POSSIBLE SECONDARILY TERRESTRIAL LIFESTYLE IN THE EUROPEAN PHYTOSAUR NICROSAURUS KAPFFI (LATE TRIASSIC, NORIAN): A PRELIMINARY STUDY

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1 306 Tanner, L.H., Spielmann, J.A. and Lucas, S.G., eds., 2013, The Triassic System. New Mexico Museum of Natural History and Science, Bulletin 61. POSSIBLE SECONDARILY TERRESTRIAL LIFESTYLE IN THE EUROPEAN PHYTOSAUR NICROSAURUS KAPFFI (LATE TRIASSIC, NORIAN): A PRELIMINARY STUDY JULIEN KIMMIG Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada, j.kimmig@usask.ca Abstract The Late Triassic (Norian) phytosaurs Nicrosaurus kapffi and Mystriosuchus planirostris appear at about the same time in central Europe. Both have very distinctive snout shapes and are thought to occupy different ecological niches. The depositional environment and post-cranial morphology suggest a secondarily terrestrial lifestyle for Nicrosaurus. Some phytosaurs might have exploited coastal habits based on fossils occuring in saline water deposits, whereas most are found in fresh to brackish water deposits. INTRODUCTION Phytosaurs are Late Triassic (Carnian to Rhaetian) archosaurs, with a body plan similar to modern crocodilians (Fig. 1). They are primarily known from the United States and Europe, but have also been found in Africa, South America and Asia. The deposits in which phytosaurs occur range from marine to swamp-like deposits, but most species are found in semi-aquatic environments (Long and Murry, 1995; Kimmig and Arp, 2010). Phytosaurs are considered to be close to the base of the crocodilian branch of evolution (Hutchinson, 2001; Irmis et al., 2007; Brusatte et al., 2010), the closest relatives being aetosaurs, rauisuchians and members of the Ornithosuchidae. Even though more recent phylogenetic analyses reinforce this relationship, there is no consensus on the precise origin of phytosaurs (Ballew, 1989; Hungerbühler, 2002). Given that the taxonomy of the Phytosauria is based on cranial characters and that most of the postcranial material has not been found associated with skulls, research on phytosaur postcrania is spare (Parrish, 1986a, b) and mostly reduced to comparisons with other archosaurs (Hutchinson, 2001; Holliday et al., 2010; Padian et al., 2010). Due to their resemblance to crocodilians, a lifestyle similar to that of extant crocodiles has long been suggested for phytosaurs ( Parrish, 1986b; Hunt, 1989). This led to the popular image of phytosaurs as semi-aquatic animals, which may be true for most taxa, although data analyzed here suggest that a more terrestrial lifestyle developed in at least one species. The first support for this conclusion came from an analysis of the European phytosaur-bearing deposits, which indicated that Nicrosaurus lived in swamp-like environments and spent less time in the water than did Mystriosuchus or most other phytosaurs. A similar habitat was suggested for the North American Redondasaurus (Heckert et al., 2001; Kimmig and Arp, 2010). To evaluate the lifestyles of various phytosaurs an analysis of the most commonly-preserved postcranial bones (ilia, femora, and humeri) of all available species was undertaken. The results of this analysis support the idea of the Early Norian phytosaur Nicrosaurus as primarily terrestrial, not semi-aquatic. In contrast, Mystriosuchus from the same deposits developed a lifestyle more aquatic than all other phytosaurs, a conclusion reinforced by the recovery of the remains of Mystriosuchus from marine strata in Austria, Italy and Switzerland (Buffetaut, 1993; Renesto and Paganoni, 1998). Institutional abbreviations: BMNH, The Natural History Museum, London, United Kingdom; NMMNH, New Mexico Museum of Natural History and Science, Albuquerque, NM, USA; Staatliches Museum fur Naturkunde Stuttgart, Stuttgart, Germany; UCMP, University of California Museum of