Palaeoecology of Triassic stem turtles sheds new light on turtle origins

Abstract

Competing hypotheses of early turtle evolution contrast sharply in implying very different ecological settings-aquatic versus terrestrial-for the origin of turtles. We investigate the palaeoecology of extinct turtles by first demonstrating that the forelimbs of extant turtles faithfully reflect habitat preferences, with short-handed turtles being terrestrial and long-handed turtles being aquatic. We apply this metric to the two successive outgroups to all living turtles with forelimbs preserved, Proganochelys quenstedti and Palaeochersis talampayensis, to discover that these earliest turtle outgroups were decidedly terrestrial. We then plot the observed distribution of aquatic versus terrestrial habits among living turtles onto their hypothesized phylogenies. Both lines of evidence indicate that although the common ancestor of all living turtles was aquatic, the earliest turtles clearly lived in a terrestrial environment. Additional anatomical and sedimentological evidence favours these conclusions. The freshwater aquatic habitat preference so characteristic of living turtles cannot, consequently, be taken as positive evidence for an aquatic origin of turtles, but must rather be considered a convergence relative to other aquatic amniotes, including the marine sauropterygians to which turtles have sometimes been allied.

Shell bone histology of solemydid turtles (stem Testudines): palaeoecological implications

“…Eileanchelys waldmani Anquetin, Barrett, Jones, Moore-Fay and Evans, 2009 and Herckerochelys romani Sukhanov, 2006: Scheyer et al 2014; also Condorchelys antigua Sterli, 2008: Cerda et al 2015, the majority of stem turtles showed terrestrial habitat preference (e.g. Joyce and Gauthier 2004;Sterli et al 2007). …”

“…Joyce et al (2011), on the other hand, argued for terrestrial habits of solemydid turtles based on anatomical data, such as the presence of limb ossicles. Unfortunately, the described solemydid turtles so far did not include articulated forelimbs, which could otherwise be used to elucidate their palaeoecology, as has been done previously for Triassic stem turtles Proganochelys quenstedti Baur, 1887 and Palaeochersis talampayensis Rougier, de la Fuente and Arcucci, 1995 (Joyce and Gauthier 2004).…”

A review of the fossil record of turtles of the clade Thalassochelydia

“…Complete limbs are mostly known for "eurysternids" and "plesiochelyids." In general, these are characterized by a moderately elongated stylopod, a shorter zeugopod, and a relatively elongated autopod similar in proportion to extant pleurodires and trionychids (Joyce and Gauthier 2004). Well-developed articular surfaces reveal that thalassochelydians did not possess stiffened paddle as in extant marine turtles, but the flippers of Neusticemys neuquina were described as having been less mobile than those of trionychids (de la Fuente and Fernández 2011).…”

Specialization for underwater hearing by the tympanic middle ear of the turtle,Trachemys scripta elegans

“…Closed middle ear cavities are not seen until the Eocene [11]. On the other hand, front limb proportions, shell shape and histology suggest that other near-crown stem turtles were terrestrial [46,47]. In addition, Eunotosaurus and successive out-groups lack obvious aquatic adaptations and are only known from terrestrial sediments [48].…”