Proterochampsids: A key group in continental Triassic ecosystems
Dr Maria Trotteyn (Argentina) and Dr Julia Desojo (Argentina)
Although the most famous and intensely studied fossilised vertebrate (backboned) animals are dinosaurs, these reptiles coexisted during a segment of geologic time -the Triassic Period – with numerous groups of equally interesting, but less well-known species. Most of these fossilised animal shave not been included in films, nor recreated by toy-designers, but many were as spectacular as dinosaurs and comprised an important part of the ecosystem during the Triassic.
The Late Triassic was an especially important time in vertebrate evolution for several reasons, as the older (often highly specialised and bizarre) early reptiles and amphibians were replaced by more familiar groups of reptiles and amphibians, in more modern ecosystems. Moreover, lizards, turtles, modern amphibians and mammals appeared (or can be inferred to have been present) in the Late Triassic. Among the reptiles, one of the most peculiar groups is the Proterochampsidae.
Proterochampsids are not a famous group of Archosauriformes, and few researchers have suggested they were a successful group of aquatic or semi-aquatic species. However, this article will try to shed some light on this interesting and largely neglected group of tetrapods, which lived close to water and coexisted with the first dinosaurs in Triassic times.
Proterochampsids are a group within the Archosauriformes, which are a huge assemblage of reptiles from the Triassic that encompass a transitional phase of vertebrate evolution during the Late Triassic. They are a group of diapsids reptiles, which evolved from a common ancestor, sometime in the Late Permian, approximately 250 million years ago. This ‘archosauromorph’ – a branch of the diapsids – includes several peculiar groups: the trilophosaurids, rhynchosaurs, prolacertiforms and archosaurs.
The first two groups are known from Triassic sediments, but the prolacertiforms arose in the mid-Permian and the archosaurs by the Early Triassic. The archosaurs are a very important group, because their evolution in the Triassic was critical for the later history of vertebrate life on land and the radiation of the dinosaurs, crocodiles and birds. Archosauriformes include the large reptile group Archosauria and all its descendants (Fig. 2). During the Triassic, Archosauriformes were present on all continents, and have been found in both terrestrial and aquatic sediments. Several groups of Archosauriformes became herbivorous, like some aetosaurs and dinosaurs, whereas most of the clades of archosaurs remained carnivorous.
Proterochampsids are an endemic (native) group from South America, present in the sediments of Brazil (Santa Maria Formation) and Argentina (Ischigualasto and Chañares formations) (Fig. 3). During the 1960s, the proterochampsids were associated with different groups of basal archosaurs, such as the crocodiles and the extinct phytosaurs, a group of superficially crocodile-like archosauriforms. In 1971, Rolf Romer and Jose Bonaparte discarded this hypothesis.
The palaeontologist, William Sill, erected the family Proterochampsidae and Romer included in this family the taxa that actually are considered proterochampsids, an arrangement which has been accepted in subsequent systematic studies. In the 1990s, some authors proposed proterochampsids as the sister group (most closely related) to Archosauria or Euparkeria, which was supported by posterior phylogenetic hypothesis obtained by several analyses (Parker and Barton, 2008; Dilkes y Sues, 2009; Nesbitt et al., 2009; Trotteyn, 2011 a,b) (Fig. 2).
In a more recent contribution, the proterochampsid Chanaresuchus was proposed as the sister group of the Archosauriforme, Doswellia kaltenbachi (from the northern hemisphere).The importance of this group, in the phylogenetic context, is that the proterochampsids should be the Archosauria sister group and share with Archosauria the same close ancestors the rest of the Archosauriformes, the same evolution of different lifestyles (for example, aquatic, semi-aquatic and terrestrial), as well as the common ancestor of living birds and crocodiles. Proterochampsids were one of the most important components of Triassic ecosystems and knowledge of this group improves our understanding of the construction and evolution of ecosystems during this key period in vertebrate evolution.
What is a proterochampsid?
