User:Borophagus/sandbox

Borophagus/sandbox Temporal range: Early Miocene
Skull and jaw elements of Eucholoeops ingens
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Pilosa
Family:	†Megalonychidae
Genus:	†Eucholoeops Ameghino, 1887
Type species
†Eucholoeops ingens Ameghino, 1887
Synonyms
Synonyms of E. ingens †E. externus Ameghino, 1891 †E. fissognathus Ameghino, 1891 †E. fronto Ameghino, 1891 †E. lafonei Mercerat, 1891 †E. latifrons Mercerat, 1891 †E. latirostris Ameghino, 1891 †E. litoralis Ameghino, 1891 †E. curtus Ameghino, 1894

Eucholoeops (sometimes incorrectly spelled Eucholaeops) is a genus of megalonychid ground sloth that lived in southernmost Argentina. Fossils have been recovered from the Santa Cruz Formation of Patagonia, which has been dated to the Early Miocene. The type species, E. ingens, was named by Florentino Ameghino in 1887 based on now-lost skull remains from the collection of his brother. Eight other species were named, throughout the late 19th century, though all are now believed to represent the same taxon as E. ingens, and the lost type species has been replaced with a neotype mandible (lower jaw).

Though fairly small compared to the giant ground sloths of the Pliocene and Pleistocene, Eucholoeops was larger than any modern species, and has an estimated body mass of 80 kg (180 lb). In many ways it resembles Hapalops, to the point where the two genera were briefly regarded as one and the same. It is distinguished by its broad muzzle, very deep mandible, and the shape of its upper molariforms (teeth comparable to the molars of other mammals), among other characteristics.

Taxonomy

Early history

The type specimen of Eucholoeops was a complete skull and mandible, largely embedded in matrix, discovered on the banks of the Santa Cruz River of southern Patagonia, Argentina. The strata from which it was recovered are part of the early-to-late Miocene Santa Cruz Formation. The specimen ended up in the collection of palaeontologist Carlos Ameghino, and was described and named by his brother, Florentino, in 1887, as part of a larger two-part paper discussing fossils in Carlos' collection. The type specimen has since been lost, and was never figured, as it was never fully extracted from the matrix that encompassed it.

Inner systematics and proposed synonyms

Five additional species of Eucholoeops (E. externus, E. fissognathus, E. fronto, E. latirostris, E. litoralis) were named four years later, also by Florentino Ameghino; he went on to name another species, E. curtus, three years later. Two more, E. lafonei and E. latifrons, had been named by Alcide Mercerat, in the same year as Ameghino's first paper. These were synonymised with existing species by Ameghino. In 1894, Richard Lydekker suggested that most of the Santa Cruz ground sloth genera could be whittled to just two: Eucholoeops and Pseudhapalops. Hapalops, named by Ameghino in 1887, was considered a junior synonym of Eucholoeops. This extreme taxonomic lumping not been followed by subsequent authors.

The validity of most of the species named after E. ingens has been called into question, and they are currently treated as junior synonyms. The type specimen of E. fronto consists of a maxilla and a mandible, and in the absence of the original type specimen, the mandible (MPM-PV 3401) has been designated the neotype of E. ingens.

Description

Eucholoeops was smaller than the giants of the Pliocene and Pleistocene (such as Eremotherium and Megatherium). It was, however, still large in comparison to modern sloths. The smallest Eucholoeops skull (MACN-A6413), measured from the upper caniniform teeth to the posterior (rear) margin of the occipital condyles, was a little over 11.5 cm (4.5 in) in length. The largest skull, MPM PV3401, measures around 16.5 cm (6.5 in) in length. It has been estimated that Eucholoeops weighed around 80 kg (180 lb).

Skull and dentition

The skull of Eucholoeops was similar to that of other megalonychids. Dorsally (from above), it was somewhat convex, in a fashion similar to other megalonychids. The muzzle was fairly robust, more so than in close relatives, which partly contributed to erroneous depictions of Eucholeops with a prominent depression between the viscerocranium (facial part of the skull) and the neurocranium (cranial part of the skull). The skull overall is broader and heavier than that of the related Hapalops, with larger sagittal and occipital crests. Typically among sloths, the premaxillae were only loosely connected to one another and the maxilla. Unlike in other sloth genera, the facial portion of the maxilla stops at the caniniform (a tooth found exclusively in sloths, analogous to the canines of other mammals). The main body of the mandible, the mandibular corpus, was abnormally massive. The coronoid process of the mandible was similar to that of Hapalops, though with a condyle that is slightly higher.

Eucholoeops had a dental formula of 5/4: there were four molariforms, flat teeth analogous to the cheek teeth of other mammals, and one caniniform on the upper jaw; there were three lower molars and one lower caniniform on the lower jaw. All of the teeth are separated by small gaps (diastemata). The caniniform was very large, triangular in cross-section, and projected forwards. This is unlike the condition seen in other megalonychids, where the caniniform was small, sometimes cylindrical, and non-projecting. The molariforms, mostly resembled those of other ground sloths, though were notable in that they, especially the first and second molariforms of the upper jaw, were expanded transversely (across), and in some cases were medially and distally compressed. Some individiual variation was present. The smallest molariform on the upper jaw was the fourth.

Postcranial skeleton

Life restoration of the related ground sloth Megalonyx.

The postcrania of Eucholoeops are represented by limb elements. The humerus is large, with a proximal portion (that close to the body) that is subcylindrical and widen into a flattened distal portion (that far from the body): this condition is seen in all non-mylodontid sloths. Similar to Hapalops, the lesser tubercle is larger than the greater tubercle. As in several other extinct sloth genera, the tubercles are widely separated. The radius has a less steeply inclined head than in other megalonychids. The ulna is not preserved in any specimen. Most of the carpal elements assigned to Eucholoeops are catalogued under FMNH P13125; however, they may belong to Hapalops. The capitate bone of the wrist is wider distally (far from the body axis) than it is proximally, a condition also seen in Megalonyx and members of Nothrotheriidae. The first metacarpal is around half the length of the others. The second and fourth metacarpals are about as robust and long as the third; this differs from the typical sloth condition, second the third and third metacarpals are roughly equal in size, but shorter than the fourth and fifth. The phalangeal (digit) elements do not meaningfully differ from those of other ground sloths. The femur is known only from a single specimen. It is wide and flat, typical for ground sloths, and in many ways, it resembles that of Acratocnus.