Paleontology, Berkeley, CA, USA; ZPAL, Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland. FIGURE 1. Comparison of the skeletons of A, Leptosuchus crosbiensis (after Long and Murry 1995), and B, Crocodylus niloticus (after Lydekker, 1879). MATERIAL AND METHODS For this study, phytosaur postcrania (ilia, femora and humeri) were compared to their closest relatives: aetosaurs, rauisuchians, and other archosaurs. The comparison was primarily based on published material, but also included some museum specimens. These three elements were chosen, because they are the portions of the phytosaur postcrania most commonly referred to the species-level. The published literature on crocodilian locomotion was used for reconstructing phytosaur locomotion. TAXONOMIC BACKGROUND Phytosaur taxonomy is highly debated and it is unlikely that a consensus opinion will be reached in the near future (Long and Murry, 1995; Hungerbühler, 2002; Stocker, 2010). In this paper I use the taxonomy of Hungerbühler (2002) and Kimmig (2009), which provides the most recent revision of the entire group. This revision concludes that all derived phytosaurs demonstrate sexual dimorphism, which is mainly based on the presence of a cranial crest in the male specimens. Phytosaurs are close to the base of the archosaurian tree (Fig. 2), but still a member of the crown Archosauria (Irmis et al., 2007; Brusatte et al., 2010). CROCODILIAN ECOLOGY All extant crocodilians are semi-aquatic animals, which has led to the general assumption that phytosaurs were also semi-aquatic. However, over the long history of the crocodilian order there were a variety of species that were terrestrial. Indeed, the earliest crocodylomorphs were terrestrial, and throughout crocodile evolution terrestrial forms evolved several times (e.g., Brochu, 2001).

2 FIGURE 2. Archosaur phylogeny based on Brusatte et al. (2010) and phytosaur phylogeny based on Hungerbühler (2002). Crocodilians are known for their crawling locomotion on land, but they can also move in a more mammal-like high walk by which they reach speeds of up to 5 kph (Guggisberg, 1972). The fastest method of moving is the gallop, where crocodiles can reach a speed of up to 17 kph (Webb and Gans, 1982). The high walk and the gallop are supposedly inherited from their ancestors, so it is likely that other members of the Crurotarsi also had the ability to perform these locomotion styles. Crocodilians feed on everything from fish and crustaceans to large mammals (Ross and Magnusson, 1989). This variation in feeding is mainly expressed by the snout form, which can partly provide an analog for phytosaurs (Hunt, 1989). ECOLOGICAL INDICATORS OF A TERRESTRIAL LIFESTYLE IN NICROSAURUS Hungerbühler (2002) mentioned that most phytosaur fossils of the Stubensandstein are found in sheet-flood sandstones at the transition from distal alluvial plain to playa lake. As phytosaur specimens in these sediments are mostly restricted to disarticulated specimens, it is likely that the fossils were subject to fluvial transport. Considering that most Mystriosuchus specimens outside of the Stubensandstein were found in saltwater deposits, it is likely that the carcasses were transported within the basin and that the animals lived close to the place where they were buried (Renesto and Paganoni, 1998; Hungerbühler, 2002; Kimmig and Arp, 2010). Nicrosaurus, in contrast, is mostly found in reworked palaeosols (Salzgitter) and marginal-lacustrine sandstones or breccias resting on sub-aerial exposure planes (Çal Dag; Krähenberg). This indicates that the animals spent a large amount of time on land and most likely lived in swamps or similar areas (Kimmig and Arp, 2010). Unfortunately most of the German basin outcrops from which the phytosaur fossils were collected are no longer accessible, so modern facies analysis is not possible. Another factor suggesting different lifestyles for these two phytosaur genera