Stegotherium Skull

''Stegotherium'' is an extinct genus of long-nosed armadillo, belonging to the Dasypodidae family alongside the nine-banded armadillo. It is currently the only genus recognized as a member of the tribe Stegotheriini. It lived during the Early Miocene of Patagonia and was found in Colhuehuapian rocks from the Sarmiento Formation, Santacrucian rocks from the Santa Cruz Formation,''Stegotherium''
at Fossilworks.org and potentially also in Colloncuran rocks from the Middle Miocene Collón Curá Formation. Its strange, almost toothless and elongated skull indicates a specialization for myrmecophagy, the eating of ants, unique among the order Cingulata, which includes pampatheres, glyptodonts and all the extant species of armadillos.

History

''Stegotherium tessellatum'' was described originally in 1887 by Florentino Ameghino based on the remains of a carapace collected by his brother Carlos in the Santa Cruz Province of Argentina. The same paper also described another genus and species of armadillo, ''Scaetops simplex'', known from a fragmentary mandible. In 1894, ''Stegotherium'', at that time only known from osteoderms, was temporarily considered by Lydekker as a synonym of '' Peltephilus''. This status was contested and proven wrong a year later by Ameghino.

In 1902, after a skull of ''Scaetops simplex'' was found in association with ''Stegotherium tessellatum'' osteoderms, Ameghino considered the two species synonymous, and proposed a new species ''Stegotherium variegatum'' based on osteoderms found in Chubut Province. In 1904, after the discovery of additional remains of ''S. variegatum'', William Berryman Scott re-evaluated ''Scaetops simplex'' as a species of ''Stegotherium'' different from ''S. tessellatum''.

In 2008, two important studies on the genus were published. The first, led by Fernicola and Vizcaíno, reviewed the material and species assigned to the genus. They proposed two new species, ''S. caroloameghinoi'', with MACN-A 10443a, an osteoderm from the dorsal carapace, as holotype, and ''S. pascuali'' using MACN A-12680d, an osteoderm from the dorsal carapace, as holotype. This review also kept, not without some doubt, ''S. simplex'' as a valid taxon. The second study from 2008, led by González Ruiz and Scillato-Yané, proposed ‘’''Stegotherium tauberi''’’ as a species, based on YPM PU 15565, a fairly complete specimen including a fragmentary dorsal carapace, a complete skull, several vertebra and a right foot, previously assigned to ''S. tessellatum''.

In 2009, another species was named by González Ruiz and Scillato-Yané, ''S. notohippidensis'', with the holotype being MLP 84-III-5-10, a collection of 130 osteoderms from Argentina.

Description

''Stegotherium'' was an unusual armadillo, whose most striking feature was the elongated skull, often compared to the skull of an anteater. The posterior area of the jaws, the only one to bear teeth, was compressed compared to '' Dasypus'', while the nasal area and the anterior parts of both jaws, completely toothless, were long and slender. The teeth were cylindricals and greatly reduced, both in number and in size, and were all contained in the posterior area of the lower and upper jaws. While ''S. tauberi'' had six teeth in its lower mandible, the dubious ''S. simplex'' only had two.

The body of ''Stegotherium'' was roughly the size of the modern species of ''Dasypus'', and its carapace was composed of at least 23 mobile bands of osteoderms. The osteoderms of ''Stegotherium'', 3 to 7.5 mm thick and 20 mm long, were characterized by the presence of a number of piliferous foramina around their posterior and lateral margins, a granular appearance, and a compact bone structure.

Species

The genus ''Stegotherium'' is unambiguously known from six species, ''S. tessellatum'', ''S. variegatum'', ''S. caroloameghinoi'', ''S. pascuali'', ''S. tauberi'' and ''S. notohippidensis''. A seventh species, ''S. simplex'', is generally considered too fragmentary, but has generally been considered valid with reservations by most recent scholars. As osteoderms are the most abundant fossils of ''Stegotherium'' known, they are commonly used as the main determinate of which species a given fossil belongs too.

''Stegotherium tessellatum''

''S. tessellatum'' is the type species of ''Stegotherium''. Fossils of it have been recovered in the Santacrucian of the Santa Cruz Formation. It had quadrangular osteoderms, with a single large foramen in the exterior margin, devoid of longitudinal ridge of any kind in the central region. While non-osteoderm remains have been historically referred to this species in literature, they are now assigned to ''S. tauberi''.

''Stegotherium simplex''

''S. simplex'' is only known from its holotype, a fragmentary mandible with two alveoli, found in the Santa Cruz Formation and dated from the Santacrucian period. It is the only species in the genus whose osteoderms, usually considered diagnostic for armadillo fossils, are unknown. Its only diagnosis characteristic could be the presence of two molariform teeth on the mandible, while ''S. tessellatum'' had six; the validity of the species has been debated since 1902, and the holotype is probably lost.

