Cranial Morphology and Phylogenetic Relationships of Trigonostylops wortmani, an Eocene South American Native Ungulate

Abstract

In 1933 George G. Simpson described a remarkably complete skull of Trigonostylops, an Eocene South American native ungulate (SANU) whose relationships were, in his mind, quite uncertain. Although some authorities, such as Florentino Ameghino and William B. Scott, thought that a case could be made for regarding Trigonostylops as an astrapothere, Simpson took a different position, emphasizing what would now be regarded as autapomorphies. He pointed out a number of features of the skull of Trigonostylops that he thought were not represented in other major clades of SANUs, and regarded these as evidence of its phyletic uniqueness. Arguing that the lineage that Trigonostylops represented must have departed at an early point from lineages that gave rise to other SANU orders, Simpson reserved the possibility that Astrapotheriidae might still qualify (in modern terms) as its sister group. Even so, he argued that the next logical step was to place Trigonostylops and its few known allies in a separate order, Trigonostylopoidea, coordinate with Astrapotheria, Notoungulata, Litopterna, and Pyrotheria. Simpson's classification was not favored by most later authors, and in recent decades trigonostylopids have been almost universally assigned to Astrapotheria. However, his evaluation of the allegedly unique characters of Trigonostylops and its allies has never been systematically treated, which is the objective of this paper. Using computed tomography, the skull of Trigonostylops is compared, structure by structure, to a variety of representative SANUs as well as extant perissodactylans (which together comprise the clade Panperissodactyla) and the "condylarthran" Meniscotherium. In addition to placing Simpson's character evaluations in a comparative context, we also provide detailed assessments of many vascular and pneumatization-related features of panperissodactylans never previously explored. Overall, we found that this new assessment strengthened the placement of Trigonostylops within a monophyletic group that includes Astrapotherium and Astraponotus, to the exclusion of other SANU clades. Although Trigonostylops cannot be considered as morphologically distinct or unusual as Simpson thought, our comparative and phylogenetic analyses have helped to generate a number of hypotheses about character evolution and function in SANUs that may now be fruitfully tested using other taxon combinations.

Transverse Canal Foramen and Pericarotid Venous Network in Metatheria and Other Mammals

“…RBVs converge on, and presumably drain, a structure-the TBS-that is ontogenetically programmed to produce erythrocytes in young stages of at least some species (see below). This arrangement recalls the drainage of red marrow by vena diploica, which are characteristically situated in cranial cancellous tissues (see Warwick and Williams, 1973;Cline and Maronpot, 1985;MacPhee et al, 2021). Most diploic veins discharge endocranially into dural channels, but in some taxa, including Homo and Canis, certain veins designated as vena diploica connect directly to external vasculature (e.g., occipital or asterional diploic vein; Evans and Christensen, 1979: 793).…”

At the origin of Neanderthals and modern humans - Cerebral complexity and encephalization

“…This led to regression of the cranioorbital sinus, with the sigmoid becoming the dominant discharge pathway for the transverse sinus. In mammals in which the neopallium does not cover the cerebellum, conditions typical of dural drainage in the embryo are retained, as is the cranioorbital sinus 10 .…”

“…Our research is based on the study of the cranial transosseous channels that connect the intracranial dural venous sinuses with the extracranial venous networks whose presence/absence and distribution could shed light on the evolution of the venous and cerebral systems in the different clades 10 . The emissary veins, being the rst to develop during venous vascular ontogeny, play a decisive role in cerebral variations as they can act as a shunt in the presence of developmental anomalies of the dural network 10 . Therefore, the emissary veins could play a decisive role in the reorganization of the cranial venous networks but above all in the process of encephalic evolution 10 .…”

“…The emissary veins, being the rst to develop during venous vascular ontogeny, play a decisive role in cerebral variations as they can act as a shunt in the presence of developmental anomalies of the dural network 10 . Therefore, the emissary veins could play a decisive role in the reorganization of the cranial venous networks but above all in the process of encephalic evolution 10 .…”

Eocene mammals from volcaniclastic deposits of the Somun Cura Plateau: biostratigraphic implications for north Patagonia Paleogene

“…6 D, E), although they definitively do not match with any known Oligocene astrapothere. A third astrapothere taxon is represented by an isolated upper molar referable to Trigonostylops, much smaller than the formers (see Supplementary Information S2) and with a typical trigonodont pattern (see Simpson, 1967a;MacPhee et al, 2021). The specimen could not be found in MJHG collection, but a drawing was provided by Carbajal et al (1977: fig.…”