Morrosaurus

Morrosaurus is an extinct genus of herbivorous elasmarian dinosaur that lived in the late Cretaceous in Antarctica. The only known species is the type Morrosaurus antarcticus.

Discovery
In 2002, the Argentine paleontologist Fernando Novas reported the discovery of a partial skeleton of a euornithopod in Antarctica. In 2016 these remains were the basis for naming the type species Morrosaurus antarcticus, named and described by Sebastian Rozadilla, Federico Lisandro Agnolin, Fernando Emilio Novas, Alexis Rolando Aranciaga Mauro, Matthew J. Motta, Juan Manuel Lirio Marcelo, and Pablo Isasi. The genus name refers to the site of El Morro on James Ross Island, where the remains of the species were found. The specific name refers to Antarctica.

The holotype specimen MACN Pv 197, was found in a layer of the Snow Hill Island Formation (Cape Lambe, previously assigned to the Lopez de Bertodano Formation), dating to the Maastrichtian age. The remains consists in a right hind leg, including the top of the femur, the lower end of the femur, the upper part of the tibia, the underside of the tibia, the upper half of the foot, the bottom of the midfoot and the top of the first joint of the third toe.

Description
The only known specimen, thought to represent an adult, is estimated to have belong to an individual 4.5-5 m in length. This makes it larger and more robust in skeletal build than many of its relatives, including the fellow Antarctic genus Trinisaura, only 1.5 m in length. The fragmentary nature of the material precludes saying much about the preserved regions. It is united anatomically with other elasmarians by the strongly compressed outer front edge of the, lacking the distinct cleft and thick edge seen in other types of ornithopod. Other uniting traits of elasmarians seen in the and  cannot be evaluated in Morrosaurus due to their lack of preservation in the genus. The seems to have been rather gracile, though more stout than in Trinisaura, as was the elongate foot, with the central third  being larger than the others.

A series of distinctive features characterizes the genus. Two of these are autapomorphies, completely unique traits acquired by the taxon. In bottom view, the greater trochanter near the top of the femur forms an S-shape, with the back edge thick across and the front edge thin, similar to the condition seen in other elasmarians Anabisetia, Notohypsilophodon, and Gasparinisaura but displayed to a more extreme extent. The fourth metatarsal bone has a prominent rearward projection that wraps around the third metatarsal, likewise similar but more developed to the condition seen in Gasparinisaura as well as Kangnasaurus, a genus with overall similar morphology and proportions to Morrosaurus. In addition, there is a unique combination of two features that by themselves are not unique features. In the femur, the is positioned forward and to the outside of the greater trochanter. In the tibia, medial malleolus has a triangular shape from the front and a concave surface from the same angle.

Phylogeny
Morrosaurus was classified in the group Iguanodontia, as a basal member of Euiguanodontia. This in turn formed a clade with other ornithopods of Patagonia and Antarctica, particularly Trinisaura, Gasparinisaura, Anabisetia, Notohypsilophodon, Talenkauen and Macrogryphosaurus in a group called Elasmaria, whose members are distinguished by their adaptation to a running lifestyle which would be reflected by the narrow foot with a thin fourth metatarsal which indicates a high speed running; subsequently expanded chevrons, a feature that is associated with a greater surface area for attachment of the lateral muscles of the tail, which would give good control of the movements of this; and a curved humerus which demonstrates the absence of a deltopectoral ridge and therefore that the front leg was not used for walking. It cannot be determined, however, if Morrosaurus itself possessed these characteristics due to their limited remains. The existence of this clade may indicate that Patagonia, Antarctica and Australia shared the same type of fauna. The exact phylogenetic relationships within this clade could not be identified, except for Gasparinisaura, which proved to be the most basal member of group.

Cladogram based in the phylogenetic analysis of Rozadilla et al., 2016:

Palaeobiology
Based on the hindlimb anatomy of Morrosaurus and other elasmarians, notably the slim, it's thought they were very capable runners, distinguishing them from other herbivorous dinosaurs they lived alongside. Elements of relatives not preserved in Morrosaurus such as expanded and distinctive bony intercostal plates along the torso would've further aided their cursorial abilities by improving tail balance and musculature control as well as breathing efficiency. This developed tail musculature and running ability of elasmarians is noted as more developed in elasmarian taxa than in other ornithopods such as Hypsilophodon.

In 2020 a study was conducted on the growth rate of Morrosaurus, and its relative Trinisaura, through use of histological study of thin samples from the holotype and fourth. Ornithischian dinosaurs living in polar regions in the Northern Hemisphere (such as Edmontosaurus and Pachyrhinosaurus typically show different growth rates than relatives from lower latitutes, owing to the extreme temperatures of their environments, but studies have failed to find the same signs in Antarctic animals. The study confirmed this pattern in elasmarians, with comparison with South American elasmarian Gasparinisaura finding incredibly similar histology to the Antarctic genera. All three show rapid growth with periodic interruptions, variation in growth marks, and changes in growth rate across their life cycle, growing slower as they aged. Similar growth strategies are reported in other ornithopods from much lower latitudes. This indicates that rather than adapting their growth physiology in response to the cold, southern environments they lived in, elasmarians were able to succeed in such ecosystems due to the existing possession of necessary adaptations to thrive there. Higher growth rate was noted in Morrosaurus as compared to Trinisaura, consistent with being a larger species. Additionally, it was found the animals reached their sexual maturity well before their skeletal maturity, and ceased growth at a later point than did, consistent with other dinosaurs. Neonatal fossils of elasmarians from Australia, which offer strong evidence of elasmarians living year round in high-latitude conditions rather than being migratory in nature, were similarly found to display growth rates very similar to those of other ornithopods such as Orodromeus.

Palaeobiogeography
The discovery of Morrosaurus was concurrent with the first evidence of a previously hypothesized uniting clade of ornithopod dinosaurs from the continents that had comprised Gondwana. The distinction between fauna of this part of the world as compared with the northern hemisphere equivalent Laurasia had long been noted, but evidence was traditionally lacking for a biogeographic link for dinosaurs between the different Gondwanan regions. Brazil and Northern Africa shared a fauna distinct from that of Patagonia, and each of these distinct from that of India and Madagascar; likewise, Australia, New Zealand, and Antarctic seemed connected to the exclusion of the others. This contrasted with existing recognition of links between the flora and marine invertebrate fauna between Patagonia, Antarctica, Australia, and New Zealand, termed the Weddellian Bioprovince.

The recognition of Morrosaurus and Trinisaura as Antarctic elasmarians closely related to animals from South America constituted important evidence of such ties between these continents existing in dinosaurs as well. This would be bolstered by later studies producing firmer evidence of Australian ornithopods as also belonging to the group, as well as possibly Kangnasaurus from Africa. Similar biogeographic patterns have been observed in megaraptoran theropods, diamantinasaurian sauropods, and parankylosaurian ankylosaurs. These discoveries of connected endemic Gondwanan ecosystems have overturned the traditional view of the southern continents, including the fauna seen in Late Cretaceous Antarctic, acting as a refugium for animals more successful elsewhere earlier in the Cretaceous.