Teleosauroidea was a clade of ancient crocodylomorphs that were a key element of coastal marine environments during the Jurassic. Despite a 300-year research history and a recent renaissance in the study of their morphology and taxonomy, macroevolutionary studies of teleosauroids are currently limited by our poor understanding of their phylogenetic interrelationships. One major problem is the genus Steneosaurus, a wastebasket taxon recovered as paraphyletic or polyphyletic in phylogenetic analyses. We constructed a newly updated phylogenetic data matrix containing 153 taxa (27 teleosauroids, eight of which were newly added) and 502 characters, which we analysed under maximum parsimony using TNT 1.5 (weighted and unweighted analyses) and Bayesian inference using MrBayes v3.2.6 (standard, gamma and variation). The resulting topologies were then analysed to generate comprehensive higher-level phylogenetic hypotheses of teleosauroids and shed light on species-level interrelationships within the clade. The results from our parsimony and Bayesian analyses are largely consistent. Two large subclades within Teleosauroidea are recovered, and they are morphologically, ecologically and biogeographically distinct from one another. Based on comparative anatomical and phylogenetic results, we propose the following major taxonomic revisions to Teleosauroidea: (1) redefining Teleosauridae; (2) introducing one new family and three new subfamilies; (3) the resurrection of three historical genera; and (4) erecting seven new generic names and one new species name. The phylogeny infers that the Laurasian subclade was more phenotypically plastic overall than the Sub-Boreal-Gondwanan subclade. The proposed phylogeny shows that teleosauroids were more diverse than previously thought, in terms of morphology, ecology, dispersal and abundance, and that they represented some of the most successful crocodylomorphs during the Jurassic.

Teleosauroids were a successful group of semi-aquatic crocodylomorphs that were an integral part of coastal marine/lagoonal faunas during the Jurassic. Their fossil record suggests that the group declined in diversity and abundance in deep water deposits during the Late Jurassic. One of the few known teleosauroid species from the deeper water horizons of the well-known Kimmeridge Clay Formation is ‘Teleosaurus’ megarhinus Hulke, 1871, a poorly studied, gracile longirostrine form. The holotype is an incomplete snout from the Aulacostephanus autissiodorensis Sub-Boreal ammonite Zone of Kimmeridge, England. The only other referred specimen is an almost complete skull from the slightly older A. eudoxus Sub-Boreal ammonite Zone of Quercy, France. Recently, the validity of this species has been called into question. Here we re-describe the holotype as well as the referred French specimen and another incomplete teleosauroid, DORCM G.05067i-v (an anterior rostrum with three osteoderms and an isolated tooth crown), from the same horizon and locality as the holotype. We demonstrate that all specimens are referable to ‘Teleosaurus’ megarhinus and that the species is indeed a valid taxon, which we assign to a new monotypic genus, Bathysuchus. In our phylogenetic analysis, the latest iteration of the ongoing Crocodylomorph SuperMatrix Project, Bathysuchus megarhinus is found as sister taxon to Aeolodon priscus within a subclade containing Mycterosuchus nasutus and Teleosaurus cadomensis. Notably Bathysuchus has an extreme reduction in dermatocranial ornamentation and osteoderm size, thickness and ornamentation. These features are mirrored in Aeolodon priscus, a species with a well-preserved post-cranial skeleton and a similar shallow and inconspicuous dermal ornamentation. Based on these morphological features, and sedimentological evidence, we hypothesise that the Bathysuchus + Aeolodon clade is the first known teleosauroid lineage that evolved a more pelagic lifestyle.