This is a question that palaeontologists asked after the discovery of Rhadinosuchus gracilis in 1938 by von Huene in Brazil, and its subsequent re-examination. These animals have a crocodile-like form (Fig. 5), but are not closely related to extant crocodiles. They belong to a group of reptiles known as Archosauriformes (from the Greek for ‘ruling lizards’). In general, many resemble living crocodiles. They are approximately 1.5m in length, with a sprawling, ‘elbows-out’ stance, and long snouts. They were also probably semi-aquatic predators. The Archosauriformes include the Proterosuchidae and more advanced forms, which survived the catastrophic Permian/Triassic extinction.
Proterochampsids resemble extant crocodiles in their general anatomy, but can be further characterised by three cranial morphotypes that may represent different lifestyles:
- Strongly compressed skulls towards the back and sides (Chanaresuchus, Proterochampsa).
- Tall skulls towards the back and sides (Gualosuchus).
- Moderately compressed skulls (Cerritosaurus, Rhadinosuchus and Tropidosuchus).
History of proterochampsid research and species
Traditionally, five genera have been regarded as proterochampsids: Proterochampsa, Chanaresuchus, Gualosuchus, Tropidosuchus and Cerritosaurus. Two of these genera are made up of two species each (Proterochampsa nodosa, Proterochampsa barrionuevoi, and Chanaresuchus bonapartei, Chanaresuchus ischigualastensis) and the rest have only one species each.
The first record of a specimen assigned as a proterochampsid was made by von Huene in 1938. This specimen (Rhadinosuchus gracilis) was found in Brazil and was referred to as a rauisuchian by von Huene. (Rauisuchians were a group of predatory and mostly large Triassic archosaurs.) The holotype of Rhadinosuchus, which is the only specimen of this species, consists of a skull and fragments of the postcranial skeleton and is under revision by one of us (JBD).
In 1946, Price named a new genus and species of Triassic reptile (Cerritosaurus binsfeldi) based on a single specimen of a skull and cervical vertebrae, (which is the vertebra just below the skull; Fig. 6). C. binsfeldi is a proterochampsid from Brazil, with a similar size and proportions to Tropidosuchus and Rhadinosuchus. The roof of the skull of Cerritosaurus has an ornamentation composed of wrinkles, more visible in the back region of the skull roof, especially in the space between the nasal to the parietal.
The upper temporal fenestra (large holes in the side of the skull behind the eye) is small and similar in length to the antorbital fenestra (holes in the skull just in front of the eyes). The external nostrils are small and oval-shaped, surrounded by slit-like depressions. The osteoderms (bony plates in the skin, for example, of crocodiles) are oval-shaped and located on the neural spines, forming one row running along the back of the animal. This species is considered a proterochampsid by several authors, but this assignment is not accepted at present by all authors.
The discovery of a large, highly distinctive skull in the Ischigualasto Formation in Argentina, reported by Reig in 1958, led to the naming of a new Triassic reptile (Proterochampsa barrionuevoi). This species was (MCZ 3408) known until recently by only three skulls and some cervical vertebrae. However, in 1996, Sill found a nearly complete skeleton that (PVSJ 606) is now housed at the Instituto y Museo de Ciencias Naturales of San Juan University in Argentina. The later was recently studied by one of the authors of this article (MJT). While most of the specimen is well preserved, the pelvis of the specimen PVSJ 606 is badly crushed and it is not possible to distinguish with certainty the acetabulum (that is, the distinctive concave surface of the pelvis) or the individual bones of the pelvis.
In 1971, Romer described two new proterochampsids: Chanaresuchus bonapartei, represented by a complete skeleton, and Gualosuchus reigi, consisting only of a skull. These species were recovered from the Chañares Formation in Argentina. C. bonapartei (UNLR 07; MCZ 4035; MCZ 4026; MCZ 4037; MCZ 4038; PVL 4575; PVL 4586; PVL 4647; MACN s/n) is a medium-sized reptile, compared to the other proterochampsids.
The skull is not quite triangular, with a notable ornamentation made up of crests that run along the top of the snout and a radial pattern on the frontal and parietal bones (which are the fused bones making up the front and sides of the skull). The postcranium is characterised by the presence of wedge-shaped osteoderms, which are narrow at the top and broader towards the bottom.
