Sunday, April 12, 2020

[PaleoMammalogy • 2020] Cranial Anatomy and Paleoneurology of the Extinct Sloth Catonyx tarijensis (Xenarthra, Mylodontidae) From the Late Pleistocene of Oruro, Southwestern Bolivia

Catonyx tarijensis (Gervais & Ameghino, 1880)

in Boscaini, Iurino, Quispe, et al., 2020.
Artwork by Dawid A. Iurino.

Extinct scelidotheriine sloths are among the most peculiar fossil mammals from South America. In recent decades, the external cranial anatomy of Pleistocene scelidotheres such as Scelidotherium, Catonyx, and Valgipes has been the subject of numerous studies, but their endocranial anatomy remains almost completely unknown. Today, computed tomographic (CT) scanning methodologies permit the exploration of previously inaccessible anatomical areas through a completely non-destructive process. For this reason, we undertook an analysis of the external and internal cranial anatomy of Catonyx tarijensis from the late Pleistocene of the Department of Oruro, in southwestern Bolivia. One particularly well-preserved specimen allowed detailed observation of all the main cranial osteological features, including the ear region and an almost complete hyoid apparatus, previously unknown for this taxon. Moreover, CT-scanning and subsequent elaboration of digital models of this specimen allowed observation of the brain cavity and cranial sinuses, and reconstruction of the trajectory of the main cranial nerves for the first time in an extinct scelidotheriine sloth. Additionally, we recovered the first three-dimensional reconstructions of the nasal cavity and the turbinates of an extinct sloth. In contrast to the usual depiction, the combined information from the external and internal anatomy suggests reduced lingual protrusion in Catonyx tarijensis, or at least a consistently more limited protrusion of the tongue in comparison with other mylodontid sloths such as Glossotherium robustum. The new morphological information recovered from this extinct sloth is compared with the available information for both extant and extinct forms, providing insights in the paleobiology of the extinct species. The present study reveals the importance of applying these novel non-destructive techniques to elucidate the evolutionary history of sloths.

Keywords: Xenarthra, scelidotheriine sloth, Catonyx tarijensis, skull, anatomy, endocast, hyoid apparatus

FIGURE 1: Map of the locality in which the remains of Catonyx tarijensis MNHN-Bol V 13364 have been recovered. Circles: cities; Star: fossil locality.

Systematic Paleontology

XENARTHRA (Cope, 1889)
PILOSA (Flower, 1883)
FOLIVORA (Delsuc et al., 2001)


Catonyx (Ameghino, 1891)

Catonyx tarijensis (Gervais and Ameghino, 1880)

Holotype: MNHN.F.TAR1260, skull and mandible from the Pleistocene of Tarija Valley (southern Bolivia).

Distribution: Pleistocene deposits of Bolivia, Argentina and Uruguay (McDonald and Perea, 2002; Corona et al., 2013; MiñoBoilini, 2016). 

FIGURE 7: Hypothetical life reconstruction of Catonyx tarijensis showing its inferred feeding behavior. The reconstruction is based on the skull MNHN-Bol V 13364 from the Pleistocene of Oruro (Bolivian Altiplano).
Artwork by Dawid A. Iurino.

We report novel data on the external and internal cranial anatomy of the scelidotheriine sloth Catonyx tarijensis, further extending knowledge on the cranial morphology of South American extinct sloths. This discovery, from late Pleistocene deposits of the Department of Oruro (southwestern Bolivia), allowed us to extend the paleobiogeographic range of C. tarijensis to more northern latitudes, as well as to the high altitudes of the Bolivian Altiplano.

The specimen described in the present study, a particularly well-preserved skull with associated mandible and hyoid apparatus, corresponds to a subadult individual of Catonyx tarijensis. Combined information from the external and the internal anatomy, obtained through CT-scanning followed by digital modeling techniques, allowed us to analyze several anatomical regions that were unknown for this taxon.

Among these, the ear region, the nasopharyngeal area and the hyoid elements revealed several phylogenetically and functionally informative features. Digital models permitted observations of the brain cavity, neurovascular grooves and cranial sinuses, and comparisons of these features with other Pleistocene mylodontids.

The information presented in this report confirms previous hypotheses on inferred modes of food intake among extinct scelidotheriine sloths. According to the data now available, C. tarijensis was likely a browsing species, which tore vegetation mainly using its strong lips, rather than the tongue. This habit was probably common among Scelidotheriinae and contrasts with that present in its sister clade, Mylodontinae, whose members were predominantly grazing species with smaller lips and more strongly protruding tongues.

The present study represents a further step in assembling broader morphological comparisons of digital endocranial models among extinct sloths, and emphasizes the importance of applying these new methodologies for understanding the evolution of this mammalian group.

Alberto Boscaini, Dawid A. Iurino, Bernardino Mamani Quispe, Rubén Andrade Flores, Raffaele Sardella, François Pujos and Timothy J. Gaudin. 2020. Cranial Anatomy and Paleoneurology of the Extinct Sloth Catonyx tarijensis (Xenarthra, Mylodontidae) From the Late Pleistocene of Oruro, Southwestern Bolivia. Front. Ecol. Evol.  DOI: 10.3389/fevo.2020.00069