On the use of osteoderm features in a phylogenetic approach on the internal relationships of the Chroniosuchia (Tetrapoda: Reptiliomorpha)

Abstract

Chroniosuchians are an enigmatic Permian to Triassic group of crocodile-like basal tetrapods. Their conspicuous dorsal osteoderm systems include most of the group's yet documented postcranial morphological variability but have hardly been considered in cladistic approaches. Aiming at the clarification of the internal relationships of the Chroniosuchia, we have carried out a parsimony analysis including, among others, 23 morphological and osteohistological osteoderm characters and 12 chroniosuchian taxa. According to the most parsimonious trees, taxa usually referred to Chroniosuchidae form a paraphyletic succession with Madygenerpeton pustulatus and Chroniosaurus dongusensis as the basalmost chroniosuchians and Uralerpeton tverdochlebovae as the sister group of Bystrowianidae (hypothesis A). However, the concurrent hypothesis of a basal split into monophyletic subtaxa Chroniosuchidae and Bystrowianidae (hypothesis B) is only slightly less parsimonious and supported by an alternative analysis which includes embolomeres as the only reptiliomorph outgroup.Searching for the better hypothesis, we compare the respective order of branching to the order of first occurrences in the fossil record, demonstrating that hypothesis A provides a better stratigraphic fit than hypothesis B. Accordingly, the last common ancestor of the yet known chroniosuchians had a series of broad complexly interlocking 'chroniosuchid' osteoderms that served as a protection carapace apart from supporting the trunk during terrestrial locomotion. The later evolution of chroniosuchian carapaces was marked by a stepwise increase in flexibility and size reduction, which resulted in a loss of protective function and in a reduction in trunk support function. The flexibility increase is paralleled by the evolution of the Crocodylomorpha whose extant members do not possess as extensively interlocking osteoderm systems as some of their Mesozoic relatives.

Microstructural features of the femur in early ophiacodontids: A reappraisal of ancestral habitat use and lifestyle of amniotes

“…6). However, if Chroniosaurus is nested within embolomeres, as previously suggested (Buchwitz et al, 2012;Laurin, 2000), its terrestrial lifestyle is probably convergent with that of Please cite this article in press as:Laurin, M., de Buffrénil, V., Microstructural features of the femur in early ophiacodontids: A reappraisal of ancestral habitat use and lifestyle of amniotes. C. R. Palevol (2015), http://dx.doi.org/10.1016/j.crpv.2015.01.001 seymouriamorphs and crown-tetrapods (Laurin, 2010: fig.…”

Comparative data on the differentiation and growth of bone ornamentation in gnathostomes (Chordata: Vertebrata)

“…In addition, this process is a general feature of the pseudosuchians (Buffrénil et al, ), including the phytosaurs (Scheyer et al, ; Buffrénil et al, ), the aetosaurs (Cerda and Desojo, ), and the Dosweliidae (a taxon of Triassic archosauriformes: Cerda et al, ). Moreover, the photographs of sections in the osteoderms of the chroniosuchians Chroniosuchus dongusensis and Bystrowiella schumanni published in Buchwitz et al () and Witzmann and Soler‐Gijòn (2010) suggest that ornamentation in these taxa could also be due, at least partly, to superficial remodeling. It is noteworthy that the specimen of Bystrowiana used in the present study does not show evidence of a resorption process, as is also the case for the chroniosuchian Uralerpeton tvedoschlebovae described by Buchwitz et al ().…”

Laosuchus nagagen. et sp. nov., a new chroniosuchian from South-East Asia (Laos) with internal structures revealed by micro-CT scan and discussion of its palaeobiology

“…Phylogenetically important variability occurs in the shape of their intercentra, which could be disc-, horseshoe-or ball-shaped. This variability would explain why this Chroniosuchidae does not always form a monophyletic group according to the phylogenetic analyses of Buchwitz et al (2012).…”

“…The chroniosuchians were a clade of non-amniotic tetrapods that survived the Permian Triassic mass-extinction (Shishkin et al 2000;Buchwitz et al 2012). They occupied fresh water environments and some species were potentially terrestrial as their system of osteoderms suggests an adaptation to terrestrial locomotion (Clack & Klembara 2009;Buchwitz et al 2011).…”

“…The first chroniosuchian was discovered in 1929: it consists of ribs and osteoderms from the upper Permian of the Russian fore-Ural region and was first attributed to the seymouriamorph Kotlassia prima by Hartmann-Weinberg (1935). Later, Vjushkov (1957a, b) recognized chroniosuchians as a distinct group (Buchwitz et al 2012). The clade Chroniosuchia was erected by Tatarinov (1972) who classified them among anthracosaurs (=Anthracosauromorpha sensu Ivachnenko & Tverdochlebova, 1980), but a recent study placed them among stemamniotes, close to lepospondyls and diadectomorphs (Schoch et al 2010).…”