It doesn’t always have to be dinosaurs – a short review of rauisuchian archosaurs
Stephan Lautenschlager (Germany) and Dr Julia Brenda Desojo (Argentina)
Among the multitude of fossil animals, dinosaurs have always been the most popular and fascinating. Loved by six-year-olds, Hollywood directors, toy-designers and scientists alike, they not only dominated most of the Mesozoic Era, but still dominate our understanding of palaeontology. However, only a few people are aware that, before the dinosaurs started their 150-million-year-long global dominion, there was an equally successful and remarkable group of fossil reptiles – the ‘rauisuchians’ (Fig. 2). In this article, we will try to shed some light on these enigmatic and commonly unknown tetrapods, which were as adapt and predominant in their time – and, to be honest, as cool – as the dinosaurs.
A history of rauisuchian research
The first rauisuchian fossil was found in 1861 by the German naturalist Hermann von Meyer. It consisted of a single maxilla of Teratosaurus suevicus and was identified as an early dinosaur. The same happened to the next to be found, Poposaurus gracilis, after its discovery in Wyoming in 1915. This specimen was subsequently described as a theropod dinosaur, a primitive stegosaurid and also an ornithopod. Only when the German palaeontologist, Friedrich von Huene, collected numerous Triassic fossils from the Santa Maria Formation of Brazil in 1928, did things begin to change. His detailed studies of the material revealed that most of it did not belong to any known reptile groups.
In 1938, in gratitude to a local medical doctor called Wilhelm Rau, von Huene established the species Rauisuchus tiradentes as well as the family Rauisuchidae and the actual group of Rauisuchia. In the following years, several new fossils were found, including the nearly complete and articulated Ticinosuchus ferox. The study of this species laid the foundation for rauisuchian palaeontology. Since then, rauisuchian fossils have been discovered and recognised all over the world, and with them, new information and confusion in equal amounts.
With each new fossil, new families and systematics have been introduced and specimens added to or removed from the rauisuchians. One reason for this is the fragmentary fossil record. Specimens are mostly incomplete and badly preserved. Crucial skeletal elements holding diagnostic features are often missing. Another reason is the widely-spread distribution of the fossils and their availability. While terrestrial Triassic sediments are widespread in South America and Africa, they are rare in Europe and Asia as the respective regions are dominated by marine sequences.
But what is a rauisuchian?
This is a question a lot of palaeontologists have asked themselves since the discovery of rauisuchian fossils nearly 150 years ago. The answer is that we’re not yet completely sure. The fact is that the rauisuchians belonged to a group of reptiles known as archosaurs. Apart from living crocodiles and birds, the archosaurs also comprise dinosaurs, pterosaurs and several groups of extinct reptiles. Literally translated, their name means “ruling lizards”, as most of the group’s members did and, in some parts, still do dominate the earth.
Among other characteristics, the archosaurs are united by the presence of an antorbital and a mandibular fenestra, that is, additional openings in the skull located between the nasal and the orbital cavity and the lower jaw respectively. The archosaurs are divided into two main groups or lineages, characterised by the way their ankle joint is formed. One lineage informally known as the “bird-line”, consists of the dinosaurs and pterosaurs, eventually leading to the birds. The other lineage, the so-called “crocodile-line”, contains the extant crocodiles, their ancestors and a variety of basal and peculiar forms, including the rauisuchians. Therefore, they are more closely related to crocodiles than dinosaurs.
The rauisuchians themselves can be split into a confusing assemblage of families, which have been proposed and sometimes equally quickly revoked over the years. This highlights the fact that palaeontologists have not yet completely understood the full concept of rauisuchians systematics. However, two different morphological types can generally be distinguished in rauisuchians. Therefore, both groups will be discussed in the rest of this article, without going much deeper into taxonomy.
The first group, conventionally termed rauisuchids (after the family Rauisuchidae and the type species Rauisuchus tiradentes respectively), comprises large and stoutly-built animals that averaged 4m to 8m in length. However, some specimens, like Fasolasuchus tenax from Argentina, may have been even larger. Their heads were comparatively large and narrow, and their powerful jaws held numerous, laterally flattened and recurved teeth. These teeth had two finely serrated edges along the anterior and posterior margin, which probably meant that these animals were carnivorous.
Cranial remains also suggest a continuous and lifelong tooth replacement. Insofar as the respective elements are preserved, the majority of rauisuchids possessed an additional antorbital fenestra in the upper jaw, located between the anterior part, the premaxilla and the posterior part, of the maxilla. The distribution of this opening in the rauisuchians and its function are not completely understood. However, it has been suggested that it held a kinetic joint to allow the skull to move or that it formed an opening for nerves and blood vessels.
The forelimbs were moderately shorter than the hind limbs and it is assumed that the rauisuchids were quadruped. In contrast to their sprawling ancestors, the rauisuchids had developed an improved articulation between the pelvis and the femur, which allowed them to centre their legs beneath the body and made erect gait possible. The body of the rauisuchids was covered by a double row of osteoderms (that is, 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. In living crocodiles, such plates are common and numerous, covering most of the dorsal (back), ventral (belly), and appendicular (legs) parts of the animal.
