of the proximal processes, the distal articulation areas, and the closure of the ectepicondylar groove to a foramen. Some turtle species retain juvenile features into adulthood and provide evidence for paedomorphic evolution. We review major changes of turtle humerus morphology throughout the evolution of its stem group.

A high diversity of land vertebrates is known from the Late Cretaceous deposits of the Bauru Basin, Brazil, including at least five turtle taxa, all belonging to the clade Podocnemidoidae. Some of the richest fossil sites of this basin are in the area of Monte Alto, which yielded several squamate, dinosaur, and crocodyliform taxa. Yet, the single turtle reported so far from this area was only briefly described. Here, we further describe that specimen, a complete but very crushed shell, as well as a partial skull, both found in outcrops of the Adamantina Formation. Comparison of the shell to other podocnemidoid taxa reveals its affinities to Roxochelys wanderleyi, a turtle originally described from that same stratigraphic unit. The comparative description of the skull and its inclusion in a phylogenetic study, supports the proposal of a new taxon, Yuraramirim montealtensis, representing a lineage (Peiropemydidae) so far known only from the Marília Formation of the Bauru Basin and the early Paleocene of Bolivia. The digitally reconstructed endocast and inner ear of the new taxon were also described, as not previously done for a fossil pleurodire.

The Bauru Basin (south-central Brazil) fossils have largely contributed to understand the faunal composition of South American Cretaceous. Among those, several turtle specimens were retrieved from those deposits, all belonging to Podocnemidoidae, the single group known from the Basin. On the other hand, only incomplete shell elements indicate large turtles such as ‘Peirópolis A’. Another shell fragment, a large peripheral plate from the Marília Formation, is described here. Its lack of surface ornamentation, and deep sulci are generally typical for podocnemidoids. The plate exhibits a narrow knob slightly projected onto the pleuro-marginal sulcus, absent in any other Bauru Basin turtle. According to extant and fossil podocnemidoids measures, we estimated this individual reached 1 m of carapace length, exceeding in size all other taxa known from the Basin. This new report reveals a broader size variation among podocnemidoids from the Late Cretaceous of Bauru Basin and a morphological diversity previously unknown for the period, comparable to those of the Amazon Basin today, in which inhabits the largest extant podocnemidoid Podocnemis expansa. Furthermore, the occurrence of such large turtles implies the presence of perennial streams on the northeastern portion of the Bauru Basin during the Maastrichtian and a richer environment than previously thought.