Acrotitan ulanshirehensis Ye, 1983

Mihlbachler, Matthew C., 2008, Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla), Bulletin of the American Museum of Natural History 311 (1), pp. 1-475 : 36-38

Acrotitan ulanshirehensis Ye, 1983

HOLOTYPE: IVPP V6686 , a partial mandible with left p3 and p4.

TYPE LOCALITY: Ulan Shireh Formation, Inner Mongolia, China.

AGE: Middle Eocene (Irdinmanhan land mammal ‘‘age’’)

DIAGNOSIS: Acrotitan ulanshirehensis is a small brontothere with two pairs of large incisors that arch anterior to the canines. The posterior margin of the symphysis extends to the anterior margin of the p3. There is a short postcanine diastema and a long p1–p2 diastema. The molar ribs are weak and the lingual enamel is thinner than the labial enamel.

Acrotitan ulanshirehensis is one of two brontotheres known to have a reduced number of lower incisors. The other, Megacerops coloradensis , is substantially larger and lacks mandibular diastemata. Additionally, Acrotitan ulanshirehensis has an elongate p1– p2 diastema and an obliquely oriented p1.

DESCRIPTION

MANDIBLE AND LOWER DENTITION: The holotype of Acrotitan ulanshirehensis (IVPP V6686) is a partial mandible of a small brontothere. The mandibular symphysis is elongate and narrow. The inferior margin of the mandibular symphysis is shallow and forms an angle less than 45 °. The posterior margin of the symphysis extends to the anterior margin of the p3. The crowns of the anterior dentition are not preserved; however, the alveoli indicate a reduced number of incisors (two pairs). Justification for the dental formula as interpreted here (2-1-4-?) is labeled in fig. 12. From the anterior view of the specimen, two pairs of small alveoli are seen between a pair of very large alveoli. The right pair of small alveoli are empty, although the roots remain in the left pair. The large pair of alveoli also contain roots. The two pairs of small alveoli at the front of the jaw are clearly incisor alveoli. The pair of much larger alveoli that are lateral to the smaller incisor alveoli could either represent a third pair of very enlarged incisors, or, more likely, the canines. Immediately behind the canines is a smaller anteriorly angled dental alveolus that undoubtedly held the p1, which, in brontotheres, is a small, single rooted tooth. Moving posteriorly, the additional alveoli held the anterior and posterior roots of the p2.

The orientation and size of the incisor alveoli indicate relatively small and procumbent incisors that are ovoid in cross section and were probably arranged in a small arch. The central incisors are separated by a short median diastema. The canines were relatively large with rounded cross sections. The postcanine diastema was rather short. The p1 alveolus indicates a simple, single-rooted tooth with a somewhat anteriorly directed orientation. There is a diastema between the p1and p2 that is substantially longer than the postcanine diastema.

The trigonids of p3 and p4 are slightly shorter and narrower than their respective talonids. The paralophid of p3 is short and angled anterolingually. The protolophid is straight and angled about equally lingually and posteriorly. The paralophid and protolophid of p4 are each angled more strongly lingually, creating a large molariform trigonid notch. The lingual enamel of the premolars is thinner than the labial enamel. A portion of the lingual enamel of the p3 is broken off and it is difficult to discern whether this tooth had a metaconid. Typically, brontothere premolars that have strongly lingually angled protolophids, such as this p3, also have metaconids, but it is difficult to confirm that in this specimen. Likewise, the lingual enamel of p4 is broken away. However, from the lateral view of the tooth, the protolophid rises lingually as it stretches from the protoconid toward the junction with the cristid obliqua. This strongly suggests that a p4 metaconid was present. The cristids obliqua and hypolophids of p3 and p4 are long and obliquely angled, thus creating broad and nearly molariform talonid basins. The labial premolar cingulids are distinct although lingual cingulids appear to be absent.

REMARKS

Acrotitan ulanshirehensis is based on a small and rather peculiar brontothere mandible (IVPP V6686). No other specimens have ever been assigned to this species. The brief description provided by Ye (1983) of A. ulanshirehensis is puzzling. Ye (1983) described the posterior margin of the symphysis as terminating anterior to the p2, although the symphysis clearly extends to the anterior margin of the p3. More puzzlingly, the dental formula was described as 3-1-4-3, although there only appear to be two pairs of lower incisors. Moreover, no molars are preserved with the specimen. Thus, the dental formula is 2-1-4-?. Among those brontotheres for which lower incisors (or mandibles with alveoli) are known, only one other brontothere, the large North American Chadronian (late Eocene) species Megacerops coloradensis , is characterized by a similar mandibular dental formula.

In addition to Acrotitan , the following taxa retain a p1–p2 diastema: Eotitanops , Palaeosyops , Mesatirhinus junius , Dolichorhinus hyognathus , and Desmatotitan tukhumensis . Additionally a p1–p2 diastema is occasionally present in Metatelmatherium ultimum and Telmatherium validus . The postcanine diastema of IVPP V6686 is rather long (16 mm), while the postcanine diastemata of these other taxa are typically shorter (usually less than 10 mm), despite the much larger size of most of these species. The p1–p2 diastema of Dolichorhinus , however, ranges up to 19 mm. However, diastema length is quite variable within brontothere species. For example p1– p2 diastema length ranges from 7 mm to 18 mm within D. hyognathus . Therefore, the unusual length of the postcanine diastema of IVPP V6686 might be a characteristic of this particular individual rather than a character of the species, Acrotitan ulanshirehensis . Brontotheres that are similar in size to Acrotitan ulanshirehensis , such as Microtitan , Metatitan , and Pygmaetitan , all lack p1–p2 diastemata.

In addition to its small size and reduced number of incisors, a potentially diagnostically important characteristic of Acrotitan ulanshirehensis is the oblique orientation of p1. In nearly all brontotheres, p1 is typically rooted vertically in the jaw. However in IVPP V6686, the alveolus of p1 is slanted, indicating a posteroventrally rooted p1. The holotype specimen of Desmatotitan tukhumensis (AMNH 21606), a larger brontothere with three incisors, has a similarly angled p1, suggesting these taxa may have a sister relationship.