Araripesuchus wegeneri

Restoration of adult A. wegeneri.

Araripesuchus is an extinct genus of notosuchian crocodyliform that lived in Gondwana during the Cretaceous Period. The genus includes at least five species found throughout Western Africa and South America. The relationship of the individual species within this genus is hotly debated, specifically, the placement of Araripesuchus wegeneri.^[1][2][3][4] A. wegeneri is a small bodied crocodyliform characterized by relatively elongate limbs, an upright gait, and marked heterodonty.^[3] The tooth morphology, when combined with tooth wear and orientation, suggests that this animal may have been an omnivore or herbivore. Additionally, the gracile limbs and their orientation imply that this was a terrestrial animal. These features demonstrate that Araripesuchus wegeneri stands in stark contrast to its modern day relatives, crocodiles and alligators.

Geologic setting

Block containing multiple specimens of A. wegeneri

Araripesuchus wegeneri was originally recovered from northeastern Niger in 1966 from the Gadoufaoua locality.^[5] Sereno and Larsson (2009) described additional skeletal material from the Elrhaz Formation of Niger, located along the western margin of the Ténéré Desert. The Elrhaz Formation, part of the larger Tegma Group, is Lower Cretaceous (Aptian-Albian) in age and is approximately 110 million years old.^[3] The Elrhaz Formation is composed of primarily cross-bedded fluvial sandstones and has yielded a diverse vertebrate fossil assemblage, including theropods, sauropods, ornithischians, and crocodyliforms.^[6]

Description

Diagnosis

Araripesuchus wegeneri is a small-bodied (< 1.0 m) crocodyliform characterized by a distinct heterodont dentition and an upright quadrupedial posture. It was originally published on in 1979 and formally described from a partial skull and lower jaw two years later.^[5][7] Sereno and Larsson (2009) revised this diagnosis with additional material, including a complete skull. Many of the unique morphological features, or autapomorphies, of Araripesuchus wegeneri are found on the cranium. For example, A. wegeneri is characterized by a hole, or foramen, in the premaxillary bone in front of the first premaxillary tooth and a small fossa on the jugal, though many other autapomorphies exist.^[3]

Skull

Araripesuchus wegeneri is distinguishable by its striking skull shape. The snout is highly reduced relative to living crocodiles and accounts for less than half the total skull length. The snout is also narrow, less than 5 cm at its widest margin. The back portion of the skull is expanded and characterized by large orbits and fenestrae. The nares are located on the front-most portion of the skull. The premaxilla is mostly smooth, which stands in stark contrast to the other portions of the skull, and contains five teeth. Other skull bones are distinctly textured on their outside surfaces, including the maxillae, frontal, and parietal. Many of the cranial sutures are interdigitated, though to a lesser degree than A. tsangatsangana.^[3] There is a distinct crest present on the frontal, which is located between the orbits. Lastly, the squamosal is triradiate when viewed dorsally and possess a distinct posterior process. This process is longer and oriented more dorsally than other species of Araripesuchus.^[3] Unfortunately, the lower jaw of A. wegeneri is less well known, with only the dentary being well-preserved. The teeth of A. wegeneri are unique and deserve special note. There are five premaxillary, 14 maxillary, and 16 dentary teeth. Sereno and Larsson (2009) assign three distinct functional types for these teeth: incisiforms, caniniforms, and postcaniniforms. Incisiform teeth have a bulbous base with an asymmetrical, recurved crown. The teeth also possess fine denticles. The crowns are often wrinkled and rounded. Caniniform teeth are distinguished by their size; they are larger than the surrounding teeth, but are otherwise similar in shape to incisiform teeth. Postcaniniforms are found behind the caniniform teeth and have a diverse range in shapes; they can be asymmetric, conical, or leaf-shaped. All premaxillary teeth are incisiform in shape and are approximately the same size. Maxillary teeth are more diverse. There are two incisiform, one caniniform, and 11 postcaniniform teeth, many of which display distinct patterns of wear. Dentary teeth follow the same basic pattern as maxillary teeth, with representatives of all forms being present. A. wegeneri dentition is unmistakably heterodont, possessing more than one morphology, and is therefore more similar to mammals than living crocodiles and their relatives. Additionally, the shape of many of these teeth is similar to prosauropod dinosaurs and living iguanas. This distinct tooth morphology and evidence for precise tooth to tooth occlusion suggests an omnivorous or herbivorous lifestyle.^[3] Unfortunately, tooth wear studies on these specimens have not been conducted and, therefore, have not confirmed this interpretation.

