Olivier Rieppel

An ancestral turtle from the Late Triassic of southwestern China

The origin of the turtle body plan remains one of the great mysteries of reptile evolution. The anatomy of turtles is highly derived, which renders it difficult to establish the relationships of turtles with other groups of reptiles. The oldest known turtle, Proganochelys from the Late Triassic period of Germany, has a fully formed shell and offers no clue as to its origin. Here we describe a new 220-million-year-old turtle from China, somewhat older than Proganochelys, that documents an intermediate step in the evolution of the shell and associated structures. A ventral plastron is fully developed, but the dorsal carapace consists of neural plates only. The dorsal ribs are expanded, and osteoderms are absent. The new species shows that the plastron evolved before the carapace and that the first step of carapace formation is the ossification of the neural plates coupled with a broadening of the ribs. This corresponds to early embryonic stages of carapace formation in extant turtles, and shows that the turtle shell is not derived from a fusion of osteoderms. Phylogenetic analysis places the new species basal to all known turtles, fossil and extant. The marine deposits that yielded the fossils indicate that this primitive turtle inhabited marginal areas of the sea or river deltas.

A new pistosaurid (Reptilia: Sauropterygia) from the Middle Triassic of Nevada and its implications for the origin of the plesiosaurs

ABSTRACT We describe a new pistosaurid sauropterygian, Augustasaurus hagdorni, gen. et sp. nov., from the Middle Triassic of Nevada. The specimen was collected in Muller Canyon, Augusta Mountains, from a late Anisian laminated mudstone unit in the Fossil Hill Member of the Favret Formation. Augustasaurus hagdorni is based on a partial articulated skeleton consisting of the posterior neck, trunk, shoulder girdle, and both forelimbs. In comparison to Pistosaurus from the Muschelkalk Beds of central Europe, the neural spines of the new pistosaurid are lower and longer with autapomorphic saw-cut rugosities and double facets of unfinished bone. Augustasaurus hagdorni has reduced front limbs with relatively short distal elements and a phalangeal formula of 1-1-3-3-1. This condition is very different from that in plesiosaurs, the presumed sister-group of pistosaurids.

Studies on Skeleton formation in reptiles. IV. The homology of the reptilian (amniote) astragalus revisited

ABSTRACT In extant turtles and lepidosaurs, the astragalus forms from a single ossification center in a tarsale proximale resulting from fusion of originally separate chondrogenic condensations in the proximal tarsus. In crocodiles, the astragalus also ossifies from a single center although the cartilaginous precursors of astragalus and calcaneum do not fuse to form a tarsale proximale. Developmental stages of the parareptilian Mesosaurus document a similar ossification pattern, as do ontogenic series of a number of fossil diapsid reptiles. There is no consistent evidence of ontogenetic fusion of a “tibiale,” intermedium, and centrale to form the astragalus of Captorhinus. The development of the amniote astragalus contrasts with variable patterns of fusion of originally separate proximal tarsal ossifications in anthracosaurian amphibians and in Diadectes. It is concluded that the reptilian (and probably also the mammalian) astragalus is a neomorph, which resulted from ontogenetic repatterning and is diagn...

Rejecting “the given” in systematics

How morphology and systematics come together through morphological analysis, homology hypotheses and phylogenetic analysis is a topic of continuing debate. Some contemporary approaches reject biological evaluation of morphological characters and fall back on an atheoretical and putatively objective (but, in fact, phenetic) approach that defers to the test of congruence for homology assessment. We note persistent trends toward an uncritical empiricism (where evidence is believed to be immediately “given” in putatively theory‐free observation) and instrumentalism (where hypotheses of primary homology become mere instruments with little or no empirical foundation for choosing among competing phylogenetic hypotheses). We suggest that this situation is partly a consequence of the fact that the test of congruence and the related concept of total evidence have been inappropriately tied to a Popperian philosophy in modern systematics. Total evidence is a classical principle of inductive inference and does not imply a deductive test of homology. The test of congruence by itself is based philosophically on a coherence theory of truth (coherentism in epistemology), which is unconcerned with empirical foundation. We therefore argue that coherence of character statements (congruence of characters) is a necessary, but not a sufficient, condition to support or refute hypotheses of homology or phylogenetic relationship. There should be at least some causal grounding for homology hypotheses beyond mere congruence. Such causal grounding may be achieved, for example, through empirical investigations of comparative anatomy, developmental biology, functional morphology and secondary structure.

Phylogeny and paleobiogeography of Triassic Sauropterygia: problems solved and unresolved

Abstract This comparison of the geographic and stratigraphic distribution of the secondarily marine stem-group sauropterygian reptiles from the Triassic with the phylogenetic relationships within this group indicates an incompleteness of the fossil record for the clade of almost 50%, which limits the reconstruction of the biogeographical history of its component taxa. Given their restriction to nearshore habitats or shallow epicontinental seas, it is unlikely that stem-group sauropterygians had the capacity for trans-oceanic dispersal. Dispersal along coastlines may have been a component of their biogeographical history, however, as is indicated by the immigration of sauropterygians into the Anglo-Germanic basin at the transition of the Lower to Middle Triassic, and by the population of the Alpine Triassic by sauropterygians after the evolution of the appropriate intraplatform basin habitat during the Middle Triassic. Conversely, trans-Panthalassan relationships of pistosaur sauropterygians appear best explained as a consequence of vicariance due to the accretion of exotic terranes at opposite margins of the Pacific Basin. A vicariant event resulting from the opening of the southern branch of the Neotethys may also account for the sister-group relationships of nothosaurs from the northern Gondwanan shelf and the Anglo-Germanic basin.