On the other hand, G. reigi is known by only two specimens (UNLR 05; PVL 4576). These skeleton are very incompletely preserved, consisting of a complete skull, together with a complete jaw, an articulated presacral vertebral column (the part of the backbone above the sacrum), sacrum (the triangular-shaped bone at the bottom of the spine) articulated with the pelvis, and a complete forelimb, but without the ‘hand’ (manus), femur, tibia or fibula.
The Brazilian researcher M. C. Barberena described a new species in 1982 Proterochampsa nodosa, only known by the skull. The overall shape of this new taxon resembles that of Proterochampsa barrionuevoi, but P. barrionuevoi differs through certain features such as an abrupt narrowing of the snout at the top.
In 1990, Andrea Arcucci described the smallest known proterochampsid (Tropidosuchus romeri) from the Chañares Formation in Argentina. This genus has the largest number of specimens (seven) for any proterochampsid and the most complete skeletons found in Argentinean Triassic deposits (Fig. 8). The skull is triangular when viewed from above, with proportionally the largest orbits among proterochampsids, and a curved premaxilla (which is a small bone at the very end of the upper jaw) at the tip of the snout.
The skull surface is ornamented with two strong crests running lengthways over the skull roof, between the frontal and parietal bones (Fig. 9), without the radial pattern of ossification that is present in Chanaresuchus and Gualosuchus on the last bone.Tropidosuchus is the smallest and most graceful proterochampsid. Its osteoderms are much larger than in Chanaresuchus and Gualosuchus, and consist of one on each vertebra, with a prominent crest.
In 1994, Sill reported a new proterochampsid from continental deposits of Argentina. The material includes an almost complete skeleton that is an exemplar of the genus Chanaresuchus ischigualastensis (Fig. 10). In general, this species is similar to Chanaresuchus bonapartei, with some differences in anatomy.
However, the major difference is the stratigraphical origin: C. bonapartei is from the Chañares Formation, and C. ischigualastensis is from the Ischigualasto Formation. A gap of five million of years separates these, so this genus must have existed for at least this length of time and its discovery changes our understanding of the pattern of macro-evolutionary history of proterochampsids during the Middle to Late Triassic in south-western Pangaea.
Geographical and stratigraphical distributions
Proterochampsids are an endemic group of Archosauriformes from South America. All species have been found in continental Triassic sediments from Brazil and Argentina (Fig. 11). The best known proterochampsids come from north-western Argentinean deposits. In particular, two stratigraphic units, the Chañares Formation (Ladinian to early Carnian) and the earlier Ischigualasto Formation (late Carnian to earliest Norian), both found within the Ischigualasto-Villa Unión Basin, formed during the breakup of Gondwana.
These two stratigraphic units have been interpreted as continental river sediments, but differ in age by approximately five million years and in their general faunal assemblages. The tetrapod assemblage from the Chañares Formation has usually been interpreted to differ from that of the Ischigualasto Formation at a generic level, but this traditional view is not the case with proterochampsids.
In the Chañares Formation, the proterochampsid species Chanaresuchus bonapartei, Gualosuchus reigi and Tropidosuchus romeri have been found, whereas, in the Ischigualasto Formation, the species Proterochampsa barrionuevoi and Chanaresuchus ischigualastensis is present. Therefore, the replacement of proterochampsids occurs at the level of species, rather than genus among the proterochampsids from Argentinean formations.
Proterochampsid palaeobiology
Factors, such as taphonomy (the study of decaying organisms over time and how they become fossilised), sedimentology and anatomy, can help to infer the way of life of the animals in the fossil record.
Taphonomical evidence includes the observation that the specimens from the Ischigualasto Formation are preserved articulated, suggesting that the animals probably died in the same environment as the sediments that actually contain the many the bodies were not transported a significant distance after their death. Sedimentological evidence suggests river deposits. Therefore, it is likely that proterochampsids in the Ischigualasto probably spent some part of their lives close to rivers and were perhaps aquatic or semi-aquatic. The Chañares and Santa Maria Formations (where we also find proterochampsids) are also river deposits, supporting a hypothesis of at least a semi-aquatic lifestyle.
