Léa C. Girard

Softshell turtles ( Pan-Trionychidae ) have a particularly rich fossil record in North America, but only a few fragmentary remains have previously been reported from the Oligocene. A review of the literature suggests that Trionyx punctiger and Trionyx leucopotamicus , two taxa previously linked to the Oligocene, should be considered nomen dubia. We report well-preserved trionychid remains from the Early Oligocene (Orellan–Whitneyan) Brule Formation of southwestern North Dakota, including a complete cranium, mandible, and three partial shells that represent a new species of apalonine, Rafetus lysoni . The new species resembles other large-bodied apalonines from the Cenozoic of North America in skull morphology but differs notably in the poor development of carapacial and plastral callosities.

The pan-trionychids Aspideretes beecheri and Helopanoplia distincta were named at the beginning of the 20 ^th century based on fossils collected from the Late Cretaceous (Maastrichtian) Lance Formation of Wyoming. Interestingly, even though numerous additional pan-trionychid remains from Late Cretaceous formations of the Upper Great Plains have since been referred to Helopanoplia distincta based on its unique surface texture, no additional specimens have been referred to Aspideretes beecheri . We figure and describe all material previously referred to Aspideretes beecheri and select specimens referred to Helopanoplia distincta . Interspecific and intraspecific variation is assessed by reference to extant trionychids as a basis for taxonomic revision. We re-assign all specimens historically referred to Aspideretes beecheri to Axestemys splendidus and Axestemys infernalis . An unusual plastron previously thought to represent Helopanoplia distincta is referred to Hutchemys walkerorum . Although the type of Helopanoplia distincta is characterized by a distinct, tuberculate shell surface texture, material previously referred to this taxon suggests that this texture is only occasionally found in some parts of the shell. The holotype of Aspideretes beecheri otherwise shows characteristics consistent with being the subadult of material previously referred to Helopanoplia distincta . We thus conclude that the two taxa are synonymous and form the new combination Helopanoplia beecheri . We urge caution when identifying fragmentary pan-trionychid remains, as a tuberculate surface texture pattern is found in at least two distinct taxa. We support the conclusion, nonetheless, that Helopanoplia beecheri is one of only a few turtles to have gone extinct at the Cretaceous–Paleogene boundary.

Softshell turtles ( Pan‐Trionychidae ) are an early branching clade of hidden‐necked turtles ( Cryptodira ) with a rich fossil record extending back to the Early Cretaceous. The evolutionary history of softshell turtles is still unresolved because of their conservative morphology combined with high levels of polymorphism related to morphological plasticity and ontogeny. We present a revised list of characters focused on the skull and hyoid of extant trionychids. To better assess the polymorphism present in extant softshell turtles, we assembled a set of photographs and μCT scans that document the morphology of 312 specimens, individually scored to better document all polymorphism. Several matrices were then assembled that variously reduce observed polymorphism. To explore which matrix performs best under which setting, we carried out numerous parsimony analyses and established phylogenetic accuracy by comparing results to the emerging molecular consensus. The matrix excluding low‐frequency polymorphism and ontogenetically related polymorphism is shown to utilize the greatest amount of character information and performs best with ordered characters and moderate implied weighting, but many relationships remain incorrectly resolved. Use of early fossil trionychids or Carettochelys insculpta as outgroups affects the polarity of the trees but leaves the topology largely unchanged. Although our revised list of characters is an improvement upon previous ones, we strongly advise the use of a molecular backbone or total evidence analysis, ideally within a Bayesian framework. A succinct description is provided for the skull and hyoid of all extant clades and species of softshell turtles.

Hutchemys rememdium is a poorly understood softshell turtle ( Trionychidae ) from the mid Paleocene of the Williston Basin of North America previously known only from postcranial remains. A particularly rich collection of previously undescribed material from the Tiffanian 4 North American Land Mammal Age (NALMA) of North Dakota is here presented consisting of numerous shells that document new variation, some non-shell postcrania, and cranial remains, which are described based on 3D models extracted from micro-CT data. Although the observed shell variation weakens previously noted differences with the younger species Hutchemys arctochelys from the Clarkforkian NALMA, the two taxa are still recognized as distinct. Parsimony and Bayesian phylogenetic analyses reaffirm the previously challenged placement of Hutchemys rememdium within the clade Plastomenidae , mostly based on novel observations of cranial characters made possible by the new material and the micro-CT data. The new topology supports the notion that the well-ossified plastron of plastomenids originated twice in parallel near the Cretaceous/Paleogene boundary, once in the Hutchemys lineage and once in the Gilmoremys / Plastomenus lineage. Hutchemys rememdium is notable for being the only documented species of trionychid in the mid Paleocene of the Williston Basin. The presence of multiple individuals in a carbonaceous claystone indicates this taxon lived in swamps and lakes and its expanded triturating surface suggests it had a durophagous diet.

Denazinemys nodosa is a Late Cretaceous representative of the North American turtle clade Baenidae diagnosed, among others, by a shell surface texture consisting of raised welts. We provide a detailed description of a partial skeleton from the late Campanian Kaiparowits Formation of Utah, USA, including bone-by-bone analysis of its cranium based on images obtained using micro-computed tomography. A revised phylogenetic analysis confirms placement of Denazinemys nodosa close to Eubaena cephalica and Boremys spp. within the clade Eubaeninae. Comparison with a second skull from the Kaiparowits Formation previously assigned to Denazinemys nodosa questions its referral to this taxon. An assortment of specimens from the Early to Late Campanian of Mexico and the USA had previously been referred to Denazinemys nodosa based on shell surface texture alone, even though this characteristic is known to occur in other baenids. Our review of all available material concludes that Denazinemys nodosa is currently only known from the Late Campanian of New Mexico and Utah.

Metriorhynchids are marine crocodylomorphs found across Jurassic and Lower Cretaceous deposits of Europe and Central and South America. Despite being one of the oldest fossil families named in paleontology, the phylogenetic relationships within Metriorhynchidae have been subject to many revisions over the past 15 years. Herein, we describe a new metriorhynchid from the Kimmeridgian of Porrentruy, Switzerland. The material consists of a relatively complete, disarticulated skeleton preserving pieces of the skull, including the frontal, prefrontals, right postorbital, nasals, maxillae, right premaxillae and nearly the entire mandible, and many remains of the axial and appendicular skeleton such as cervical, dorsal, and caudal vertebrae, ribs, the left ischium, the right femur, and the right fibula. This new specimen is referred to the new species Torvoneustes jurensis sp. nov. as part of the large-bodied macrophagous tribe Geosaurini. Torvoneustes jurensis presents a unique combination of cranial and dental characters including a smooth cranium, a unique frontal shape, acute ziphodont teeth, an enamel ornamentation made of numerous apicobasal ridges shifting to small ridges forming an anastomosed pattern toward the apex of the crown and an enamel ornamentation touching the carina. The description of this new species allows to take a new look at the currently proposed evolutionary trends within the genus Torvoneustes and provides new information on the evolution of this clade.