In this article, we will refer to the other group of rauisuchians as poposaurids, after Poposaurus gracilis. As the species name indicates, these animals were slender and had gracile forms. With an overall length of 2m to 3m, they were considerably smaller than the rauisuchids. Their heads were small and connected to a slightly elongated and slim neck.
Although the majority of the poposaurids was probably carnivorous, the tooth pattern of some specimens (for example, Shuvosaurus) suggests that some might also have been herbivores. Whether the poposaurids also possessed a second antorbital fenestra is presently unknown, as the relevant cranial bones have not been preserved in any poposaurid that has been found – a problem well known in rauisuchian palaeontology.
Judging by the comparatively short forelimbs, the majority of the poposaurids probably was facultatively bipedal, meaning they would have been able to walk on two or four legs. Like the rauisuchids, they had developed an erect gait. In contrast to the rauisuchids, they lacked the body osteoderms, although some had developed strongly elongated neural spines, which formed a sail back.
Recent studies reveal that the function of these specialised neural spines was not thermoregulation or display as originally thought. Rather, with this configuration, the sail-backed poposaurids were able to effectively transmit the power necessary for bipedal locomotion, especially at low speeds. This was a revolutionary concept at that time. Consequently, these animals were probably the first vertebrate group to achieve a bipedal locomotion – far earlier than the dinosaurs.
The relationships between the rauisuchids and the poposaurids are mostly unknown and it is still unclear whether both groups derived from a common ancestor. According to recent studies, both are tentatively treated as sister groups. In general, there is some indication that the rauisuchids constituted a more primitive state of evolution. In many ways, the poposaurids were specialised rauisuchians and resembled somewhat the small and agile theropod dinosaurs of the Late Triassic. However, this is simply a phenomenon of convergent evolution, as both groups probably filled the same ecological niche, but are not closely related.
Geographic and stratigraphic distribution
As the dominant predators in most ecological systems, the rauisuchians were globally distributed in the Triassic. At present, nearly 40 different rauisuchian species have been found on nearly every continent, with the exception (so far) of Australia and Antarctica. For example, Europe has yielded two of the most interesting rauisuchian specimens. One of them is Ticinosuchus ferox, which is represented by a small but fully articulated specimen from the Anisian black shale sequence of Monte San Giorgio, Switzerland.
The carcass of this normally terrestrial animal was washed into the Triassic sea, where it was buried. The anoxic conditions on the sea floor prevented the carcass from being torn apart by scavengers, while bacterial films and slow sedimentation preserved the skeletal remains. Although the specimen was considerably flattened during fossilisation, it revealed a lot about rauisuchian anatomy.
The other one, Batrachotomus kupferzellensis, was found in a small village near Stuttgart, Germany in the late seventies at a highway construction site. Numerous bones, osteoderms and teeth of up to five specimens were found in the marlstones of the Upper Ladinian (about 237 million years ago). The good preservation and the great number of skeletal elements allowed a detailed and scientifically up-to-date reconstruction of the animal (Fig. 3).
Apart from these, further rauisuchian remains have been found in England, Italy, and Poland. A particularly rich collection of rauisuchian fossils of more than ten species comes from Russia, although it is still questionable if these taxa really constitute rauisuchian archosaurs or more basal and primitive forms such as Erythrosuchians.
Several Indian and Chinese specimens are known from Asia and, even though they are far from complete, each of these fossils helps us to improve our knowledge on rauisuchian phylogeny and distribution.
In North America, the United States has proved to be a good location for rauisuchian fossils, especially poposaurids. Examples include Poposaurus gracilis and Arizonasaurus babbitti, the latter being one of the peculiar sail-backed forms (Fig. 3).
In the southern hemisphere, Brazil and Argentina have become important fossil locations for rauisuchian palaeontologists. More than ten different rauisuchian species are known from South America, including the name-bearing taxon, Rauisuchus tiradentes. Africa has so far only yielded a few, but no less important rauisuchian fossils. Some show a close resemblance to South American forms and it seems that there may be distinct morphological and evolutionary differences between the rauisuchian taxa from Gondwana and those from Laurasia.
The first rauisuchians are known from the Middle Triassic (Anisian) (about 245 million yeas ago) but their origin lies probably in the Lower Triassic or even the Permian. One of the first rauisuchians to appear was Ticinosuchus ferox. During the Anisian, most ecological systems were still dominated by primitive reptiles and the rauisuchians were a comparatively rare faunal element. This changed towards the Ladinian (about 237 million years ago), when they became the top predators, such as Batrachotomus kupferzellensis. Tooth and bite marks, which have found on several bones of Mastodonsaurus, a 6m long primitive amphibian, show that Batrachotomus ranged at the top of the food chain. In the Carnian (about 228 million years ago), the number of rauisuchian taxa reached its peak with more than 15 different species.
