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Trialestes rises again!!

Tuesday, October 4, 2016 18:02
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Lecuona et al. 2016
redescribe in greater detail Trialestes (Reig 1963; Figs. 1, 2), more than 50 years after its original discovery and publication. Glad to see this! Data used to nest Trialestes in the LRT as the proximal outgroup for the Dinosauria consisted of a few old drawings (Fig. 1), nothing more. The new data do not move the nesting much.

Figure 1. Tracings from old drawings are the data used to create this reconstruction of Trialestes, which nested it basal to the Dinosauria.

Figure 1. Tracings from old drawings are the data used to create this reconstruction of Trialestes, which nested it basal to the Dinosauria. New data from Lecuona et al. 2016 greatly reduce the guesswork here. 

From the Lecuona et al. abstract:
“Here, we describe in detail all the material assignable to the species and test its phylogenetic relationships using a comprehensive data matrix focused on early archosaurs. We support the referral of PVL 3889 to Trialestes and reject the presence of a mesotarsal ankle joint in this specimen. We recovered Trialestes within Crocodylomorpha, closer to Crocodyliformes than Pseudhesperosuchus, Hesperosuchus, Dromicosuchus and Sphenosuchus. Therefore, Trialestes represents the most completely known of the earliest non-crocodyliform crocodylomorph taxa known to date.”

They report, “In contrast, Reig’s third taxon, ‘Triassolestes’ romeri has received relatively little attention. In his original publication, Reig (1963) interpreted this taxon as a theropod dinosaur of the family Podokesauridae, a group now considered roughly equivalent to Coelophysoidea (Holtz 1994). Because of this interpretation, Reig considered only part of PVL 2561 as the holotype of the dinosaur ‘Triassolestes’, including an incomplete cranium and mandible, four cervical vertebrae, and 16 caudal vertebrae. Other postcranial remains associated with PVL 2561 were excluded from this genus, including a scapula, humerus, ulna, radius, carpus and proximal metacarpus, which were interpreted by Reig (1963, p. 15) as crocodilian elements because of the presence of an elongated radiale and ulnare.”

FIgure 2. Assembly of the many Trialestes parts featured in Lecuona et al. 2016. This is an odd combination of robust cervicals and gracile limbs and girdles.

FIgure 2. Assembly of the many Trialestes parts featured in Lecuona et al. 2016. This is an odd combination of robust cervicals and gracile limbs and girdles.

Those proximal wrist elements
(radiale and ulnare) are never elongated in dinosaurs, but they are elongated in dinosaur ancestors like Trialestes. The name ‘Triassolestes’ was preoccupied by an Australian Triassic dragonfly, hence the change we know today.

Lecuona et al. continue: “Clark (in Benton & Clark 1988, fig. 8.6) hypothesized Trialestes as the sister taxon of Crocodylomorpha, but expressed some caution given the structure of the ankle and the poor knowledge of the specimens.”

“Novas (1989) recognized that the referred specimen PVL 2559 contained elements belonging to two individuals of different sizes (Reig 1963, fig. 4b part.) and assigned both of them to Herrerasauridae indet. (Novas 1989, 1993); presently, they can only be assigned to an indeterminate saurischian dinosaur.”

“Clark et al. (2000) questioned the referral of PVL 3889 to Trialestes romeri, suggesting that this specimen was more likely assignable to a basal dinosaur. Nevertheless, these authors could not reject the alternative explanation that PVL 2561 and PVL 3889 belong to one taxon with a combination of crocodylomorph and dinosaurian character states.”

“In addition, Trialestes has not been included in a quantitative phylogenetic analysis so far and, thus its affinities remain untested using modern methodologies.”

Well, the large reptile tree did that several years ago. But let’s keep an open mind moving forward.

Under materials and methods, Lecuona et al report,
“Herein we study the two specimens assigned to Trialestes romeri, the holotype PVL 2561 and PVL 3889; we exclude PVL 2559 because we agree with previous authors that it represents an indeterminate saurischian.”

Lecuona et al. based their analysis on a cladogram originated by Nesbitt (2011) which had several problems listed here. They also included data from Butler et al. (2014) and added Carnufex, a related taxon.

The Trialestes fossils have a pristine beauty. The authors did not create a reconstruction, so I attempt one here (Fig. 2).

A little background data
Trialestes romeri (Bonaparte 1982)= Triassolestes (Reig, 1963/Tillyard, 1918) Carnian, Late Triassic ~235 mya is known from scattered parts. Clark, Sues and Berman (2000) redescribed the known parts and admitted the possibility that this taxon combined dinosaurian and crocodylomorph characters. As it nests here, Trialestes was derived from a sister to Carnufex. This clade phylogenetically preceded Herrerasaurus and the Dinosauria. We looked at this heretical relationship several years ago here.

Distinct from Pseudhesperosuchus,
the skull of Trialestes had a larger antorbital fenestra and a deeper rostrum. The mandibular fenestrae (yes, there are two!) were smaller.

The vertebral centra had excavated lateral surfaces, for bird-like air sacs. The radius was longer than the humerus, a character otherwise known only in dinosaurs. The radiale was smaller than the ulnare, matching the radius and ulna. The fingers were tiny.

The pelvis was semi-perforated, as in basal dinosaurs, with a well-developed supraacetabular crest. The dorsal pelvis was straight, as in Gracilisuchus. The femoral head was not inturned, suggesting a variable posture, promoted by that really long forearm. The ankle joint had a crocodile normal configuration and a functionally pentadactyl pes. Most crocs lose pedal digit 5, but not those basal to dinos, like PVL4597.

Bonaparte JF 1982. Classification of the Thecodontia. Geobios Mem. Spec. 6, 99-112
Clark JM, Sues H-D and Berman DS 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20(4):683-704.
deFranca MAG, Bittencourt JdS and Langer MC 2013. Reavaliação taxonomica de Barberenasuchus brasiliensis (Archosauriformes), Ladiniado do Rio Grande do Sul (Zona-Assembleia de Dinodontosaurus). Palaenotogia em Destaque Edição Especial Octubro 2013: 230.|
Irmis RB, Nesbitt SJ and Sues H-D 2013. Early Crocodylomorpha. Pp. 275–302 in Nesbitt, Desojo and Irmis (eds). Anatomy, phylogeny and palaeobiology of early archosaurs and their kin. The Geological Society of London. doi:10.1144/SP379.24.
Kischlat EE 2000. Tecodôncios: a aurora dos arcossáurios no Triássico. Pp. 273–316 in Holz and De Ros (eds.). Paleontologia do Rio Grande do Sul. Porto Alegre: CIGO/UFRGS.
Lecuona A, Ezcurra MD and Irmis RB 2016. Revision of the early crocodylomorph Trialestes romeri (Archosauria, Suchia) from the lower Upper Triassic Ischigualasto Formation of Argentina: one of the oldest-known crocodylomorphs. Papers in Palaeontology (advance online publication). DOI: 10.1002/spp2.1056
Nesbitt S 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Reig, OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.

Riff D et al. 2012. Crocodilomorfos: a maior diversidade de répteis fósseis do Brasil. TERRÆ 9: 12-40, 2012.

‘Origin of Dinosaurs’ video. 



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