Ferugliotherium is known from isolated teeth, the assignment of some of which is controversial. The material from the Los Alamitos Formation, which is mostly in the
Museo Argentino de Ciencias Naturales Bernardino Rivadavia (MACN) in
Buenos Aires, with one tooth in the
Museo de La Plata (MLP) in
La Plata, Argentina, has been thoroughly described; while there are additional
Ferugliotherium fossils from the La Colonia Formation, they have not been described in detail. Although the fragmentary nature of the known fossils of
Ferugliotherium makes it impossible to determine its
dental formula with certainty, Gurovich suggested that it had one
incisor (possibly two in the upper jaw), no
canines, one or two premolars, and two molars on each side of the lower and upper jaws. However, on the basis of comparisons with
Sudamerica, which is known to have had four lower molariforms (molar-like teeth, either premolars or molars) in its lower jaw, Pascual and Ortiz-Jaureguizar suggested in 2007 that
Ferugliotherium may also have had four lower molars.
Ferugliotherium was much smaller than the sudamericids
Gondwanatherium and
Sudamerica, and its body mass is estimated to have been about 70 g (2.5 oz). Unlike the
hypsodont (high-crowned) sudamericids,
Ferugliotherium has
brachydont (low-crowned) molariform teeth that are supported by at least two roots, not a single massive root. The direction of wear on the teeth indicates that
Ferugliotherium,
Gondwanatherium, and
Sudamerica all had
palinal jaw movement (i.e., the lower jaw moved backwards during the power stroke of chewing)—a feature otherwise only seen in multituberculates among mammals.
Incisors Three fragmentary
Ferugliotherium lower incisors (MACN Pv-RN 701A, 701B, and 701C) are known from the Los Alamitos Formation. Another incisor, MACN Pv-RN 970, was assigned to
Ferugliotherium by Bonaparte in 1990, but it is much larger than the other three incisors, which are otherwise similar, and probably represents
Gondwanatherium instead. Only the tips of the three incisors are preserved. They are laterally compressed, with an estimated width of 1.3 mm and height of 2.4 mm in 701A. The medial side (towards the middle of the mouth) is flat, but the lateral side (towards the sides) is convex. There is
enamel only on the lower (ventral) side. A large
wear facet is present at the tip, forming an angle of about 35° with the ventral margin in 701A. The three incisor fragments are identified as
Ferugliotherium because of their size and provenance and the presence of a restricted enamel band. They show features, such as lateral compression, an acute angle at the tip, small curvature, and an irregular cross section, that are usually seen in lower, not upper incisors in mammals with
procumbent incisors, such as rodents and
taeniolabidoid multituberculates. Four specimens (MACN Pv-RN 702A through 702D) are thought to represent second upper incisors (I2) of
Ferugliotherium. 702A (height 1.5 mm; width 1.1 mm) and 702B are slightly larger than 702C (height 1.2 mm; width 0.9 mm) and 702D. The smaller incisors cannot be lateral incisors (I3), because 702C's wear facet is stronger than would be expected in an I3; therefore, all four upper incisors are identified as central incisors (I2). To explain the size difference, Krause and colleagues suggested that
Ferugliotherium was variable in size or that the smaller incisors were
deciduous precursors of the larger permanent tooth. They considered it less likely that multiple species with similar incisors were present. The upper incisors have enamel only on the dorsal side. The wear facet at the tip is preserved only in 702C, forming an angle of 52° with the dorsal side, and is more concave than the facet in the lower incisors. 702A–D are recognizable as upper incisors because they have a less acute angle at the tip and are less laterally compressed, more curved, and elliptical in cross section. Incisors of
Ferugliotherium and
Gondwanatherium are similar in overall shape and share a restricted band of enamel—a feature otherwise seen only in multituberculates among Mesozoic mammals. The incisors of
Sudamerica are also similar.
