Too Kewl Not to Share: Dr Bill Parker's Aetosaurs!

Revised phylogenetic analysis of the Aetosauria (Archosauria: Pseudosuchia); assessing the effects of incongruent morphological character sets

Author:

Parker

Abstract:

Aetosauria is an early-diverging clade of pseudosuchians (crocodile-line archosaurs) that had a global distribution and high species diversity as a key component of various Late Triassic terrestrial faunas. It is one of only two Late Triassic clades of large herbivorous archosaurs, and thus served a critical ecological role. Nonetheless, aetosaur phylogenetic relationships are still poorly understood, owing to an overreliance on osteoderm characters, which are often poorly constructed and suspected to be highly homoplastic. A new phylogenetic analysis of the Aetosauria, comprising 27 taxa and 83 characters, includes more than 40 new characters that focus on better sampling the cranial and endoskeletal regions, and represents the most comprenhensive phylogeny of the clade to date. Parsimony analysis recovered three most parsimonious trees; the strict consensus of these trees finds an Aetosauria that is divided into two main clades: Desmatosuchia, which includes the Desmatosuchinae and the Stagonolepidinae, and Aetosaurinae, which includes the Typothoracinae. As defined Desmatosuchinae now contains Neoaetosauroides engaeus and several taxa that were previously referred to the genus Stagonolepis, and a new clade, Desmatosuchini, is erected for taxa more closely related to Desmatosuchus. Overall support for some clades is still weak, and Partitioned Bremer Support (PBS) is applied for the first time to a strictly morphological dataset demonstrating that this weak support is in part because of conflict in the phylogenetic signals of cranial versus postcranial characters. PBS helps identify homoplasy among characters from various body regions, presumably the result of convergent evolution within discrete anatomical modules. It is likely that at least some of this character conflict results from different body regions evolving at different rates, which may have been under different selective pressures.

Dr Parker has his own post over at his own blog, Chinleana.

Gorgetosuchus pekinensis: a new Basal Desmatosuchine Aetosaur From Norian Triassic North Carolina

A new aetosaur (Archosauria, Suchia) from the Upper Triassic Pekin Formation, Deep River Basin, North Carolina, U.S.A., and its implications for early aetosaur evolution

Authors:

Heckert et al

Abstract:

Aetosaurs are an extinct clade of quadrupedal, heavily armored archosaurs that had a worldwide distribution during the Late Triassic. Aetosaur fossils from the Upper Triassic Pekin Formation in the Deep River Basin of North Carolina (U.S.A.) consist primarily of isolated osteoderms and, rarely, more associated material. Here we describe a new genus and species, Gorgetosuchus pekinensis, based on an associated incomplete anterior carapace, consisting of a total of 19 nearly complete paramedian and lateral osteoderms from the first 10 rows of armor as well as some associated fragments. An important feature of Gorgetosuchus is that an articulated fifth row of cervical osteoderms almost encloses the neck, with prominent spines on both the dorsal and lateral osteoderms. This is a novel configuration among aetosaurs. Otherwise, NCSM 21723 preserves a mosaic of character states found in Longosuchus, Lucasuchus, or both taxa while simultaneously preserving several more plesiomorphic character states, such as cervical osteoderms that are wider than long. Our reevaluation of other Pekin Formation specimens that various authors have assigned to Desmatosuchus, Longosuchus, or Lucasuchus confirms that some possess characteristics of Lucasuchus, whereas others are not generically determinate. Incorporating Gorgetosuchus into existing phylogenies of aetosaurs results in a reshuffling of basal aetosaur relationships, but a variety of analyses consistently place Gorgetosuchus as a basal desmatosuchine. Using current taxonomic practices, there are at least three aetosaur genera in the Pekin Formation: Lucasuchus, Coahomasuchus, and Gorgetosuchus.

Sexual Dimorphism Discovered in Upper Triassic Aetosaur Scagliali casamiquela?

INTRASPECIFIC VARIATION IN AETOSAUROIDES SCAGLIAI CASAMIQUELA (ARCHOSAURIA: AETOSAURIA) FROM THE UPPER TRIASSIC OF ARGENTINA AND BRAZIL: AN EXAMPLE OF SEXUAL DIMORPHISM?

Authors:

Taborda et al

Abstract:

Aetosaurs are a group of quadrupedal, armoured pseudosuchian archosaurs from the Upper Triassic. They are characterized by dorsal and ventral carapaces, and appendicular osteoderms, all of them ornamented. Aetosaurs have been proposed as index fossils based in large part on the distinctiveness of some osteoderms; therefore, it is important to understand the intraspecific variation of these elements in the clade. In the present contribution, we describe three types of ornamentation on the medial area of paramedian osteoderms in the dorsal armour of Aetosauroides scagliai: the “radial pattern” consists of radial grooves and ridges, with small pits inside the grooves; the “anastomosing pattern” is composed of anastomosing crests and proportionately larger, more irregular pits; and an “intermediate” or “transitional pattern” between the “radial” and “anastomosing patterns”. The articulated dorsal armour preserved in specimens PVL 2059 and PVL 2073 possess all three patterns of ornamentation, but they differ in the position of each throughout the carapace. Recent studies of histological thin-sections in osteoderms of A. scagliai show the absence of remodeling of osteoderm tissues, allowing the estimation of age by counting LAGs (lines of arrested growth). PVL 2073 is slightly longer (~10% longer centra) but also ontogenetically younger (5 LAGs) than PVL 2059 (10 LAGs; slightly wider osteoderms). Combining this information and comparing it with that of living crocodiles (where the male specimens are typically relatively larger than the females of the same age), we conclude that the intraspecific variation observed in A. scagliai is compatible with the hypothesis of sexual dimorphism. These results show the need to explore the sources of intraspecific variation in aetosaurs.

