“Living fossils” and the mosaic evolution of characters

The modern discussion of living fossils turns mostly on the persistence of archaic, or ancestral, traits in extant organisms. Prime examples mentioned by Darwin already—who also coined the term “living fossil”—include the platypus and the extant lungfishes. However, the identification of archaic traits in extant organisms requires a basis of comparison, i.e., it requires an estimate of the phylogenetic interrelationships of the living fossil in question. Phylogenetic relationships are determined not on the basis of the persistence of archaic traits, but on the basis of shared derived characters. The identification of persistent traits in an organism requires the same organism to also exhibit advanced, or specialized traits that allow its proper classification. The occurrence of such a mixture of primitive (plesiomorphic) or derived (apomorphic) traits in an organism, or species, is called the heterobathmy of characters. Willi Hennig recognized the heterobathmy of characters as quite a universal phenomenon, and made it the basis of his phylogenetic systematics

The earliest herbivorous marine reptile and its remarkable jaw apparatus

Newly discovered fossils of the Middle Triassic reptile Atopodentatus unicus call for a radical reassessment of its feeding behavior. The skull displays a pronounced hammerhead shape that was hitherto unknown. The long, straight anterior edges of both upper and lower jaws were lined with batteries of chisel-shaped teeth, whereas the remaining parts of the jaw rami supported densely packed needle-shaped teeth forming a mesh. The evidence indicates a novel feeding mechanism wherein the chisel-shaped teeth were used to scrape algae off the substrate, and the plant matter that was loosened was filtered from the water column through the more posteriorly positioned tooth mesh. This is the oldest record of herbivory within marine reptiles

Separating sexual dimorphism from other morphological variation in a specimen complex of fossil marine reptiles (Reptilia, Ichthyosauriformes, Chaohusaurus).

The Early Triassic Chaohu Fauna from Anhui Province, China, contains the oldest record of Mesozoic marine reptiles, such as Cartorhynchus and Sclerocormus. Most specimens from the fauna belong to the ichthyosauriform Chaohusaurus, more specifically resembling C. chaoxianensis. However, a wide range of morphological variation exists within about 40 skeletons that have been prepared, likely reflecting mixed signals from both sexual and taxonomic differences. We test whether the sexual and taxonomic signals are separable based on quantification, aided by the knowledge of sexual dimorphism in extant marine tetrapods. There are two different suites of dimorphism that divide the specimens differently from each other yet consistently within each suite, resulting in four morphotypes in combination, likely representing two sexes of two taxa. Presumed males have larger organ of prehension sensu Darwin, specifically limbs in the present case, for a given body length. This sexing criterion is supported by the only specimen of a gravid female, which belongs to the morphotype with short limbs. Males also have larger skulls for the trunk length compared to females. This study demonstrates that sexual and taxonomic signals are separable in fossil reptiles, with a sufficient sample size and careful analyses

Triassic marine reptile

34 p. : ill. ; 26 cm.Includes bibliographical references (p. 30-32).We here describe the complete skeletal anatomy of Anshunsaurus huangguoshuensis that previously remained unknown. The description includes a review of the dorsal side of the skull as well as the first description of the ventral side of the skull and of the postcranial skeleton. A new phylogenetic analysis supports the monophyly of Askeptosauridea and Thalattosauridea; Endennasaurus is found to be closely related to Askeptosauridae

Repeated evolution of durophagy during ichthyosaur radiation after mass extinction indicated by hidden dentition.

Marine tetrapods quickly diversified and were established as marine top predators after the end-Permian Mass extinction (EPME). Ichthyosaurs were the forerunner of this rapid radiation but the main drivers of the diversification are poorly understood. Cartorhynchus lenticarpus is a basal ichthyosauriform with the least degree of aquatic adaptation, holding a key to identifying such a driver. The unique specimen appeared edentulous based on what was exposed but a CT scanning revealed that the species indeed had rounded teeth that are nearly perpendicular to the jaw rami, and thus completely concealed in lateral view. There are three dental rows per jaw ramus, and the root lacks infoldings of the dentine typical of ichthyopterygians. The well-developed and worn molariform dentition with three tooth rows supports the previous inference that the specimen is not of a juvenile. The premaxilla and the corresponding part of the dentary are edentulous. Molariform dentition evolved three to five times independently within Ichthyosauriformes in the Early and Middle Triassic. Convergent exploitation of hard-shelled invertebrates by different subclades of ichthyosauriforms likely fueled the rapid taxonomic diversification of the group after EPME