Mandible with lower premolar MACN Pv-RN 975, a fragment of the
mandible (lower jaw) preserving one premolar, was discovered in 1991 and tentatively identified as
Ferugliotherium by Kielan-Jaworowska and Bonaparte in 1996, but this assignment remains controversial. The poorly preserved and worn premolar is a bladelike tooth, resembling multituberculate fourth lower premolars (p4). The premolar is 4.8 mm long and bears eight faint ridges on both the labial (towards the lips) and lingual (towards the tongue) sides. On the labial side, the four ridges at the back are more widely separated than the four in front of them. The back and front margins of the tooth are parallel and there is no small cusp on the labial side. There are two roots; the one at the front is larger than the one at the back and bears a furrow. The lower border of the enamel cover is marked by two semicircular extensions of the enamel on the front side, but there is only one such extension at the back. By its size, the number of ridges, and apparently greater length than height, it differs from all known multituberculate first, second, and third lower premolars, indicating that it is a p4. The dentary (lower jaw bone) itself is robust and short. The length axis of the p4 makes an angle of about 58° with the length axis of the jawbone. The bone is concave on the lingual, but convex on the labial side. There is a
diastema (gap) between the p4 and the incisor that would have been in front of it, as in the jaw of
Sudamerica. Gurovich estimated the length of the diastema as 2.5 mm. There is a rounded
mental foramen (an opening in the labial side of the jawbone), with a diameter of 0.7 mm, located about 0.8 mm below the dorsal margin of the bone and 1.5 mm in front of the p4. Although the incisor itself is not preserved, its
alveolus (the housing of the root) is in part. As in
Sudamerica, it extends far into the dentary, passing below p4. The alveolus is 1.5 mm wide below the front root of p4 and 1.4 mm at the back of the jaw fragment. Although the height of the alveolus cannot be determined because the lower side is broken away, the incisor must have been quite deep. When it was discovered that
Sudamerica had four molariform teeth and no bladelike premolar in its lower jaw, Pascual, Kielan-Jaworowska, and colleagues removed MACN Pv-RN 975 from
Ferugliotherium, which they expected to have the same dental formula as its fellow gondwanathere
Sudamerica, and identified it as an indeterminate multituberculate instead. Pascual and colleagues argued that molariform teeth as seen in
Sudamerica could not have evolved from the bladelike p4 of
Ferugliotherium, and that it was unlikely that additional molars had been added in
Sudamerica. In 2004 and 2007, Kielan-Jaworowska and colleagues aligned the dentary with the multituberculate suborder "
Plagiaulacida" because the p4 is rectangular in labial view, not curved as in the suborder
Cimolodonta. This feature was also used to distinguish MACN Pv-RN 975 from the single p4 assigned to
Argentodites, which was tentatively placed in Cimolodonta. Gurovich, Guillermo Rougier, and colleagues, on the other hand, maintain that the dentary is referable to
Ferugliotherium and that the p4s of
Argentodites and MACN Pv-RN 975 are very similar. The alveolus of MACN Pv-RN 975 fits the lower incisors attributed to
Ferugliotherium in size and the blade-like premolar is of the size expected for an animal with molariforms the size of
Ferugliotherium teeth. If the dentary and premolars (whose identification has been similarly controversial; see below) do not belong to
Ferugliotherium, then, Gurovich and Beck argue, the Los Alamitos Formation would contain two mammals (
Ferugliotherium and a multituberculate) similar in size and morphology, and therefore presumably occupying similar
ecological niches—and one of those would be represented only by molariforms and incisors and the other only by premolars and a jaw fragment among the available fossils. Furthermore, they noted that the transition from blade-like to molariform premolars had actually been observed in the fossil record of the extinct
sthenurine kangaroos, and that the first molariform in
Sudamerica and
Gondwanatherium is laterally compressed, suggesting that it may have derived from a blade-like tooth. Gurovich and Beck attributed the difference in shape between the MACN Pv-RN 975 and
Argentodites p4s to the extensive wear of the former, and suggested that the two are similar enough that they probably represent at least closely related species.
