| Stegotherium | |
|---|---|
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| Skeleton ofStegotherium tauberi (without carapace) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Cingulata |
| Family: | Dasypodidae |
| Subfamily: | Dasypodinae |
| Genus: | †Stegotherium Ameghino, 1887 |
| Type species | |
| †Stegotherium tessellatum Ameghino, 1887 | |
| Species | |
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| Synonyms | |
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Stegotherium is anextinctgenus of long-nosedarmadillo, belonging to theDasypodidae family alongside thenine-banded armadillo. It is currently the only genus recognized as a member of the tribe Stegotheriini. It lived during theEarly Miocene ofPatagonia and was found inColhuehuapian rocks from theSarmiento Formation,Santacrucian rocks from theSanta Cruz Formation,[2] and potentially also inColloncuran rocks from theMiddle MioceneCollón Curá Formation.[3] Its strange, almost toothless and elongated skull indicates a specialization formyrmecophagy, the eating of ants, unique among the orderCingulata, which includespampatheres,glyptodonts and all the extant species of armadillos.[4]
Stegotherium tessellatum was described originally in 1887 byFlorentino Ameghino based on the remains of a carapace collected by his brotherCarlos in theSanta Cruz Province of Argentina. The same paper also described another genus and species of armadillo,Scaetops simplex, known from a fragmentary mandible.[5] In 1894,Stegotherium, at that time only known fromosteoderms, was temporarily considered byLydekker as a synonym ofPeltephilus. This status was contested and proven wrong a year later by Ameghino.[citation needed]
In 1902, after a skull ofScaetops simplex was found in association withStegotherium tessellatum osteoderms, Ameghino considered the two species synonymous, and proposed a new speciesStegotherium variegatum based on osteoderms found inChubut Province.[6] In 1904, after the discovery of additional remains ofS. variegatum,William Berryman Scott re-evaluatedScaetops simplex as a species ofStegotherium different fromS. tessellatum.[7]
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, andS. 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.[8] 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 toS. tessellatum.[1]
In 2009, another species was named by González Ruiz and Scillato-Yané,S. notohippidensis, with theholotype being MLP 84-III-5-10, a collection of 130 osteoderms from Argentina.[4]
Stegotherium was an unusual armadillo, whose most striking feature was the elongated skull, often compared to the skull of ananteater. The posterior area of the jaws, the only one to bear teeth, was compressed compared toDasypus, 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.[7] WhileS. tauberi had six teeth in its lower mandible, the dubiousS. simplex only had two.[8]
The body ofStegotherium was roughly the size of the modern species ofDasypus,[5][8] and its carapace was composed of at least 23 mobile bands of osteoderms.[1] The osteoderms ofStegotherium, 3 to 7.5 mm thick[8] and 20 mm long,[6] were characterized by the presence of a number ofpiliferousforamina around their posterior and lateral margins, a granular appearance, and a compact bone structure.[9]
The genusStegotherium is unambiguously known from six species,S. tessellatum,S. variegatum,S. caroloameghinoi,S. pascuali,S. tauberi andS. 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 ofStegotherium known, they are commonly used as the main determinate of which species a given fossil belongs too.[citation needed]
S. tessellatum is the type species ofStegotherium.[5] 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 toS. tauberi.[8]
S. simplex[5] is only known from its holotype, a fragmentary mandible with twoalveoli, 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 twomolariform teeth on the mandible, whileS. tessellatum had six;[10] the validity of the species has been debated since 1902,[8][1][4] and the holotype is probably lost.[10]
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.[8]
S. caroloameghinoi is known from the Sarmiento Formation ofArgentina, 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.[This paragraph needs citation(s)]
The specific name,caroloameghinoi, is meant to honourCarlos Ameghino, who discovered the holotype ofStegotherium and was a prominent figure in the history of paleontology in Patagonia.[8]
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 inS. variegatum andS. caroloameghinoi, and a ridge absent inS. tessellatum, were present on the osteoderms, completing the diagnostic characteristics.[8]
It was named to honour the Argentinian paleontologistRosendo Pascual.[8]
S. tauberi is known from the Santa Cruz Formation, in rocks dated from the Santacrucian period. It is distinguished from other species ofStegotherium by osteoderms more rugged and with a sharper ridge thanS. 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 describeS. tauberi was assigned by Fernicola and Vizcaíno toS. tessellatum; both species are, however, considered valid by the current consensus.[This paragraph needs citation(s)]
Its species name,tauberi, honours Adán Alejo Tauber, an Argentinian paleontologist who worked on the Santa Cruz Formation.[1]
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 thanS. variegatum andS. tauberi. In addition, the longitudinal ridge present in the osteoderms of other species ofStegotherium was absent inS. notohippidensis.[This paragraph needs citation(s)]
The species name, "notohippidensis" means, inNeo-Latin, "from the Notohippidian", which was itself named after the large herbivoreNotohippus, considered to be characteristic of this period.[4]
The morphology of the jaws ofStegotherium 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 considerStegotherium as a specializedmyrmecophage, similar ecologically toanteaters and to the less specializedgiant armadillo.[7]
The area whereStegotherium lived was, during the Early Miocene, a forestedsavannah with a mild climate.[7] It lived alongside a diversity of relatedcingulates, such as theEuphractineProzaedyus, the basalChlamyphoridProeutatus, theDasypodidStenotatus, thehorned armadilloPeltephilus and several genera ofglyptodonts, such asAsterostemma,Propalaehoplophorus,Cochlops andEucinepeltus.[11]
The specialisation ofStegotherium 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 therise of theAndes, causing anaridization that may have caused the rarefaction ofant andtermite colonies it fed upon, andcooling making it harder for the animal to regulate its own body temperature.[7] After the Santacrucian, the genus is only known by oneColloncuran fossilized osteoderm, MLP 91-IV-1-66 from theCollón Curá Formation, tentatively assigned toStegotherium sp. and different from all currently known species ofStegotherium, although otherColloncuran osteoderms of indeterminate Stegotheriini have also been recovered in theChubut Province.[3]