Axial skeleton

Little has been published on the axial skeleton of A. wegeneri, as much of this skeletal material is obscured by osteoderms, covered by matrix, or missing. The preserved centra are amphicoelous, meaning both ends of the centra are concave.^[3] Additionally, much of the rear portion of the body was covered by two paired rows of bony dermal armor, which likely protected the animal in life.

Appendicular skeleton

The appendicular skeleton of A. wegeneri is well exposed. The long bones (humerus, radius, ulna, femur, tibia, and fibula) are straight and relatively gracile, which stands in contrast to living crocodiles and their relatives. This species is also characterized by elongate carpals (wrist bones) relative to the metacarpals (hand bones). These skeletal features suggest that this animal had an upright, quadrupedal posture, more similar to living mammals than to living crocodiles. This posture, in addition to many skeletal features, indicates that Araripesuchus wegeneri likely lived in a terrestrial environment.

Phylogeny

The phylogenetic position (placement on the evolutionary tree) of Araripesuchus wegeneri has been a subject of much debate. It was original referred to the genus Araripesuchus based on many of the skeletal features discussed previously,^[5][7] but Ortega et al. (2000) called into question this phylogenetic placement. Ortega et al. (2000) placed the genus Araripesuchus outside of the notosuchian clade as a sister group of Neosuchia. Additionally, they assert that A. wegeneri does not belong in the Araripesuchus clade due to the presence of a fifth premaxillary tooth, denticles on the tooth margins, and a number of other cranial characteristics. Later, Turner (2006) found that the genus Araripesuchus, including A. wegeneri, does form a monophyletic clade. Turner also found that the Araripesuchus genus was more closely related to neosuchians than to notosuchians, though weakly so. The addition of new, well-preserved A. wegeneri material allowed for a more accurate assessment of phylogenetic placement. Sereno and Larsson (2009) found that the genus Araripesuchus was part of the notosuchian clade, contradicting many previous studies. They also found that the genus Araripesuchus was not monophyletic, but paraphyletic. These conclusions are supported by the phylogenetic analysis of Turner and Sertich (2010). This paraphyletic arrangement is attributed to the weak support of the basal nodes within notosuchians.^[8] The placement of this genus continues to fluctuate as new notosuchian skeletal material is discovered. Recently the well-preserved skull of Uruguaysuchus allowed for a new phylogenetic assessment to be made. In this 2011 study the authors found that the Araripesuchus genus formed a clade within the Uruguaysuchidae^[9] a position that was upheld recently.^[4] As new material is discovered the phylogenetic position of this clade and its monophyletic placement will likely continue to fluctuate.

Biogeography

From the initial discovery, paleontologists recognized the importance of the Araripesuchus clade for the field of biogeography. The presence of this genus on Africa and South America demonstrated that these two continents had not yet fully separated by the Aptian (approximately 125-113 million years ago).^[5] Additionally, this taxon was used to test the effect of vicariance on crocodyliforms during the break up of Gondwana. Turner (2004)^[10] found that crocodyliform diversity, including the Araripesuchus clade, is driven by the separation of the continents during the Cretaceous Period.