Other premolars Krause and colleagues identified a single tooth, MACN Pv-RN 251, as a possible deciduous anterior (i.e., not p4 or dp4, the deciduous version of p4) lower premolar of
Ferugliotherium. It is minuscule, with a length of 0.85 mm and width of 0.5 mm (assuming the tooth is oriented correctly). It bears two serrations (small projections) at the tip of the crown—one around the middle of the crown and the other at what may be the back of the crown, where it is highest. Two prominent ridges descend from each serration towards the front down the sides of the tooth. No roots are preserved, but the rounded surface of the lower side of the tooth suggests they may have been resorbed, which would indicate that the tooth is deciduous. Krause and colleagues suggested that the tooth may have been the frontmost premolar, whether deciduous or permanent. However, Kielan-Jaworowska and Bonaparte wrote that this tooth does not match the partial jaw MACN Pv-RN 975, which has no alveoli in front of p4, and Pascual and colleagues agreed in 1999 that the tooth probably does not belong to
Ferugliotherium. Bonaparte had identified another tooth, MACN Pv-RN 252, as a possible
Ferugliotherium lower premolar in 1990, but this fossil is very fragmentary and according to Krause and colleagues, it cannot even be proven to be a mammalian tooth. Krause and colleagues identified two teeth, MACN Pv-RN 249 and 250, as anterior upper premolars. 249 bears two longitudinal rows of cusps. One row (row A; possibly the lingual one) includes four cusps, the other (row B) includes at least two, but is damaged. In row A, there are three ridges (at the front, middle, and back) extending from the tip of the base of each cusp. The second and third cusps are largest and most widely separated from each other. In row B, one cusp bears three ridges, of which one extends towards the other cusp in the row and the two others towards row A) and the other cusp is damaged. 250 is more fragmentary, but bears at least five cusps and may represent the same tooth position as 249, though it would come from the opposite side of the mouth. The microstructure of the enamel of this tooth has been studied. With a width of about 55 μm near the tip of a cusp, the enamel is thin. The
enamel prisms are straight, small, and rounded and there is little material between the prisms. Small, rounded prisms are also seen in
Gondwanatherium,
Sudamerica, and other gondwanatheres, but in few multituberculates. Even in those multituberculates that do have small prisms, the prism sheath is closed, but the sheath is incomplete in
Gondwanatherium and possibly
Ferugliotherium. Krause and colleagues wrote that these two teeth resemble multituberculate deciduous anterior upper premolars, particularly second and third premolars (P2 and P3), and used this as one of their arguments for identifying
Ferugliotherium as a multituberculate. However, as with the dentary MACN Pv-RN 975, the two upper premolars were excluded from
Ferugliotherium and identified as multituberculates by Kielan-Jaworowska and colleagues after the discovery of the jaw of
Sudamerica. Gurovich continues to identify them as
Ferugliotherium on the basis of their size and provenance and other similarities between
Ferugliotherium and multituberculates.
Lower molariforms Five putative lower molariforms of
Ferugliotherium are known from the Los Alamitos Formation (MACN Pv-RN 20, 174, 175, and 253 and MLP 88-III-28-1). These teeth include the
holotypes of
Ferugliotherium windhauseni (MACN Pv-RN 20, the only second lower molariform, or m2) and
Vucetichia gracilis (MACN Pv-RN 174). The best-preserved mf1 is MLP 88-III-28-1. The crown is unworn and complete and there are no roots, suggesting that the tooth had not yet erupted when its owner died. Krause, who first described the tooth in 1993, identified it as a right molar, but the subsequent discovery of the jaw of
Sudamerica made it clear that
Ferugliotherium molariforms had been reversed, and MLP 88-III-28-1 is actually from the left side of the jaw. The tooth is 2.2 mm long and 1.5 mm wide. The crown is roughly rectangular, with rounded corners, and bears two longitudinal rows of cusps. The lingual row consists of four cusps, which are smaller and lower than the three labial ones. The cusps in this row become smaller and lower from the front to the back. Two ridges descend from the tip of each cusp to the lingual and labial sides. The labial ridges on the first and fourth cusp only reach the base of the cusp, but those on the second and third cusps join ridges descending from the first and second labial cusp. In the first three cusps, the lingual ridge extends to near the lingual margin of the tooth and then turns backward; the end of the ridge is lingual to the next cusp. In the fourth cusp, the ridge hardly extends posteriorly, but rather labially, forming the posterior margin of the tooth and joining a ridge descending from the last labial cusp. The labial cusp row includes three, larger