Teleosaurids were a clade of crocodylomorphs that attained near-global distribution during the Jurassic Period. Within Teleosauridae, one particular sub-clade of durophagous/macrophagous taxa achieved large body sizes and were apex predators in shallow marine environments during the Late Jurassic and Early Cretaceous in Europe and around the coast of the Tethys Seaway. Unfortunately, the origins of this clade are still poorly understood. 'Steneosaurus' obtusidens is a little-studied macrophagous species from the Oxford Clay Formation (Callovian, Middle Jurassic) of the United Kingdom and near Migné-les-Lourdines (middle Callovian) in France. Despite being considered a sister taxon of the Late Jurassic taxon Machimosaurus, the taxonomy of 'S.' obtusidens remains unclear. Although three different synonymies have been proposed (variously a subjective synonym of other taxa), these taxonomic hypotheses have not been based on detailed anatomical comparisons and thus have not been tested. Here we re-describe of the holotype of 'S.' obtusidens, demonstrate that it is indeed a valid taxon, restrict the referred specimens to a fragmentary skeleton, nearly complete skull, and partial rostrum, and establish a new monotypic genus, Lemmysuchus. Our re-description reveals five autapomorphies for L. obtusidens, and nine apomorphic characters that support the tribe Machimosaurini (Lemmysuchus + Machimosaurus).

Teleosauroidea is a clade of ancient crocodylomorphs that were integral components of coastal marine environments throughout the Jurassic. For nearly two centuries, one of the most familiar genera of teleosauroids has been Steneosaurus, encompassing nearly every teleosauroid species at some point. However, no type species has been designated for Steneosaurus under ICZN Code rules; the type specimen of the presumed type species S. rostromajorGeoffroy Saint-Hilaire, 1825 (MNHN.RJN 134c-d) is a chimera that has been largely neglected in the literature. Moreover, there is confusion as to which teleosauroid species it pertains to, and the genus Steneosaurus is often recovered as paraphyletic or polyphyletic in phylogenetic analyses. As such, the validity of Steneosaurus is uncertain. Here we formally designate S. rostromajor as the type species of Steneosaurus, designate a lectotype and re-evaluate MNHN.RJN 134c-d. We compare it with several well-known teleosauroids, including Lemmysuchus and ‘S.’ edwardsi. Due to lack of autapomorphic characters, poor preservation and a tortured taxonomic history, we find MNHN.RJN 134c-d to be an undiagnostic and unreliable specimen. Thus, we consider S. rostromajor as a nomen dubium and propose that the genus Steneosaurus is undiagnostic. This has profound implications for teleosauroid phylogenetics, which we will clarify in an upcoming paper.

Teleosauroidea was a clade of successful, morphologically diverse, ancient crocodylomorphs that were integral in coastal marine/lagoonal environments during the Jurassic. Within Teleosauroidea, the macrophagous/durophagous tribe Machimosaurini evolved specialized feeding strategies (e.g. hypertrophied jaw musculature and blunt, heavily ornamented dentition) and large body sizes (> 7 m), becoming an important component of Middle and Late Jurassic ecosystems. These ocean-dwelling giants are well known from the Callovian (Lemmysuchus) of Europe and the UK, and from the Kimmeridgian–Tithonian (Machimosaurus) of Europe and northern Africa. There are reports of fragmentary machimosaurin material from the Bathonian of Africa, but the overall Bathonian teleosauroid material is poorly understood. While multiple specimens were described during the 19th and 20th centuries, little research has been done since. Here we re-describe two historically important Bathonian species from near Oxford, UK. We demonstrate that both ‘Steneosaurus’ larteti and ‘Steneosaurus’ boutilieri are valid taxa and we establish neotypes for both species and two new genera, Deslongchampsina and Yvridiosuchus. Our cladistic analysis finds Yvridiosuchus boutilieri as a basal member of Machimosaurini and Deslongchampsina lartetito be closely related to Steneosaurus heberti. Interestingly, four distinct teleosauroid ecomorphotypes are present in the Bathonian of Europe and teleosauroid ecomorphological diversity continued throughout the Callovian and Kimmeridgian/Tithonian in Europe and England.

