Monday, 23 March 2020

Dynamosuchus collisensis: A new species of Ornithosuchid Pseudosuchian from the Late Triassic Santa Maria Formation of Rio Grande do Sul, Brazil.

The radiation of Pseudosuchian Archosaurs during the Triassic Period is characterized by the origin and extinction of several peculiar and disparate clades, such as Aetosauria, Erpetosuchidae, Gracilisuchidae, and Ornithosuchidae. The latter is one of the most enigmatic clades. Coined by Friedrich von Huene in 1908, the Ornithosuchidae have a long and controversial taxonomic history. The clade has been placed in different phylogenetic positions across the Archosauria. Nevertheless, most hypotheses have converged on a position near the base of Pseudosuchia. Three species form the clade: Ornithosuchus woodwardi, from the late Carnian–early Norian Lossiemouth Sandstone Formation of Scotland, Venaticosuchus rusconii, from the late Carnian Ischigualasto Formation of Argentina, and Riojasuchus tenuisceps, from the Norian Los Colorados Formation of Argentina. All members of this group are interpreted to have been carnivorous, putatively scavengers and facultatively bipedal during fast movement. Nevertheless, the fossil record of the group is geographically limited and so far, no remains outside Ischigualasto-Villa Union Basin pf Argentina or the Lossiemouth Sandstone Formation of Scotland have been identified, thus leaving a large gap in their potential biogeographic distribution.

In a paper published in the journal Acta Palaeontologica Polonica on 31 January 2020, Rodrigo Müller of the Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia at the Universidade Federal de Santa Maria, Maria Belén Von Baczko and Julia Desojo of the División Paleontología de Vertebrados at the Museo de La Plata, and Sterling Nesbitt of the Department of Geosciences at Virginia Tech describe the first partial skeleton of an Ornithosuchid from the Late Triassic sediments of Brazil and explore its phylogenetic affinities and implications for the evolution of the group.

(A) Location map of the Janner site and the surface distribution of the geologic units in the area. (B) Stratigraphic column of the Janner site depicting its fossiliferous content: (1) Exaeretodon; (2) Hyperodapedon; (3) Pampadromaeus; (4) Trucidocynodon; (5) Bagualosaurus; and (6) the new Ornithosuchid. Müller et al. (2020).

The specimen comes from the Santa Maria Formation at the the Janner site, which is located at the base of the Agudo Hill in the Paraná Basin of Rio Grande do Sul State, Brazil. The location is considered to be Carnian in age (237 to 227 million years old), based upon the presence of Hyperodapedon and Exaeretodon, which are also known from sites elsewhere in Brazil and Argentina with well constrained dates.

The specimen is a partial skeleton including the premaxillae, the left maxilla, both frontals, the right postfrontal, the right parietal, both squamosals, both quadratojugals, both quadrates, the parabasisphenoid, both hemimandibles, some cervical, dorsal, sacral and caudal vertebrae, several ribs, gastralia, and osteoderms, the left scapula, both forelimbs, the left ilium, the right pubis, left femur, the right tibia, and the left fibula. It is named Dynamosuchus collisensis, where 'Dynamosuchus' means 'power-Crocodile' and 'collisensis' means 'from the hill'.

Representative skeletal elements of the Ornithosuchid Archosaur Dynamosuchus collisensis (CAPPA/UFSM 0248) from Janner outcrop, Carnian, Late Triassic. (A) Selected skull bones in left lateral view. (B) Reconstruction of the skull. (C) Skull in ventral view. (D) Left quadrate and quadratojugal in posterodorsal view. (E) Parabasisphenoid in left lateral view. (F) Neural arch of an anterior cervical vertebra in anterior view. (G) Centrum of a cervical vertebra in left lateral view. (H) Right osteoderm in dorsal view. (I) Neural arch of an anterior dorsal vertebra in left lateral view. (J) Left ilium in lateral view. (K) Reconstruction of the skeleton of CAPPA/UFSM 0248 (preserved elements indicated in orange) (L) Right humerus in anterior view. (M) Right forearm in medial view. (N) Left manus in dorsal view. (O) Right (reversed) pubis in lateral view. (P) Left femur in anterior view. (Q) Left fibula in lateral view.  Scale bars 20 mm. Müller et al. (2020).

A phylogenetic analysis recovered 18 most parsimonious trees of 3356 steps each. In all the most parsimonious trees, Dynamosuchus collisensis nests within Ornithosuchidae. Ornithosuchus woodwardi lies as the basal-most member of Ornithosuchidae. The clade is supported by 19 synapomorphies, such as the alveolar margin of the premaxilla not reaching the contact with the maxilla, three premaxillary teeth, mandibular symphysis present along one-third of the lower jaw, three sacral vertebrae, a perforated acetabulum, and the presence of the anterior trochanter on the femur. Dynamosuchus collisensis was found as the sister taxon of Venaticosuchus rusconii and this is supported by the semilunar depression on the posterolateral surface of the parabasisphenoid and ascending process of the quadratojugal moderately anterodorsally to posteroventrally oriented in an angle higher than 40°. The clade including both taxa is the sister group of Riojasuchus tenuisceps. This arrangement relies in the strongly downturned main body of the premaxilla and the strongly anteroposteriorly expanded distal end of the radius. The Ornithosuchidae is found as the sister taxon of the Erpetosuchidae, whereas the clade comprising both these groups is found as the sister taxon of the Aetosauria.