cusps, each of which bears two ridges that descend lingually into the valley between the two cusp rows. The front ridge of each pair ends in the central valley, and the back ridge joins a ridge from a lingual cusp. The ridge pattern results in the presence of three transverse furrows between the main cusps. Another mf1, MACN Pv-RN 253, is almost unworn, but damaged: only the front two lingual cusps and the first two cusps and part of the third in the labial row are preserved. This tooth is similar to MLP 88-III-28-1 in all respects. However, Gurovich suggests that it may also be an m2. MACN Pv-RN 174, which is heavily worn, and MACN Pv-RN 175, which is not only heavily worn but has also undergone severe abrasion, were originally identified as upper molars of
Vucetichia gracilis by Bonaparte in 1990. The roots of MACN Pv-RN 174 are preserved; at the front and back of the tooth, there is a pair of roots, which are fused near their bases. It has small enamel prisms. Krause and colleagues suggested in 1992 that 174 and 175 were mf1s of
Ferugliotherium on the basis of similarities with 253, and Krause confirmed this in 1993 by describing the complete mf1 MLP 88-III-28-1. The related ferugliotheriid genus
Trapalcotherium is known from a single mf1, which is similar to
Ferugliotherium mf1s but different in some morphological details (see
Trapalcotherium: Relationships). The holotype, MACN Pv-RN 20, is a right mf2 according to both Krause and colleagues (1992) and Gurovich (2005), but Gurovich considered the side that Krause and colleagues thought was lingual to be labial, and vice versa. The latter interpretation is used in the following description. It is almost square, but at the front it is slightly narrower than at the back. The labial side of the tooth is taller and less worn than the lingual side. There are two rows of cusps, and each lingual cusp is connected to each labial cusp by a broad crest, with one or more fossas in the middle. One of the two labial cusps may have been divided into two smaller cusps. The two crests are separated by a deep furrow. The enamel prisms of this tooth are small, like those of the premolar MACN Pv-RN 250. Transverse ridges between the cusps, as seen in
Ferugliotherium, are known in only one multituberculate,
Essonodon, but the ridge pattern in
Essonodon is more complicated and the animal lacks the prominent furrows of
Ferugliotherium and differs in numerous other features. On the other hand, overall patterns of cusps and ridges are essentially similar among
Ferugliotherium,
Gondwanatherium, and
Sudamerica, indicating that the three are closely related.
Upper molariforms A single tooth, MACN Pv-RN 248, is currently identified as a
Ferugliotherium upper molariform. In 1992, Krause and colleagues labeled it as a right MF1, but Gurovich identifies it as a left MF1 or possibly even a right mf1.
LACM 149371, an enigmatic tooth from the
Paleogene of
Santa Rosa, Peru, may represent an upper molar of an animal related to
Ferugliotherium. Like the latter, it has cusps that are compressed from front to back and that are connected to the center of the crown by low crests. MACN Pv-RN 248 is somewhat damaged and almost rectangular, but slightly narrower at the back than at the front. The tooth bears three longitudinal rows of cusps. The middle row consists of five cusps, the labial row (assuming it is a left M1; if it is from the right, "lingual" and "labial" should be reversed) includes two or perhaps three cusps, and the lingual row includes probably four cusps. The lingual and middle rows extend across the entire length of the tooth, but the labial row is shorter, extending across about 70% of the length. The middle row is oriented obliquely with respect to the length axis of the tooth, so that it converges with the lingual row towards the back of the tooth. The front lingual corner of the tooth is missing, but it appears that the first cusps in the lingual and middle rows are connected by two ridges, one at the front margin of the tooth and one at the back of the cusps. A deep fossa (basin) lies between the two cusps and their connecting ridges. Behind these two cusps, a transverse furrow extends across the width of the tooth. The second lingual and middle cusps are also connected by a crest, which is somewhat weaker than those connecting the first cusps. Another transverse furrow extends behind the second cusps and also separates the second middle cusp from the labial row. A third furrow, behind the third lingual and middle cusps, also separates the first from the second labial cusp. Three ridges descend from the fourth lingual cusp: one connects to the fourth middle cusp, one ends blindly between the fourth lingual and middle cusps, and one connects to the fifth middle cusp. The second labial cusp, which is larger than the first one, is superficially divided into two smaller cusps by an indentation on its lingual side. There are vertical grooves at the bases of the cusps. ==Range and ecology==