Paleobiology

Diet

The unique dental morphology has led scientists to believe that Araripesuchus wegeneri was either an herbivore or an omnivore.^[3][11] The complex, heterodont dentition is the primary reason for this assertion. Many of the teeth are leaf-shaped and have distinct wear facets, which indicate direct tooth-to-tooth occlusion.^[3] Additionally, the teeth possess wrinkles and additional textures not commonly seen in obligate carnivores. Osi (2013)^[11] points out that A. wegeneri does not possess any dental features that unquestionably demonstrate herbivory, but the tooth morphology suggests a diet that included more plants than other members of the genus. In fact, the morphological diversity seen in this clade may be due to separate dietary niches.^[3]

Posture

Araripesuchus wegeneri was likely a terrestrial animal given a suite of morphological features. The straight shaft of the long bones indicates an upright posture, as opposed to living crocodiles, which are characterized by a sprawled posture and a more bowed humerus. Both the wrist and hand bones (radiale, ulnare, and metacarpals) and leg bones are lengthened, demonstrating the potential for substantial speeds on land.^[3]

Ontogeny

There is evidence that proportions of Araripesuchus wegeneri changed through ontogeny, or the development of an organism through its life. Sereno and Larsson (2009) found that the limbs of juvenile individuals are relatively longer than those seen in adults. For example, a juvenile’s (60% adult size) hind limb is 98% the length of its trunk (dorsosacral column), whereas an adults is only 75%. Therefore, the relative length of the appendages decreases through life.

References

1. Ortega F., Z. Gasparini, A. D. Buscalioni, and J. O. Calvo. 2000. A new species of Araripesuchus (Crocodylomorpha, Mesoeucrocodylia) from the Lower Cretaceous of Patagonia (Argentina). Journal of Vertebrate Paleontology 20:57–76.
2. Turner, A. H. 2006. Osteology and phylogeny of a new species of Araripesuchus (Crocodyliformes: Mesoeucrocodylia) from the Late Cretaceous of Madagascar. Historical Biology 18:255–369.
3. Sereno, P. C., and H. C. E. Larsson. 2009. "Cretaceous Crocodyliforms from the Sahara". ZooKeys 28:1–144.
4. Pol, D. Nascimento P. M., Carvalho A. B., Riccomini C., Pires-Domingues R. A., and H. Zaher. 2014. "A New Notosuchian from the Late Cretaceous of Brazil and the Phylogeny of Advanced Notosuchians". PLoS ONE 9(4): e93105. doi:10.1371/journal. pone.0093105
5. Buffetaut, E., and P. Taquet. 1979. "An early Cretaceous terrestrial crocodilian and the opening of the South Atlantic". Nature 280:486–487.
6. Sereno P. C., and S. L. Brusatte. 2008. Basal abelisaurid and carcharodontosaurid theropods from the Lower Cretaceous Elrhaz Formation of Niger. Acta Palaeontologica Polonica 53:15–46.
7. Buffetaut, E. 1981. "Die biogeographische Geschichte der Krokodilier, mit Beschreibung einerneuen Art, Araripesuchus wegeneri". Geologische Rundschau 70:611–624.
8. Turner A. H., J. J. W. Sertich. 2010. Phylogenetic history of Simosuchus clarki (Crocodyliformes: Notosuchia) from the Late Cretaceous of Madagascar. In: Krause D. W., Kley N. J., editors. Simosuchus clarki (Crocodyliformes: Notosuchia) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 30(Suppl. 1):177–236.
9. Soto, M., Pol, D., and D. Perea. 2011. A new specimen of Uruguaysuchus aznarezi (Crocodyliformes: Notosuchia) from the middle Cretaceous of Uruguay and its phylogenetic relationships. Zoological Journal of the Linnean Society 163:S173–S198.
10. Turner, A. H. 2004. Crocodyliform biogeography during the Cretaceous: evidence of Gondwanan vicariance from biogeographical analysis. Proceedings of the Royal Society of London B. 271:2003-2009.
11. Ősi, A. 2013. "The evolution of jaw mechanism and dental function in heterodont crocodyliforms". Historical Biology DOI: 10.1080/08912963.2013.777533.