Varanid evolution

15 p. : ill. ; 26 cm. Includes bibliographical references (p. 11-14).Saniwa ensidens is a pivotal taxon for understanding varanid evolution. A complete specimen of Saniwa ensidens was recently described, offering important new insights into the morphology of this taxon. We apply these new data to a broader-scale study of squamate relationships in order to understand the phylogenetic position of Saniwa ensidens and of varanids more generally. Among the other fossils included in our analysis were the Eocene taxon "Saniwa" feisti, the Miocene Varanus rusingensis, and the giant Pleistocene varanid Megalania prisca. We compare the phylogenetic hypothesis from our analysis of morphology with a recent molecular-based hypothesis and find numerous differences in the phylogenetic relationships within Varanus. We constrained our morphological data set to the phylogenetic pattern presented by the molecular data to further analyze the possible phylogenetic relationships of the fossil taxa. Our analyses show that Saniwa ensidens is the sister taxon to crown-group Varanus and that "Saniwa" feisti is a basal member of the varanid lineage, not closely related to Saniwa ensidens. Both Varanus rusingensis and Megalania prisca are members of the crown radiations of Varanus

The first Triassic vertebrate fossils from Myanmar: pachypleurosaurs (Reptilia: Sauropterygia) in a marine limestone

As ecosystems recovered from the end-Permian extinction, many new animal groups proliferated in the ensuing Triassic. Among these were the sauropterygians, reptiles that evolved from terrestrial ancestors and transitioned to a marine environment. The first sauropterygians were small, marine-adapted taxa such as pachypleurosaurs, which are known from Middle–Late Triassic deposits, particularly in the Tethyan realm of Europe, and more recently from Lagerstätten in southwestern China. Here we report two pachypleurosaurs from Myanmar, the first Triassic vertebrate fossils from the country. These specimens demonstrate that their entombing rocks in northern Shan State, which have received less study than terrestrial sediments in southern Shan State and whose ages have long been uncertain, are Triassic. The specimens may be among the oldest pachypleurosaurs globally, potentially corroborating biogeographic scenarios that posit an eastern Tethyan origin for pachypleurosaurs, and raise the potential for future discoveries of well-preserved Triassic reptiles in Myanmar

Aquatic Habits and Niche Partitioning in the Extraordinarily Long-Necked Triassic Reptile Tanystropheus

Tanystropheus longobardicus is one of the most remarkable and iconic Triassic reptiles. Mainly known from the Middle Triassic conservation Lagerstätte of Monte San Giorgio on the Swiss-Italian border, it is characterized by an extraordinarily long and stiffened neck that is almost three times the length of the trunk, despite being composed of only 13 hyper-elongate cervical vertebrae [1-8]. Its paleobiology remains contentious, with both aquatic and terrestrial lifestyles having been proposed [1, 9-12]. Among the Tanystropheus specimens, a small morphotype bearing tricuspid teeth and a large morphotype bearing single-cusped teeth can be recognized, historically considered as juveniles and adults of the same species [4]. Using high-resolution synchrotron radiation microtomography (SRμCT), we three-dimensionally reconstruct a virtually complete but disarticulated skull of the large morphotype, including its endocast and inner ear, to reveal its morphology for the first time. The skull is specialized toward hunting in an aquatic environment, indicated by the placement of the nares on the top of the snout and a "fish-trap"-type dentition. The SRμCT data and limb bone paleohistology reveal that the large morphotype represents a separate species (Tanystropheus hydroides sp. nov.). Skeletochronology of the small morphotype specimens indicates that they are skeletally mature despite their small size, thus representing adult individuals of Tanystropheus longobardicus. The co-occurrence of these two species of disparate size ranges and dentitions provides strong evidence for niche partitioning, highlighting the surprising versatility of the Tanystropheus bauplan and the complexity of Middle Triassic nearshore ecosystems.© 2020 Elsevier Inc. The attached document is the author(’s’) final accepted/submitted version of the journal article. You are advised to consult the publisher’s version if you wish to cite from it

A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction

Contrary to the fast radiation of most metazoans after the end-Permian mass extinction, it is believed that early marine reptiles evolved slowly during the same time interval. However, emerging discoveries of Early Triassic marine reptiles are questioning this traditional view. Here we present an aberrant basal ichthyosauriform with a hitherto unknown body design that suggests a fast radiation of early marine reptiles. The new species is larger than coeval marine reptiles and has an extremely small head and a long tail without a fluke. Its heavily-built body bears flattened and overlapping gastral elements reminiscent of hupehsuchians. A phylogenetic analysis places the new species at the base of ichthyosauriforms, as the sister taxon of Cartorhynchus with which it shares a short snout with rostrally extended nasals. It now appears that ichthyosauriforms evolved rapidly within the first one million years of their evolution, in the Spathian (Early Triassic), and their true diversity has yet to be fully uncovered. Early ichthyosauromorphs quickly became extinct near the Early-Middle Triassic boundary, during the last large environmental perturbation after the end-Permian extinction involving redox fluctuations, sea level changes and volcanism. Marine reptile faunas shifted from ichthyosauromorph-dominated to sauropterygian-dominated composition after the perturbation