Today I’m delighted to bring you a guest post by Stephan Lautenschlager. Pterosaur researchers might remember Stephan from the Munich Flugsaurier meeting where he was generous enough to help me arrange the meeting during his last year of study there. Since then he’s moved on to start a PhD in the UK on therizinosaurs (cool!) and has just returned from a trip to Larry Witmer’s lab. Here he introduces the rauisuchians (a group of archosaurs I really should have written more about before – though the one behind him might be familiar…) and discusses his recent paper on them.
Among the multitude of archosaur groups, both extant and extinct, dinosaurs are probably the most popular. Loved by six-year-olds, Hollywood directors, toy-designers and scientists alike, they not only dominated most of the Mesozoic Era, but also a lot of the Archosaur Musings. So when Dave asked me to write a guest post on rauisuchians, he did so with the comment that he never covers these animals in his blog – which is a pity, as rauisuchians are clearly an important and fascinating, if considerably neglected, group of archosaurs. We will have a look at the reasons for this a bit later. First I’d like to provide you with a short introduction, in case you are not familiar with rauisuchians, which I certainly wasn’t before I started to work on this group as a graduate student some years ago.
What are rauisuchians?
This is a question a lot of vertebrate palaeontologists have asked themselves over the years. And to date there is not really a satisfying answer. We know that rauisuchians are a cryptic assemblage of archosaurs within the crurotarsan lineage, distantly related to crocodiles. First appearing in the Anisian, they were restricted to the Triassic period and had a nearly global distribution. Rauisuchians are generally split into two taxonomic groups: The Rauisuchoidea and the Poposauroidea (sometimes referred to as Rauisuchidae and Poposauridae, respectively, although these terms depend heavily on single phylogenetic definitions and should not be used interchangeably). The Rauisuchoidea consisted of medium-sized to large (3-6m), carnivorous animals and are considered to have been among the top predators in the Middle Triassic. They were quadrupedal animals, which – in contrast to their sprawling ancestors – had developed an improved articulation between the pelvis and the femur. This allowed them to centre their legs beneath the body and made erect gait possible. The body of the rauisuchoids was covered by a double row of osteoderms (flattened, leaf-shaped or sometimes quadrangular bony plates) running along the midline of the back and turning into a single row above the tail section.
With an overall length of 2-3m, the Poposauroidea were smaller and more gracile than the Rauisuchoidea. Although most poposauroids were probably carnivorous, the cranial morphology of some taxa (e.g. Shuvosaurus or Lotosaurus) suggests that some might also have been herbivores. Judging by the comparatively short forelimbs, the majority of the Poposauroidea probably was facultatively bipedal. In contrast to the rauisuchoids, they lacked the body osteoderms, although some had developed strongly elongated dorsal neural spines, which formed a peculiar sail back.
Phylogenetic relationships of both the Rauisuchoidea and the Poposauroidea, as well as of Rauisuchia in general are far from being well understood. This resulted in numerous phylogenetic analyses over the last 25 years, providing evidence for and against the monophyly of Rauisuchia. In the process various families and taxonomic units have been created (or equally fast been revoked), sometimes leading to more confusion than clarification. For the time being, a monophyletic Rauisuchia as a sister group to ornithosuchids is found in the most recent phylogenetic analyses.
One of the reasons for these confusing relationships and complicated taxonomy is the fragmentary fossil record of Rauisuchia. Specimens are mostly incomplete and badly preserved. Cranial elements, holding diagnostic features, are often missing. To make things worse, a majority of rauisuchian taxa are represented by single specimens only. Another factor is the documentation of the fossil material. More often than not, the original descriptions (in some cases from the early decades of the last century) are the only existing publications or otherwise inaccessible. German palaeontologist Friedrich von Huene, who collected and described several taxa (including the name-bearing taxon Rauisuchus tiradentes), did publish his results only in German. Other descriptions lack the designation of type material, not to mention missing photographs or illustrations of the specimens themselves. As a result, rauisuchian anatomy, systematics and taxonomy still remain cryptic.
Revising, revising, revising…
The recent years have seen the discovery of several new taxa, providing more information and featuring so far unknown characters. Although more information from new specimens is one of the key factors in trying to resolve the rauisuchian puzzle, the “old” but practically unknown taxa do also still hold a lot of information. So going a different way by systematically studying and revising several of these “old” taxa, in our recently published paper we (that’s my Argentinian colleague Julia Brenda Desojo and me) have focused on two of the earliest known rauisuchoid taxa, Stagonosuchus nyassicus and Ticinosuchus ferox. Both taxa are of Anisian age and therefore of special interest in terms of rauisuchian evolution. Ticinosuchus ferox was found in the bituminous shales of the “Grenzbitumenzone” (upper Anisian) of Monte San Giorgio in Switzerland, a fossil Lagerstätte, well known for its abundant (marine!) vertebrate fauna. Stagonosuchus nyassicus originated from the famous Manda Beds (upper Anisian) in Tanzania. Both taxa had been described in German only – Stagonosuchus nyassicus in 1938 by von Huene and Ticinosuchus ferox in 1965 by Krebs, respectively.
A closer study of the two specimens – in the case of Ticinosuchus ferox a nearly complete and articulated one – revealed numerous characters, which have not been described or illustrated before. So many in fact, that we considered a detailed description of these characters necessary. Among other things, we were able to identify new elements in the skull (including a partial maxilla, a nasal and a frontal) of Ticinosuchus ferox, the only part of the specimen, which is rather poorly preserved. In the post cranial skeleton, we revised and corrected the erroneous anatomy of the pectoral and the pelvic girdle, both of which had been persistently used and illustrated in publications on rauisuchian archosaurs.
The vertebrae of both taxa turned out to be of special interest, showing prominently developed vertebral laminae and fossae – features mainly seen in sauropod dinosaurs, but also present in rauisuchians, and undescribed for these two taxa. Furthermore, some of the dorsal vertebrae show additional hyposphene-hypantrum articulations and accessory neural spines are present in the caudals. These features were part of an extensive bracing system for the dorsal and caudal vertebral column. The hyposphene-hypantrum articulation between the successive vertebrae prevented lateral and transverse movement, whereas the accessory neural spines served as additional facets for ligaments. However, among rauisuchian, accessory neural spines are apparently only present in the Rauisuchoidea, but not the Poposauroidea and therefore of phylogenetic value.
The information gathered from this (and other) revisions are or will be used to supplement phylogenetic analyses, in the hope to achieve a better resolution of rauisuchian relationships and to shed some more light on these animals. In a preliminary phylogenetic analysis, Stagonosuchus nyassicus and Ticinosuchus nyassicus were retrieved as either sister taxa or closely related. For some parts this is surprising. Although both taxa share many characters, which group them together, they both show a distinctive morphology. Stagonosuchus nyassicus was a large and stoutly built animal, whereas Ticinosuchus ferox was considerably smaller and gracile, with elongated cervical vertebrae and delicate limb bones. Being among the earliest representatives of their lineage, this indicates that the diversification of Rauisuchoidea (and Rauisuchia) must have occurred early in their evolutionary history.
Browsing through the BSPG collections in Munich, we continued to study old material and took a closer look on two further rauisuchian taxa, originally collected and described by von Huene in Brazil: Rauisuchus tiradentes and Prestosuchus chiniquensis. Results of both will be published soon, hopefully…
Lautenschlager, S. & Desojo, J. B. (2011): Reassessment of the Middle Triassic rauisuchian archosaurs Ticinosuchus ferox and Stagonosuchus nyassicus. Paläontologische Zeitschrift.
DOI: 10.1007/s12542-011-0105-1 (published online)