is the skull shape and the dentition. Nicrosaurus has a 307 very massive skull with a tripartite dentition, which is similar to the skull of modern crocodiles and suggests predation on larger animals with robust bones or even bony osteoderms. Mystriosuchus, in contrast, has a very slender skull with a bipartite dentition, similar to modern gavials, which is more suited for hunting fish and smaller tetrapods. PREVIOUS RESEARCH ON PHYTOSAUR LOCOMOTION Locomotion studies on phytosaurs are restricted to Leptosuchus (also referred to as Rutiodon: Parrish, 1986a, b). These studies compared them with several other archosaurs, but as Leptosuchus happens to be found in semi-aquatic deposits it is no surprise that it shows strong similarities to extant crocodilians (Mehl, 1928; Doyle and Sues, 1995). As there are at least nine genera of phytosaurs (Hungerbühler, 2002; Stocker, 2010), and some are found in marine or playa lake deposits (Gozzi and Renesto, 2003; Kimmig and Arp, 2010), it is surprising that functional analyses have not been performed for other genera. Some research on the functional morphology of the Polish Paleorhinus angustifrons material is in progress (Bronowicz, personal commun.) and supposedly this analysis will indicate a semi-aquatic lifestyle, which is consistent with the habitat (Dzik and Sulej, 2007). The advantage is that there is a large amount of P. angustifrons cranial and postcranial material available from this locality (Dzik, 2001; Dzik and Sulej, 2007), similar to the Pseudopalatus material from the Snyder quarry in New Mexico, USA (Zeigler, 2003; Zeigler et al., 2003). In their paper on Apatopus tracks, Padian et al. (2010) concluded that phytosaurs are the likely trackmaker of this ichnotaxon and that they must have been capable of a high walk. Achieving a high walk is the first step towards a terrestrial lifestyle and supports the possibility of a secondarily terrestrial lifestyle. In the case of Apatopus, these tracks were likely produced by Rutiodon, as they are primarily known from New Jersey, and Rutiodon is the only phytosaur known from the eastern United States (Long and Murry, 1995). PHYTOSAUR POSTCRANIAL ANATOMY Humerus (Fig. 5) The humerus of phytosaurs is not very variable. The major variation is in the diaphysis, which is slender in Paleorhinus (Fig. 4A) and Pseudopalatus (Fig. 4D) and thickened in the other phytosaurs. The proximal end of the humerus is similar in all phytosaurs where the humerus is known, except for Nicrosaurus (Fig. 4C), in which it is flattened. All derived phytosaurs have a well-developed crest that covers half of the humerus from the postero-distal end to the middle of the diaphysis. The distal end displays more variation than the proximal end. Paleorhinus, Leptosuchus (Fig. 4B), Pseudopalatus and Nicrosaurus have humeri with a rounded distal end. In contrast, the humerus of Mystriosuchus (Fig. 4E) has a flattened distal end and is thicker overall (more robust) than the other humeri. The humerus of Mystriosuchus is different from that of the other phytosaurs, especially the more massive shape and the flattened top. A more extreme form of this is found in marine reptiles, for example, placodonts, nothosaurs and marine crocodiles. Overall, the variation is smaller than in the hind limb, but it indicates a different lifestyle of Nicrosaurus, Redondasaurus and Mystriosuchus when compared to other phytosaurs. Ilium (Fig. 3) The ilium is known for all phytosaur genera except Angistorhinus talainti (Hunt, 1994) and is one of the most primitive ilia of all archosaurs (Benton and Clark, 1988). Phytosaur ilia are characterized by a low and long, sub-horizontally oriented iliac blade, a horizontal acetabulum and a pubis that attaches to the antero-ventral face. The iliac blade is oriented slightly inwards (medially), as seen in articulated specimens of Angistorhinus (Lucas et al., 2002).