Polesinesuchus aurelioi: a new Aetosaur From Carnian Triassic Brazil

A new aetosaur from the Upper Triassic of the Santa Maria Formation, southern Brazil

Authors:

ROBERTO-DA-SILVA et al

Abstract:

Aetosaurs are armored pseudosuchian archosaurs widespread in Upper Triassic units. In South America, four taxa were previously recorded: Aetosauroides scagliai, Neoaetosauroides engaeus, Aetobarbakinoides brasiliensis, and Chilenosuchus forttae. Herein we describe a new Late Triassic juvenile aetosaur from the Santa Maria Formation of southern Brazil, Polesinesuchus aurelioi gen. et sp. nov., increasing the paleobiodiversity of this interesting group to five taxa in Western Gondwana. The holotype is composed of cranial (parietal and braincase) and postcranial elements (cervical, dorsal, sacral, caudal vertebrae, both scapulae, a humerus, ilium, pubis, ischium, tibia, a partial right pes, and anterior and mid-dorsal paramedian osteoderms). It belongs to a juvenile individual, as its neurocentral sutures are open in all vertebrae, and also due to its small size. However, future paleohistological investigation is necessary to fully corroborate this assumption. This new taxon is distinguished from all other aetosaurs by the presence of an unique combination of character states (not controlled by ontogeny) such as: cervical vertebrae with prezygapophyses widely extending laterally through most of the anterior edge of the diapophyses; absence of hyposphene articulations in both cervical and mid-dorsal vertebrae; presence of a ventral keel in cervical vertebrae; anterior and mid-dorsal vertebrae without a lateral fossa in their centra; expanded proximal end of scapula; anteroposteriorly expanded medial portion of scapular blade; a short humerus with a robust shaft; and a dorsoventral and very low iliac blade with a long anterior process which slightly exceeds the pubic peduncle. Regarding
its phylogenetic relationships, the present analysis placed Polesinesuchus as the sister taxon of Aetobarbakinoides and both as sister taxa of the unnamed monophyletic clade Desmatosuchinae plus Typothoracisinae.

Aetosaur Awesomeness!

Aetosauria: a clade of armoured pseudosuchians from the Upper Triassic continental beds

Authors:

1. Julia B. Desojo (a)
2. Andrew B. Heckert (b)
3. Jeffrey W. Martz (c)
4. William G. Parker (d,e)
5. Rainer R. Schoch (f)
6. Bryan J. Small (g)
7. Tomasz Sulej (h)

Affiliations:

a. CONICET, Comisión Nacional de Investigación Científica y Técnica, Sección Paleontología Vertebrados, Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Av. Ángel Gallardo 470, C1405DJR, Buenos Aires, Argentina

b. Department of Geology, Appalachian State University, ASU Box 32067, Boone, NC 28608-2067, USA

c. Department of Earth Sciences, Denver Museum of Nature and Science, 2001 Colorado Blvd, Denver, CO 80205, USA

d. Division of Resource Management, Petrified Forest National Park, PO Box 2217, Petrified Forest, AZ 86028, USA

e. Department of Geological Sciences, University of Texas, Austin, TX 78712, USA

f. Staatliches Museum für Naturkunde, Rosenstein 1, 70191 Stuttgart, Germany

g. Museum of Texas Tech University, 3301 4th St., Lubbock, TX 79409-3191, USA

h. Instytut Paleobiologii, PAN, Twarda 51/55, 00-818 Warszawa, Poland

Abstract:

Aetosauria is a clade of obligately quadrupedal, heavily armoured pseudosuchians known from Upper Triassic (late Carnian–Rhaetian) strata on every modern continent except Australia and Antarctica. As many as 22 genera and 26 species ranging from 1 to 6 m in length, and with a body mass ranging from less than 10 to more than 500 kg, are known. Aetosauroides scagliai was recently recovered as the most basal aetosaur, placed outside of Stagonolepididae (the last common ancestor of Desmatosuchus and Aetosaurus). Interrelationships among the basal aetosaurs are not well understood but two clades with relatively apomorphic armour – the spinose Desmatosuchinae and the generally wide-bodied Typothoracisinae – are consistently recognized. Paramedian and lateral osteoderms are often distinctive at the generic level but variation within the carapace is not well understood in many taxa, warranting caution in assigning isolated osteoderms to specific taxa. The aetosaur skull and dentition varies across taxa, and there is increasing evidence that at least some aetosaurs relied on invertebrates and/or small vertebrates as a food source. Histological evidence indicates that, after an initial period of rapid growth, lines of arrested growth (LAGs) are common and later growth was relatively slow. The common and widespread Late Triassic ichnogenus Brachychirotherium probably represents the track of an aetosaur.