A basal ichthyosauriform with a short snout from the Lower Triassic of China

The incompleteness of the fossil record obscures the origin of many of the more derived clades of vertebrates. One such group is the Ichthyopterygia, a clade of obligatory marine reptiles that appeared in the Early Triassic epoch, without any known intermediates. Here we describe a basal ichthyosauriform from the upper Lower Triassic (about 248 million years ago) of China, whose primitive skeleton indicates possible amphibious habits. It is smaller than ichthyopterygians and had unusually large flippers that probably allowed limited terrestrial locomotion. It also retained characteristics of terrestrial diapsid reptiles, including a short snout and body trunk. Unlike more-derived ichthyosauriforms, it was probably a suction feeder. The new species supports the sister-group relationships between ichthyosauriforms and Hupehsuchia, the two forming the Ichthyosauromorpha. Basal ichthyosauromorphs are known exclusively from south China, suggesting that the clade originated in the region, which formed a warm and humid tropical archipelago in the Early Triassic. The oldest unequivocal record of a sauropterygian is also from the same stratigraphic unit of the region.

First record of Placodontoidea (Reptilia, Sauropterygia, Placodontia) from the Eastern Tethys

DA-YONG JIANG, RYOSUKE MOTANI, WEI-CHENG HAO, OLIVIER RIEPPEL, YUAN-LIN SUN, LARS SCHMITZ, and ZUO-YU SUN Department of Geology and Geological Museum, Peking University, Beijing 100871, P. R. China, djiang@pku.edu.cn; Department of Geology, University of California, Davis, California 95616-8605, U.S.A; Department of Geology, The Field Museum, Chicago, Illinois 60605-2496, U.S.A., rieppel@fieldmuseum.org.

Paleobiogeography of Middle Triassic Sauropterygia in Central and Western Europe

Publisher Summary This chapter discusses paleobiogeography of middle Triassic sauropterygia in Central and Western Europe. An isolated eusauropterygian vertebral neural arch (MHI 1279) was found in the Formazione a gracilis near Recoaro, Val Camonda, above the classical Dadocrinus site Cava di Gesso, in the Vicentinian Alps. The zygosphene is an accessory articular process located between the prezygapophyses. It has been found that in MHI 1279, the zygosphene is fully divided by a deep anterior indentation. The zygantrum is a depression in the base of the neural arch located between the postzygapophyses and receiving the zygosphene from the successive vertebra. In Nothosaurus, the zygantrum is subdivided by a thin medial vertical septum. It is observed that in spite of the deep subdivision of the zygosphene, this vertical septum is absent in the zygantrum in MHI 1279. Further, it has been suggested that neural arches, different from those of Nothosaurus and similar in morphological detail to MHI 1279, can be reported from other lower Muschelkalk localities in the central part of the Germanic Basin.

The first record of a thalattosaur reptile from the Late Triassic of southern China (Guizhou Province, PR China)

Abstract The first thalattosaur from the marine Middle Triassic of Guizhou Province, southern China, is described, and its phylogenetic relationships analyzed. The taxon, Anshunsaurus huangguoshuensis, was found to be the sister-taxon of Askeptosaurus italicus from the southern Alpine Triassic. The paleobiogeography of thalattosaurs, now known from the Middle and Late Triassic of China, Europe, and North America, is discussed.

Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils

Viviparity in Mesozoic marine reptiles has traditionally been considered an aquatic adaptation. We report a new fossil specimen that strongly contradicts this traditional interpretation. The new specimen contains the oldest fossil embryos of Mesozoic marine reptile that are about 10 million years older than previous such records. The fossil belongs to Chaohusaurus (Reptilia, Ichthyopterygia), which is the oldest of Mesozoic marine reptiles (ca. 248 million years ago, Early Triassic). This exceptional specimen captures an articulated embryo in birth position, with its skull just emerged from the maternal pelvis. Its headfirst birth posture, which is unlikely to be a breech condition, strongly indicates a terrestrial origin of viviparity, in contrast to the traditional view. The tail-first birth posture in derived ichthyopterygians, convergent with the conditions in whales and sea cows, therefore is a secondary feature. The unequivocally marine origin of viviparity is so far not known among amniotes, a subset of vertebrate animals comprising mammals and reptiles, including birds. Therefore, obligate marine amniotes appear to have evolved almost exclusively from viviparous land ancestors. Viviparous land reptiles most likely appeared much earlier than currently thought, at least as early as the recovery phase from the end-Permian mass extinction.