3 308 FIGURE 3. A, Left ilium of Leptosuchus crosbiensis (UCMP 27200). B, Left ilium of Pseudopalatus buceros (NMMNH P-58352). C, Right ilium of Redondasaurus gregorii (NMMNH P-25662). D, Left ilium of Nicrosaurus kapffi (BMNH 38063). Parrish (1987) noted that the ilium of primitive archosaurs is characterized by a shallow, imperforate acetabulum and the lack of any anterior extension of the blade. In contrast, more derived archosaurs have a deeper acetabulum and an anterior extension of the blade. Phytosaurs have this more derived type of ilium (Fig. 3). All phytosaurs have an anterior extension of the iliac blade As can be seen in Figure 3, the more derived phytosaurs (Leptosuchus, Pseudopalatus and Redondasaurus) have a dorso-ventrally compressed ilium, whereas Nicrosaurus has a non-compressed ilium. The ratio of the length of the iliac blade to the width of the ilium is about 1.85:1 in Leptosuchus, Redondasaurus, Pseudopalatus and Angistorhinus. In contrast the ratio in Nicrosaurus is 1.5:1. The ratio is smaller in Erythrosuchus (1.3:1) and Stagonolepis (1.4:1). The ilium of Parasuchus, the most primitive among phytosaurs (Chatterjee, 1978), is characterized by a shorter iliac blade than in all other phytosaurs and no dorso-ventral compression. Indeed, it looks very similar to the ilium of the aetosaur Stagonolepis. The ilium of Angistorhinus, much like Parasuchus, is not dorso-ventrally compressed, but the iliac posterior process is elongated and the tip of the process is ventrally oriented. In Parasuchus, this tip has a dorsal orientation (Chatterjee, 1978). Both genera have a highly-developed acetabular crest that covers half of the acetabulum and shows an elongated anterior process. The ilia of Leptosuchus (Fig. 3A) and Rutiodon (McGregor, 1906) are very similar. Both are dorso-ventrally compressed, and have a slightly convex dorsal margin. The blade is posteriorly elongated and has a short anterior process. Compared with Leptosuchus, the ilium of Pseudopalatus (Fig. 3B) is dorso-ventrally compressed and has a flat ankle between the pubis and the ischium. The ilium of Redondasaurus (Fig. 3C) is similar to that of Pseudopalatus, but in contrast the iliac blade is more robust and has a shorter anterior process. As described by Brusatte et al. (2010), there are three different forms of the dorsal margin of the ilium. The first one is a straight margin, which can be found in Pseudopalatus and Redondasaurus. A convex dorsal margin is known in Angistorhinus, Rutiodon, Leptosuchus and Nicrosaurus, whereas Paleorhinus has a concave and saddle shaped one. In terrestrial archosaurs and archosauriforms the ilium tends to have a straight or convex dorsal margin. Nicrosaurus (Fig. 3D) has the most distinctive ilium of all phytosaurs. Like all derived phytosaurs it is characterized by a posteriorly elongated blade and a short anterior process, but in contrast to the ilia of other phytosaurs it is dorso-ventrally elongated and more closely resembles the ilia of the erythrosuchian Erythrosuchus africanus (Romer, 1923). The angle between the pubis and ischium is steeper than that of Leptosuchus. Due to dorso-ventral elongation the acetabulum is longer than in the other phytosaurs (Fig. 3) and the posterior blade is more massive than in Leptosuchus or Pseudopalatus. The acetabular crest is well developed, but not as strong as in rauisuchians, e.g., Batrachotomus kupferzellensis. Femur (Fig. 4) Parrish (1987) notes that the femur in erect archosaurs developed a medially-directed head, lost the adductor fossa, developed the fourth trochanter on the retractor surface of the femur and developed femoral condyles. This is true for the more derived archosaurs, but basal archosauromorphs such as Proterosuchus did not have these developments and were still able to walk in an erect gait. All formerly analyzed phytosaur femora - Leptosuchus, Rutiodon, Mystriosuchus and Paleorhinus (Parrish, 1986a; Brusatte et al., 2010) are femora with the typical s-shape. This s-shape is also found in extant crocodilians and is common in sprawling and semi-erect animals. More elongated femora indicate an erect gait. For taxa with femoral heads, decreased torsion of the femur correlates with a tendency towards erect gait (Parrish, 1986a). Parrish (1986a) suggested a sprawling or semi-erect locomotion for all phytosaurs because the torsion of the femoral head to the shaft is approximately 70 degrees. By reconsidering the torsion of the femur in phytosaurs a similar result to Brusatte et al.

4 309 FIGURE 4. A, Left femur of Leptosuchus crosbiensis (UCMP 27200) in lateral and medial view. B, Left femur of Pseudopalatus buceros (NMMNH P-56268) in lateral and medial view. C, Right femur of Redondasaurus gregorii (NMMNH P-25665) in lateral and medial view. D, Right femur of Nicrosaurus kapffi (BMNH 38054) in lateral and medial view. Abbreviations: cr, crest; faa, facies articularis antotrochanterica; ftd, fourth trochanter depression; lc, lateral condyle; mc, medial condyle; ve, ventral emargination.