In terms of their anatomy, proterochampsids are similar to extant aquatic animals. The triangular shape of the skull, the orientation of the back of the supratemporal fenestra (a hole in the skull behind the eyes, towards the top of the skull), the position of the back of the nasal passage, the presence of a secondary palate, and notable ornamentation are traditionally considered to be aquatic characteristics and all are present in proterochampsids. Also, conical and laterally compressed teeth are a character of extant crocodiles, caimans and gavials, and are related to a fish diet. Extant aquatic animals have long and laterally compressed tails, osteoderms and reduced limbs.
A reduction in the forelimbs could be related to a semi-aquatic life style, but this reduction cannot be confirmed for proterochampsids as a result of the scarcity of fossil material. The presence of osteoderms suggests an aquatic or semi-aquatic lifestyle, but dermal armour is also present in some terrestrial lepidosaurs (a large group of non-avian reptiles), so this is not a feature exclusive to aquatic animals.
Moreover, there are some aquatic animals without dermal amour, such as the Triassic mesosaurs. Compressed tails are not present in all proterochampsids and, in Tropidosuchus and Chanaresuchus, the caudal vertebrae have neural spines with proportions similar to some terrestrial squamates (which is group of reptiles that includes snakes and lizards). The lifestyle of proterochampsids is currently in revision by one of us (MJT).
The end of the proterochampsids
As more specimens of proterochampsids are discovered, it becomes clear that they were a successful aquatic or semi-aquatic group of their time. Why then did they become extinct? In the Late Triassic, the first dinosaurs were small, fast bipeds, living in the shadow of the rauisuchians. Therefore, smaller, less agile proterochampsids were probably prey rather than competition for predatory dinosaurs and rauisuchians.
These carnivorous archosaurs had evolved an effective locomotion through an erect gait, which is more effective than the quadrupedal style of proterochampsids. However, the crucial advantage of the dinosaurs may have been their possible warm-blooded metabolism. In combination with the erect gait, the first dinosaurs were not only quicker, but also more agile. These small, but efficient changes in morphology and physiology might have been a critical factor in the competition for food and space, as well as a successful feature making the proterochampsids easy prey.
At the same time, the bigger carnivorous rauisuchids were searching for small prey, which may have included a proterochampsid. While these predators did not inhabit the same aquatic places as proterochampsids, they probably could get close to bodies of water and prey on proterochampsids.
Proterochampsid research today
There are many items related to the proterochampsids that are currently under study. Detailed descriptions of some specimens, such as the new species Chanaresuchus ischigualastensis, Tropidosuchus, Gualosuchus and Cerritosaurus, are currently in progress. Data from new detailed descriptions of these species will be important for phylogenetic analyses that include new specimens and new characters.
There are numerous studies on palaeobiological aspects, particularly the way the bones of the skull move relative to each other and the microstructure or bones. Detailed descriptions of the braincase and endocast give information about the form and location of soft tissues, such as the brain and individual parts like the olfactory or optic bulbs. This will allow functional analysis of the brain and more detailed inferences of the lifestyle of proterochampsids.
Research on the microscopic anatomy of the cells and tissues of the longer bones and osteoderms also provides information about the lifestyle of these animals, and these studies are in progress. And finally, the authors (MJT and JBD) are currently working on several of these research topics and, since 2011, have conducted exploratory fieldtrips to Triassic outcrops of the Chañares Formation, looking for and extracting numerous specimens preliminarily assigned to proterochampsids.
Acknowledgments
We thank V Contreras for his comments on PVSJ 567; D Dilkes for his comments and observations on the manuscript, G Heredia for the drawings of Proterochampsa barrionuevoi, M Ezcurra for the photo of Cerritosaurus binsfeldi, and A Arcucci for the photo of Tropidosuchus romeri.
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