Introducing Stegouros elengassen

Life reconstruction of Stegouros elengassen. Image credit: Luis Perez Lopez

Ankylosauria is a clade of herbivorous, armored ornithischian dinosaurs subdivided in two major clades: the Ankylosauridae and the Nodosauridae. The most derived members of this group are characterized by shortened skulls, pyramidal squamosal horns, and tail clubs. Fossil evidence of armored dinosaurs from Gondwana is scarce. They were present primarily in Asia and North America. Stegouros elengassen, a new specimen from the Late Cretaceous Dorotea Formation of southern Chile, offers new evidence that contributes to the understanding of the relationships among the ankylosaurs from Gondwana.

Stegouros lived about 72 to 75 million years ago, and reached 2 meters in lenght (six feet). The generic name is derived from the the Greek word “stego” (roof ) and the Greek word “uros”
(tail) in reference to the covered tail. The specific name “elengassen” comes from an armoured
beast in the mythology of the Aónik’enk people.

Skeletal anatomy of the S. elengassen holotype. From Soto-Acuña et al., 2021.

The holotype (CPAP–3165), represented by a nearly complete skeleton, semi-articulated, was discovered in 2017 at the lower section of the Dorotea Formation. The new specimen exhibits an unusual combination of characters. The skull and teeth, are classically Ankylosauria, but the pelvis and slender limb bones resemble Stegosauria. The most striking feature of Stegouros is the short, bizarre tail covered by seven pairs of large osteoderms, that are fused into a flat composite unit resembling a battle axe.

Phylogenetic analyses with five different datasets indicate that Stegouros was closer to Ankylosauria than to Stegosauria. It was grouped with the basal ankylosaurs Kunbarrasaurus (from the late Lower Cretaceous of Australia) and Antarctopelta (from the Late Cretaceous of Antarctica) forming a monophyletic clade that split earliest from all other Ankylosauria. The study, led by Chilean paleontologists, proposes the clade Parankylosauriato include the first ancestor of Stegouros—but not Ankylosaurus—and all descendants of that ancestor (Soto-Acuña et al., 2021).

 

 

References:

Soto-Acuña, S., Vargas, A.O., Kaluza, J. et al. Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile. Nature (2021). https://doi.org/10.1038/s41586-021-04147-1

Forelimb posture in Chilesaurus diegosuarezi.

 

Chilesaurus holotype cast (MACN. From Wikipedia Commons. Author: Evelyn D’Esposito)

Chilesaurus diegosuarezi is a bizarre tetanuran from the Upper Jurassic of southern Chile. Holotype specimen (SNGM-1935) consists of a nearly complete, articulated skeleton, approximately 1.6 m long. Four other partial skeletons (specimens SNGM-1936, SNGM-1937, SNGM-1938, SNGM-1888) were collected in the lower beds of Toqui Formation. For a basal tetanuran, Chilesaurus possesses a number of surprisingly plesiomorphic traits on the hindlimbs, especially in the ankle and foot, which resemble basal sauropodomorphs.

All the preserved specimens of Chilesaurus show ventrally flexed arms with the hands oriented backwards, an arrangement that closely resembles the resting posture similar described in Mei long, Sinornithoides youngi, and Albinykus baatar. However, the hindlimbs of Chilesaurus are posteriorly extended, rather than ventrally flexed. So it seems that individuals of Chilesaurus were buried quickly and fossilized almost in life position during passive activity (e.g. feeding, resting).

Cast of SNGM-1937 specimen of Chilesaurus diegosuarezi in dorsal (1), 471 lateral (2), and anterolateral view (3).

Cast of SNGM-1937 specimen of Chilesaurus diegosuarezi in dorsal (1), 471 lateral (2), and anterolateral view (3). Scale: 20 mm.

The specimen SNGM-1937 shows an angular relation in the wrist that resembles that in Deinonychus. In fact, several coelurosaurs have the same resting position as the forelimbs of Chilesaurus, with the humerus and radius-ulna in perpendicular relation or elbow flexed in an acute angle, hands under the radius-ulna, and palmar surface posterodorsal and dorsomedial oriented with respect to the main body axis. The resting posture of the forelimbs has been studied in theropod species, in relation to the acquisition of flight. It was suggested that the presence of the forelimb folded structure in advanced theropods are related with soft structures, as patagial skin and muscles, present in several maniraptoran dinosaurs.

The cojoined flexion of wrist and elbow in living birds is mainly conducted by the action of a large number of tendons located within the propatagium. Although the existence of propatagium was considered as unique to modern birds, it have also been described for coelurosaurs and Pterosauria. The preserve of a flexed forearm in Chilesaurus, may be also regarded as an indirect indicative of the presence of propatagium in this taxon.

 

References:

Nicolás R. Chimento, Federico L. Agnolin, Fernando E. Novas, Martín D. Ezcurra, Leonardo Salgado, Marcelo P. Isasi, Manuel Suárez, Rita De La Cruz, David Rubilar-Rogers & Alexander O. Vargas (2017) Forelimb posture in Chilesaurus diegosuarezi (Dinosauria, Theropoda) and its behavioral and phylogenetic implications. Ameghiniana (advance online publication) doi: 10.5710/AMGH.11.06.2017.3088

Novas, F.E., Salgado, L., Suarez, M., Agnolín, F.L., Ezcurra, M.D., Chimento, N.R., de la Cruz, R., Isasi, M.P., Vargas, A.O., and Rubilar-Rogers, D. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature 522: 331-334. doi:10.1038/nature14307