5 310 FIGURE 5. A, Left humerus of Paleorhinus arenaceus (ZPAL AbIII 1516). B, Right humerus of Leptosuchus (UCMP 27094). C, Left humerus of Nicrosaurus kapffi (SMNS unnumbered). D, Left humerus of Pseudopalatus buceros (NMMNH P-20852). E, Left humerus of Mystriosuchus planirostris (SMNS 10260). (2010, character 133) was achieved and suggests that it is less than 45 degrees. The femur of Mystriosuchus planirostris has a thicker shaft and its distal end is more strongly curved than other phytosaur femora. It also demonstrates less torsion. A similar femur is known in the marine crocodile Metriorhynchus. The fibular condyle on phytosaur femora is not present, whereas all more derived members of the Crurotarsi possess it. Terrestrial animals of similar size, i.e., Proterosuchus and Erythrosuchus, are also missing the fibular condyle. This makes it a possible systematic tool, but not a necessity for an erect gait. A very small fourth trochanter can be seen in Paleorhinus, Redondasaurus and Nicrosaurus, but it is not as well developed as in more derived archosaurs. Like the ilium, the femur is known for all phytosaur genera except Angistorhinus, and the femur shows the highest degree of variation of all the postcranial bones of phytosaurs. The femur of Pseudopalatus (Fig. 3B) is very similar to that of Crocodylus niloticus. The most significant difference is a longer crest and a stronger development of the lateral condyle and medial condyle in Crocodylus. Considering the similarity between the two animals and the sediments in which Pseudopalatus is found, a similar lifestyle can be assumed. The facies articularis antotrochanterica shows strong variation in the different phytosaur taxa. The femur of Leptosuchus (Fig. 3A) is characterized by a high and nearly pointed shape and a short tip, whereas Nicrosaurus and Redondasaurus (Fig. 3C) have a high, rounded shape and a short tip. Pseudopalatus has a rounded, low facies articularis antotrochanterica and a long tip. The crest is more strongly developed in Nicrosaurus and Redondasaurus than in the other genera, as are the medial and lateral condyles. In contrast to Leptosuchus and Pseudopalatus, the femora of Redondasaurus and Nicrosaurus (Fig. 3D) show a straighter overall shape. This is not due to deformation, as there is no evidence of it on the specimens. This shape is reminiscent of that of basal terrestrial archosaurs (e.g. Nesbitt et al., 2009). Next to this the torsion in Nicrosaurus was reduced, which is an indication of animals with an erect gait (Parrish, 1986a, 1987). The femur of Mystriosuchus (Gozzi and Renesto, 2003) is very curved, even more so than in Pseudopalatus. The crest is very short, and the facies articularis antotrochanterica is flat and has a long tip (Gozzi and Renesto, 2003). DISCUSSION All derived phytosaurs have an ilium with an elongated blade and a short anterior process, but three basic forms characterize the derived phytosaur ilium. The first one, represented by Pseudopalatus and Redondasaurus, is dorso-ventrally compressed and shows a flat angle between the pubis and the ischium. Leptosuchus and Rutiodon show an intermediate form. Nicrosaurus shows similarities to terrestrial archosaurs, which is less compressed than others and is able to keep the femur closer to the body. Overall, the diversity of form in derived phytosaurs suggests that they adapted better to the lifestyle of the animal and that there must have been differences in the different genera. Three different types of femora characterize phytosaurs: a crocodile-like femur, a much straighter more basal dinosaurian or rauisuchianlike femur and the extremely curved femur of Mystriosuchus. The straighter femur is found in Nicrosaurus and Redondasaurus. This means that the ilium and femur of Nicrosaurus more resemble those of terrestrial archosaurs than those of semi-aquatic archosaurs such as extant crocodilians. In general, the hind-leg morphology gives clues to the lifestyle of animals and suggests different lifestyles in the different genera. Straight shapes of femora and ilia similar to phytosaur ilia are known in aetosaurs and rauisuchians (Long and Murry, 1995; Gower and Schoch, 2009). Both of these animals were terrestrial, and the similarity of postcrania supports a secondary terrestrial lifestyle of Nicrosaurus and Redondasaurus. The different shape of the Mystriosuchus femur from that of all other phytosaurs speaks in favor of a different lifestyle. In addition to the postcranial anatomy, several other arguments