During the last 15 to 20 million years, they had successfully diversified and risen to become the predominant group of archosaurs. However, in the Norian (about 216 million years ago), their number significantly declined. A reason for this may be the appearance of another group of archosaurs in the Carnian – the first dinosaurs. Forms like Herrerasaurus and Staurikosaurus are the first dinosaurs recorded so far. They were found in Argentina and Brazil, the latter in the same localities as Rauisuchus tiradentes and other rauisuchians, and probably posed a growing competition. Towards the end of the Norian, the rauisuchians finally became extinct. Fasolasuchus tenax, from the Late Norian of Argentina, is the last known specimen. By then, the theropod dinosaurs had already replaced the rauisuchians as top predators.
Rauisuchian ecology
Compared to other fossil groups, the knowledge of rauisuchians and their ecology and behaviour is still limited. However, some exceptional finds, such as the fossil lagerstätte of Kupferzell, Germany, where Batrachotomus kupferzellensis was found, tell us a lot about rauisuchian ecology. Studies of its bones and shape show that Batrachotomus was a quadruped with an erect gait. It was probably not a fast runner, but it could chase its prey effectively and for long periods. The dentition and the powerful jaws identify Batrachotomus as a carnivorous predator and the laterally flattened and serrated teeth permitted an efficient digestion of its prey.
These teeth grew continuously and were regularly replaced. The bite marks on a large amphibian found in the same locality gave Batrachotomus its name, which literally means “frog killer”. Plant fossils and sediments of the Kupferzell area reveal that Batrachotomus lived in a swamp-like environment, dominated by horse-tails (Neocalamites and Equisetites) and ferns, comparable to today’s mangrove swamps. The faunal elements included insects of all kinds, small reptiles and amphibians. The latter ones were probably the preferred prey of Batrachotomus.
In contrast to the wet swamp lands of the Ladinian of Germany, South America was characterised by dry lands dominated by meandering rivers and small lakes. The flora consisted of cycads and conifers. In that environment lived Rauisuchus tiradentes and Prestosuchus chiniquensis. While Prestosuchus was a big rauisuchian being about 5m long, Rauisuchus was comparatively small at between 3m and 4m in length. The presence of different rauisuchian species in one locality suggests that they had successfully adapted to different ecological niches.
Other faunal elements included rhynchosaurs, cynodonts and dicynodonts. Some of these were herbivorous reptiles, occupying a similar ecological role as modern browsing mammals. Impressions of rauisuchian teeth have been found on several of their bones, suggesting that Prestosuchus regularly hunted these animals. Rauisuchus probably went for smaller prey like amphibians and small reptiles.
Other fossil remains, like the famous tetrapod tracks of Germany from the Middle Triassic (known as Chirotherium) might be attributed to different rauisuchians. While some of the tracks could be attributed to the quadruped Ticinosuchus ferox, other tracks showed only imprints of the hind limbs.
Recent studies may show that they were created by the bipedal poposaurid, Ctenosauriscus. This genus also had a distinct sail back like Arizonasaurus and was able to walk without the support of its forelimbs. An analysis of the tracks has estimated the speed of its producer at about 12km/h.
The end of the rauisuchians
One question still remains – why did the rauisuchians become extinct, even though they were one of the most successful groups of their time? The fact that they declined towards the end of the Triassic and that the dinosaurs appeared and quickly diversified at the same time is certainly no coincidence. The first dinosaurs were small, but fast bipedals living in the shadow of the rauisuchians, probably more prey than competition – a situation that repeated itself about 150 million years later with the dinosaurs and the first mammals. Both the rauisuchians and the dinosaurs had achieved an effective locomotion by the development of an erect gait.
The crucial advantage of the dinosaurs may have been a possible warm-blooded metabolism. In combination with the erect gait, the first dinosaurs were not only quicker, but also more agile than the comparably big and heavy rauisuchians. Bipedalty was probably another advantage. The dinosaurs had achieved two-legged locomotion with a different articulation of pelvis and hind-limb and also a different constellation of the ankle joint, than the rauisuchians. Although the sail-backed poposaurids were probably the first to have invented a bipedal walk, the dinosaurs had improved this concept. These small, but efficient changes in morphology and physiology might have been a critical factor in the competition for food and space.
Rauisuchian research today
Since the discovery of the first rauisuchians and their description, a multitude of characters and diagnostic features have been proposed and described. During the last ten years, vertebrate palaeontologists have tried to resolve this problem. In fact, we have revised several specimens, including Rauisuchus tiradentes and Prestosuchus chiniquensis, and collected data from different collections world-wide in the hope of a comprehensive knowledge on rauisuchians and to solve at least some of their mysteries.
Stephan Lautenschlager studied palaeontology and geology at the University of Munich (Germany) and works for the Bavarian State Collection for Palaeontology and Geology. In his diploma thesis, he redescribed the rauisuchian archosaur Rauisuchus tiradentes.
Dr Julia Brenda Desojo has a PhD from the University of Buenos Aires (Argentina) in Biology and works as a research scientist for Conicet at the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” in Argentina. Her projects included the diversity and biogeography of non-crocodilian crurotarsal archosaurs from the continental Triassic. As a Humboldt research fellow, she currently studies the rauisuchians and basal archosaurs from the Brazilian Santa Maria Formation.