''Stegotherium variegatum''

''S. variegatum'' is known from the Colhuehuapian Sarmiento Formation. The species is mainly known from fossilized quadrangular osteoderms, whose exposed surface showed several piliferous pits around a single granulated central figure, and a longitudinal ridge surrounded, in all of its length, by depressions.

''Stegotherium caroloameghinoi''

''S. caroloameghinoi'' is known from the Sarmiento Formation of Argentina, in rocks dating from the Colhuehuapian period. It is only known from osteoderms. Those were rectangular, with a granular textured dorsal surface. Piliferous pits are placed around a central figure, crossed by a median longitudinal ridge, and one to three smaller anterior figures.

The specific name, ''caroloameghinoi'', is meant to honour Carlos Ameghino, who discovered the holotype of ''Stegotherium'' and was a prominent figure in the history of paleontology in Patagonia.

''Stegotherium pascuali''

''S. pascuali'' is known from the Colhuehuapian period in the Sarmiento Formation. It is known by fossilized osteoderms, whose various shapes all shared the same grainy-textured central figure surrounded by piliferous pits, without anterior figures. Two foramina, absent in ''S. variegatum'' and ''S. caroloameghinoi'', and a ridge absent in ''S. tessellatum'', were present on the osteoderms, completing the diagnostic characteristics.

It was named to honour the Argentinian paleontologist Rosendo Pascual.

''Stegotherium tauberi''

''S. tauberi'' is known from the Santa Cruz Formation, in rocks dated from the Santacrucian period. It is distinguished from other species of ''Stegotherium'' by osteoderms more rugged and with a sharper ridge than ''S. variegatum''. Those osteoderms had a large foramen in the anterior-central region, along with several smaller foramina assembled in a transversal row in the anterior region. The presence of a longitudinal ridge on the osteoderms also distinguishes them. Some of the non-osteoderm material used by González Ruiz and Scillato-Yané to describe ''S. tauberi'' was assigned by Fernicola and Vizcaíno to ''S. tessellatum''; both species are, however, considered valid by the current consensus.

Its species name, ''tauberi'', honours Adán Alejo Tauber, an Argentinian paleontologist who worked on the Santa Cruz Formation.

''Stegotherium notohippidensis''

''S. notohippidensis'' is found in sediments from the "Notohippidian" period (traditionally considered as the lower part of the Santacrucian period) of the Santa Cruz Formation.
Its osteoderms had several foramina in their anterior region, larger than ''S. variegatum'' and ''S. tauberi''. In addition, the longitudinal ridge present in the osteoderms of other species of ''Stegotherium'' was absent in ''S. notohippidensis''.

The species name, "''notohippidensis''" means, in New Latin, "from the Notohippidian", which was itself named after the large herbivore '' Notohippus'', considered to be characteristic of this period.

Paleoecology

The morphology of the jaws of ''Stegotherium'' shows that most of the mastication muscles were specialized for a horizontal and propalinal movement; the teeth were reduced but could still be used for masticating relatively soft food. Those important specializations pushed most scholars to consider ''Stegotherium'' as a specialized myrmecophage, similar ecologically to anteaters and to the less specialized giant armadillo.

The area where ''Stegotherium'' lived was, during the Early Miocene, a forested savannah with a mild climate. It lived alongside a diversity of related cingulates, such as the Euphractine '' Prozaedyus'', the basal Chlamyphorid ''Proeutatus'', the Dasypodid '' Stenotatus'', the horned armadillo '' Peltephilus'' and several genera of glyptodonts, such as '' Asterostemma'', ''Propalaehoplophorus'', '' Cochlops'' and ''Eucinepeltus''.

The specialisation of ''Stegotherium'' may have caused the extinction of the genus during the Santacrucian, as it may have suffered from the large-scale environmental and climatic changes occurring in Patagonia during this period, the result of the rise of the Andes, causing an aridization that may have caused the rarefaction of ant and termite colonies it fed upon, and cooling making it harder for the animal to regulate its own body temperature. After the Santacrucian, the genus is only known by one Colloncuran fossilized osteoderm, MLP 91-IV-1-66 from the Collón Curá Formation, tentatively assigned to ''Stegotherium'' sp. and different from all currently known species of ''Stegotherium'', although other Colloncuran osteoderms of indeterminate Stegotheriini have also been recovered in the Chubut Province.

References

{{Taxonbar, from=Q7606718

Armadillos
Prehistoric cingulates
Prehistoric placental genera
Miocene xenarthrans
Miocene mammals of South America
Santacrucian
Colhuehuapian
Neogene Argentina
Fossils of Argentina
Fossil taxa described in 1887
Taxa named by Florentino Ameghino
Golfo San Jorge Basin
Sarmiento Formation