6 speak in favor of a secondary terrestrial lifestyle in Nicrosaurus. The tripartite dentition of Nicrosaurus in combination with the massive skull suggests predation on larger prey: The morphology of the posterior maxillary dentition of Nicrosaurus kapffi is unique among carnivorous archosaurs. It may well indicate a unique mode of food processing among archosaurs, and can be interpreted as an excellent tool for dismembering large prey items (Hungerbühler, 2000, p. 46). As terrestrial vertebrates such as Sellosaurus mainly represent larger prey in the Stubensandstein, Nicrosaurus would have had to wait for the prey as living crocodilians do. This is difficult to explain as the presence of a second definitely semiaquatic to aquatic phytosaur in Mystriosuchus planirostris makes it improbable that two phytosaurs lived in the same environment. If Hungerbühler s taxonomy is considered it would even indicate that four phytosaurs lived in the same environment at the same time (Hungerbühler and Hunt, 2000; Hungerbühler, 2002). SUMMARY Research on phytosaur ecology and locomotion is still in its early stages and it will take a large amount of research on existing and new material to completely understand these animals. Whereas the ecology of the early (Carnian to early Norian) phytosaurs has been analyzed in the past or is being analyzed at the moment, the more derived genera have not had a lot of attention apart from their systematic position. Further investigation including in-depth functional morphology analyses of phytosaurs have to be made to understand the movement of the different species. Also, muscle reconstructions as far as possible will help to understand how far the femur and humerus were flexible to switch into high gait and to what extent a gallop movement might be possible in these animals. Reconstructing the ecology of phytosaurs is complicated, because postcranial material is rarely preserved with the skulls on which the taxonomy is based. Due to this, the material that can be used for postcranial studies is restricted and not always well preserved. 311 The fore- and hind limbs of phytosaurs show the highest adaptation to their environment and speak in favor of three different lifestyles in phytosaurs. The first one is a relatively generalist, semi-aquatic lifestyle that is known from Parasuchus, Paleorhinus, Pseudopalatus and Leptosuchus. The limbs of these four genera resemble those of extant members of the genus Crocodylus and support a similar, semi-aquatic lifestyle. A terrestrial lifestyle is definitely supported in Nicrosaurus and is partly supported for Redondasaurus. Due to the phylogenetic relationships of both genera this secondary terrestrial lifestyle must have developed somewhere between the Late Carnian and Early Norian. This may be due to the aquatic niche having been occupied by a better-developed predator; in the case of Nicrosaurus this could be Mystriosuchus. Overall, the similarities between the ilia and femora of Nicrosaurus and those of other archosaurs represent a less primitive condition than in other phytosaurs. The similarities also suggest a non-sprawling walking style and an erect walking style that has mostly been associated with a terrestrial lifestyle. In contrast to Nicrosaurus and Redondasaurus, the limbs of Mystriosuchus are evidence of a more aquatic lifestyle. This is indicated by a more s-like femur and extremely widened proximal and distal ends compared to the other phytosaurs. As documented by Kimmig and Arp (2010), Mystriosuchus remains are always found in marine or playa lake deposits. This and the fact that the remains are always found far away from the coast also support a fully aquatic lifestyle. As all close phytosaur relatives represent terrestrial animals, a secondary terrestrial lifestyle would certainly represent reversals of the semi-aquatic lifestyle back to a terrestrial lifestyle, whereas the more aquatic lifestyle of Mystriosuchus would represent a more derived state. ACKNOWLEDGMENTS I thank Justin Spielmann and Spencer G. Lucas for their reviews and comments. Sandra Chapman (BMNH), Rainer Schoch (SMNS), Pat Holroyd (UCMP), Daniel Brinkman (YPM), Robert Bronowicz (Warsaw University), Justin Spielmann (NMMNH) and William G. Parker (PEFO) are thanked for access to their collections. 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