The genus Mystriosaurus, established by Kaup in 1834, was one of the first thalattosuchian genera to be named. The holotype, an incomplete skull from the lower Toarcian Posidonienschiefer Formation of Altdorf (Bavaria, southern Germany), is poorly known with a convoluted taxonomic history. For the past 60 years, Mystriosaurus has been considered a subjective junior synonym of Steneosaurus. However, our reassessment of the Mystriosaurus laurillardi holotype demonstrates that it is a distinct and valid taxon. Moreover, we find the holotype of "Steneosaurus" brevior, an almost complete skull from the lower Toarcian Whitby Mudstone Formation of Whitby (Yorkshire, UK), to be a subjective junior synonym of M. laurillardi. Mystriosaurus is diagnosed in having: a heavily and extensively ornamented skull; large and numerous neurovascular foramina on the premaxillae, maxillae and dentaries; anteriorly oriented external nares; and four teeth per premaxilla. Our phylogenetic analyses reveal M. laurillardi to be distantly related to Steneosaurus bollensis, supporting our contention that they are different taxa. Interestingly, our analyses hint that Mystriosaurus may be more closely related to the Chinese teleosauroid (previously known as Peipehsuchus) than any European form.

Teleosaurids were a clade of marine crocodylomorphs that were globally distributed during the Jurassic Period. They evolved a wide range of body sizes, from small ( 2-3 m) to very large (> 9 m). Until now, the largest known Middle Jurassic teleosaurid was 'Steneosaurus' obtusidens, from the Oxford Clay Formation of the UK. Here, we re-examine a very large Oxford Clay specimen (ilium, ischium, and femur) that had been tentatively attributed to 'S.' obtusidens. Based on comparative anatomical study with the 'S.' obtusidens holotype and referred specimens of Steneosaurus edwardsi and Steneosaurus leedsi, we conclude that this very large individual actually pertains to S. edwardsi. Based on comparisons with the Machimosaurus mosae neotype (which has a complete femur and skeleton), we estimate a total length in excess of 7 m for this large S. edwardsi individual, making it the largest known Middle Jurassic teleosaurid. Therefore, along with the closely related genus Machimosaurus, this clade of large-bodied Middle-Late Jurassic teleosaurids were the largest species during the first 100 million years of crocodylomorph evolution.

Throughout the Jurassic, a plethora of marine reptiles dominated ocean waters, including ichthyosaurs, plesiosaurs and thalattosuchian crocodylomorphs. These Jurassic ecosystems were characterized by high niche partitioning and spatial variation in dietary ecology. However, while the ecological diversity of many marine reptile lineages is well known, the overall ecological diversification of Teleosauroidea (one of the two major groups within thalattosuchian crocodylomorphs) has never been explored. Teleosauroids were previously deemed to have a morphologically conservative body plan; however, they were in actuality morphofunctionally more diverse than previously thought. Here we investigate the ecology and feeding specializations of teleosauroids, using morphological and functional cranio‐dental characteristics. We assembled the most comprehensive dataset to date of teleosauroid taxa (approximately 20 species) and ran a series of principal component analyses (PC) to categorize them into various feeding ecomorphotypes based on 17 dental characteristics (38 specimens) and 16 functionally significant mandibular characters (18 specimens). The results were examined in conjunction with a comprehensive thalattosuchian phylogeny (153 taxa and 502 characters) to evaluate macroevolutionary patterns and significant ecological shifts. Machimosaurids display a well‐developed ecological shift from: (1) slender, pointed tooth apices and an elongate gracile mandible; to (2) more robust, pointed teeth with a slightly deeper mandible; and finally, (3) rounded teeth and a deep‐set, shortened mandible with enlarged musculature. Overall, there is limited mandibular functional variability in teleosaurids and machimosaurids, despite differing cranial morphologies and habitat preferences in certain taxa. This suggests a narrow feeding ecological divide between teleosaurids and machimosaurids. Resource partitioning was primarily related to snout and skull length as well as habitat; only twice did teleosauroids manage to make a major evolutionary leap to feed distinctly differently, with only the derived machimosaurines successfully radiating into new feeding ecologies.