Macroevolutionary patterns of Ornithosuchidae. (A) Time-calibrated reduced strict consensus tree depicting the phylogenetic position of Dynamosuchus collisensis. Numbers on nodes represent Bremer support values higher than one. Numbers associated with the Ornithosuchid branch represent the characters and states that support the clade. (B) Geographical distribution of Ornithosuchids across the time. (C) Life reconstruction of Dynamosuchus collisensis. Müller et al. (2020).

Ornithosuchids, first described at the end of the 19th century, were important for deciphering the relationships among basal Archosaurs because of their controversial anatomical features. However, no other Ornithosuchids have been found since Venaticosuchus rusconii was discovered in Argentina almost a half century ago. Furthermore, Dynamosuchus collisensis is the first unambiguous Ornithosuchid from Brazil.

The presence of Ornithosuchids in Upper Triassic beds from Brazil is not a surprise, as the group is recorded in coeval strata of Argentina (i.e. Venaticosuchus rusconii). The age of the strata that yielded Dynamosuchus collisensis is similar to that of the levels in which Ornithosuchus woodwardi from Scotland and Venaticosuchus rusconii from Argentina were found, as these strata have also yielded remains of the Rhynchosaur Hyperodapedon, which is extensively adopted as index fossil of Carnian age.The other known Ornithosuchid, Riojasuchus tenuisceps, was exhumed from the Los Colorados Formation, Argentina, which is Norian in age ( 227 to 208.5 million years old). Therefore, Dynamosuchus collisensis falls within the stratigraphic range of the oldest known Ornithosuchids.

The close relationship between Venaticosuchus rusconii and Dynamosuchus collisensis reinforces the previous proposed biostratigraphic hypotheses, indicating a similar faunal assemblage in both contemporaneous Paraná and Ischigualasto-Villa Unión basins. Moreover, the sister taxon affinity between Venaticosuchus rusconii and Dynamosuchus collisensis rejects a potential endemic radiation of ornithosuchids from the Ischigualasto-Villa Unión Basin and would better support multiple diversification events, at least at the Carnian. Indeed, the phylogenetic affinities of Proterochampsid Archosauriforms and Erpetosuchids would also suggest multiple dispersal events between the Ischigualasto-Villa Unión and the Paraná basins with the simultaneous appearance of several closely-related species in both basins (i.e. the Argentinean Proterochampsids Proterochampsa barrionuevoi and Pseudochampsa ischigualastensis, and the Brazilian Rhadinosuchus gracilis and Proterochampsa nodosa; the Brazilian Erpetosuchids Archeopelta arborensis and Pagosvenator candelariensis and the Argentinean Tarjadia ruthae). In addition, the presence of an Ornithosuchid in Brazil expands the longitudinal distribution of the group, suggesting that Ornithosuchids were more widespread than previously thought in the southern hemisphere. No records of the group have been found in lower latitudes close to the Paleo-Equator, however, age equivalent formations or localities are very scarce. Therefore, Ornithosuchids are still restricted to higher latitudes of northern and southern Pangea.

The discovery of Dynamosuchus collisensis in the Paraná Basin also provides novel paleoecological implications for this type locality (the Janner site). Strictly carnivorous Reptiles were previously absent from this site, as the early Sauropodomorph Pampadromaeus barberenai was more likely to be an omnivorous animal according to a recent ecomorphological analysis. On the other hand, Bagualosaurus agudoensis is more related to post-Carnian Sauropodomorphs, which are considered herbivorous or omnivorous. Therefore, strictly carnivorous animals are solely represented in the site through the record of Ecteniniid Cynodonts. Indeed, Ornithosuchids have been considered as top tier predators. However, recently the feeding behavior of these animals have been explored through more rigorous techniques (i.e., quantitative approaches), indicating that Ornithosuchids likely adopted scavenging feeding habits or preyed only on small Vertebrates. Such statement relies on the elevated bite force together with the low bite speed and the morphology of their constricted snouts. Therefore, the discovery of Dynamosuchus collisensis provides the first clue of a putative necrophagous vertebrate from the Janner site and expands our knowledge regarding the trophic chain of Late Triassic of Brazil. This is particularly interesting because necrophagous animals are an important part of the extant terrestrial ecosystems, interacting with the carcasses before its incorporation within the lithosphere. Therefore, identifying the animals that played such role in ancient environments is crucial for the reconstruction of reliable pictures of these. Indeed, the Janner site has yielded evidences of osteophagic behavior by Insects and the putative action of necrophagous animals that produced an accumulation with predominance of cranial elements.

Dynamosuchus collisensis is the first unambiguous member of Ornithosuchidae from Brazil, and was recovered as sister taxa of the Argentinean taxon Venaticosuchus rusconii. This sister taxon relationship would support multiple radiation events between Paraná and Ischigualasto-Villa Unión basins in agreement with previous hypothesis based on other Archosauriforms that are shared across these basins. The discovery of an Ornithosuchid in Carnian continental outcrops of Brazil provides a new shared faunal component between the basins. The Carnian Archosauromorph communities of Argentina and Brazil would therefore consist of Rhynchosaurs, Proterochampsids, Ornithosuchids, Aetosaurs, Rauisuchians, Erpetosuchids, Sauropodomorphs, and Theropods. In addition, the discovery of Dynamosuchus collisensis provides the first clue of a putative necrophagous Vertebrate from the oldest Dinosaur-bearing beds and expands our knowledge regarding the trophic structure of the Late Triassic of Brazil.

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