Proeremotherium, Carlini, Brandoni and Sanchez, 2006

Carlini, Alfredo A., Brandoni, Diego, Sánchez, Rodolfo & Sánchez-Villagra, Marcelo R., 2018, A new Megatheriinae skull (Xenarthra, Tardigrada) from the Pliocene of Northern Venezuela - implications for a giant sloth dispersal to Central and North America, Palaeontologia Electronica (16 A) 21 (2), pp. 1-12 : 3-6

PROEREMOTHERIUM Carlini, Brandoni and Sánchez, 2006

Type species. Proeremotherium eljebe Carlini, Brandoni and Sánchez, 2006 .

Geographic and stratigraphic distribution. Estado Falcón, Urumaco; El Jebe Member, Codore Formation and Vergel Member, San Gregorio Formation, Pliocene of Venezuela. cf. Proeremotherium

Referred material. AMU-CURS 184, an almost complete skull lacking mandible, jugals, the premaxillae, the left zygomatic process of the squamosal, vertical lamina of the left pterygoid, anterior part of the nasals, anterior part of the maxillae, lateral and partial anterior wall of the alveoli of right M1 and lateral and anterior wall of the alveoli of left M1, the lateral wall of those of the left tooth row, and teeth ( Figure 2 ).

Geographic and stratigraphic provenance. Twelve Km NNW from Urumaco town; Falcón Basin, Vergel Member , lower levels of San Gregorio Formation , late Pliocene.

Description

The skull AMU-CURS 184 is similar in size and gross morphology to that of Proeremotherium eljebe (represented by the type specimen AMU-CURS 126) ( Figure 2 , Table 1 ). Similarly to Proeremotherium eljebe , Eremotherium laurillardi and E. eomigrans, AMU-CURS 184 is relatively low (mainly at the anterior third), elongate and “gracile” in comparison with the Pleistocene Megatherium Cuvier, 1796 , species; with its major width on both the maxillar and the squamosal zygomatic roots ( Figure 3 ). The basicranial area is longer than in P. eljebe, and the basioccipital is more posteriorly projected relative to the condyles. The antero-dorsal and postero-dorsal edges of each temporal fossa are convex both anteriorly and posteriorly, respectively, and the edge were both fossa converge at midline is straight and form a longer sagittal crest than that of P. eljebe . Because of the latter, in AMU-CURS 184 sagittal crest starts just posterior to the level of the maxillar zygomatic root (anteriorly to the interorbital constriction), whereas in Proeremotherium eljebe , the margins converge posteriorly, at the anterior edge of the squamosal zygomatic root, and consequently form a shorter sagittal crest. The dorsal margin of the skull of AMU-CURS 184 is slightly convex at the middle third, but abruptly descends at the fronto-nasal level, where a sub-triangular concave area is present (with a side parallel to a transverse plane and its opposite angle pointed posteriorly); in P. eljebe the triangular area is much more extended and almost convex, and in the species of Eremotherium it is almost flat; in Proeremotherium eljebe and Eremotherium laurillardi the fronto-nasal area descends gradually antero-ventrally. As in P. eljebe , E. laurillardi and E. eomigrans, AMU-CURS 184 has an occiput forming a ca. 90° angle, either with the parietal dorsal plane of the skull roof and the palatal plane (different to the species of Megatherium were the angle with de palatal plane is close to 70°). As in E. laurillardi and E. eomigrans, AMU-CURS 184 has the occipital condyles posteriorly projected, hemispherical in lateral view, and bigger and with an antero-posterior axis more sub-horizontal than in P. eljebe . As in P. eljebe , E. laurillardi , and E. eomigrans , the basilar plane of AMU-CURS 184 is close to the palatal plane, surely related to the low hypsodonty reached (not as in Megatherium species where it is higher, as do the hypsodonty, see De Iuliis, 1996:154). In AMU-CURS 184, the maxillar zygomatic root is located lateral to M2, whereas in Proeremotherium eljebe lies a little more posterior, extending between the mid plane of the M2 and the mid plane of the M3. The infraorbital foramen is at 24 mm above the alveolar plane, whereas it is at 22 mm in P. eljebe . The abundant remains of Eremotherium laurillardi and Megatherium americanum Cuvier, 1796 , indicate that both the position of the root and the height of the infraorbital foramen could vary somewhat. In Eremotherium laurillardi the root tends to lies more anteriorly, extending from the level of the middle of M1 to that of the middle of M3; whereas in M. americanum , the position of the root is more variable than in Eremotherium laurillardi (see De Iuliis, 1996). In AMU-CURS 184, because the height of the squamosal zygomatic root, it is more dorsally extended and is higher than in P. eljebe . As in other megatheriines, the squamosal part of the zygomatic arch is a laminar vertical bone over the alveolar plane, but its ventral margin is sub-horizontal as in Proeremotherium eljebe and Eremotherium species (not oblique as in Megatherium species), and its anterior end reaches the level of the M4–M5 septum. The preserved palatal premolariform extension is concave, as is at the M1–M2 area, but is almost flat at the M3-M4. The posterior palatal notch is U-shaped, and its anterior margin is at the M5 level; whereas in P. eljebe it is anteriorly extended reaching the mid M4 level, and in Eremotherium laurillardi and Megatherium americanum , the position of the posterior palatal notch is variable (see De Iuliis, 1996), but always posterior to the M5 level.

The upper dental series (M1–M5) has approximately 165 mm in length. Each tooth row is convex on its lingual margin and straight-convex on the labial margin, whereas in P. eljebe the margins are much more straight either lingual/labial side ( Figure 3 ). The maximum distance between the alveolus of both tooth rows is between the preserved anterior most margin the M1 (43 mm) and the middle plane of the M5 (49 mm). Despite the fact that the specimen AMU-CURS 184 does not preserve the molariform teeth, judging from the shape of the alveoli, we hypothesize that the M1 is subtrapezoidal, the M2–M4 are sub-squared, and the M5 (the smallest) is mesiodistally compressed (as in other Megatheriinae), and nearly oval in outline.

Figures

FIGURE 2. Skulls of AMU-CURS 184 and AMU-CURS 126 (Proeremotherium eljebe type specimen). AMU-CURS 184 in 1, dorsal; 3, lateral, and 5, palatal views. Proeremotherium eljebe in 2, dorsal; 4, lateral, and 6, palatal views.

FIGURE 3. Skulls of 1: Eremotherium laurillardi, 2: Eremotherium eomigrans, 3: AMU-CURS 184, cf. Proeremotherium sp., and 4: Proeremotherium eljebe (AMU-CURS 126, type), compared in (from top to bottom) dorsal, lateral and palatal views. In light grey are the upper dental series alveoli contours of each skull; black arrows shows the outline and extension of the posterior palatal notch, and up to where it reaches in relation to the dental series (in 1 the dotted line is used because that part of the palatal notch was mechanically broken).