Duchesneodus californicus ( Stock, 1935 )

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 : 337-340

Duchesneodus californicus ( Stock, 1935)

HOLOTYPE: LACM / CIT 1398 , a mandible fragment with left p3–m2, m3 (partial), and isolated dental elements, including a two incisors, canine, left p1, and p2.

TYPE LOCALITY: LACM /CIT locality 150 in the Brea Canyon Section of the Sespe Formation, north of Simi Valley, California.

AGE: Middle Eocene (Duchesnean land mammal ‘‘age’’).

REFERRED SPECIMENS: (From LACMł CIT locality 150 in the Brea Canyon Section of the Sespe Formation, north of Simi Valley, California) LACM/CIT 973, a left maxilla fragment with P1–P3; LACM/CIT 974–975, isolated teeth; LACM/CIT 1004, a left maxilla fragment with M2–M3; LACM/CIT 1005–1030 isolated cheek teeth; LACM/CIT 1078–1098, canines and isolated cheek teeth; LACM/CIT 1110–1118, isolated cheek teeth; LACM/CIT 1119, a mandible fragment with right p3–p4; LACM/CIT 1120, a partial mandible with right c, left c, and p2–p4; LACM/CIT 1126, a partial mandible with right c–m2 and left p3; LACM/CIT 1127– 1129, isolated teeth; LACM/CIT 1132, a right P1; LACM/CIT 1135–1136, isolated teeth; LACM/CIT 1136, canine; LACM/CIT 1387–1389, partial mandibles with cheek teeth; LACM/CIT 1831–1833, isolated teeth; LACM/CIT 1834, a right maxilla fragment with C–P2; LACM/CIT 1835–1836, isolated teeth; LACM/CIT 4793–4794, partial mandibles and isolated teeth; LACM/CIT 53109– 53214, isolated teeth and partial mandible fragments with teeth.

DETERMINATION: Nomen dubium, possibly a synonym of either Protitanops curryi or Notiotitanops mississippiensis .

DESCRIPTION

The holotype of D. californicus (LACM/ CIT 1398) is an unremarkable specimen, consisting of a mandible fragment with a complete but heavily worn cheek-tooth row, several isolated small globular incisors, and a small isolated canine. The many other specimens found from the same locality are referred to this species based on the assumption of a monospecific assemblage. They are all a consistent size and do not suggest radically conflicting morphologies beyond the normal variability found in other brontothere species (e.g., variable lingual premolar morphology).

The incisors of these specimens are consistently small and globular (upper) or small and semiwedge-shaped (lowers). All of the numerous canines are small. A maxilla fragment LACM/CIT 1834 indicates a very short postcanine diastema (fig. 171a, d). The P1 is complex, with a distinct paracone, a similarly sized metacone, and a small lingual heel with a small lophlike protocone. The P2– P4 are relatively advanced with two distinct lingual cusps, a protocone and hypocone. The variation seen in the lingual premolars is similar to that seen in other species. The hypocone varies from being strongly connected from the protocone by a lingual crest (e.g., LACM/CIT 1013: fig. 171b) to being completely separated without a lingual crest (LACM/CIT 1115: fig. 171c). The P2s tend to have strong preprotocrista, while weaker preprotocrista are discernable on the P3s and P4s. Distinct anterolingual cingular cusps occur frequently on the upper premolars and can be seen on the maxilla fragment (LACM/CIT 1834), but they are occasionally absent in other specimens. Finally, the molars are like those of most other advanced brontotheres with well-developed labial shearing facets, distinct anterolingual cingular cusps, and central molar fossae (fig. 171i, j). The size of the M3 hypocone varies radically, but it is always smaller than the M2 hypocone.

A partial mandible, LACM/CIT 1120, indicates a very short incisor row positioned between the anterior margins of the canines (fig. 171h). The incisors of that specimen are not preserved, although the remaining alveoli suggest only two pairs of incisors. The symphysis extends roughly to the posterior margin of p4 or slightly past it. Close-ups of the lower premolars of LACM/CIT 1126, 1005, and 1119 are shown (fig. 171e–g). The p1 is small with a relatively broad talonid heel. The trigonids of the p2s tend to be nearly twice as long as the talonids. The p2 has a slightly lingually arched paralophid, a straight protolophid, a well-developed cristid obliqua, and hypolophid, and it lacks a distinct metaconid. The p3 are p4 are more nearly molariform with lingually arching paralophids and protolophids, broader talonids, and large, lingually positioned metaconids. The lower molars are typical with thin lingual enamel, weak labial ribs, and an elongate m3.

REMARKS

A large number of specimens consisting mostly of isolated teeth, and partial maxilla and mandible fragments were recovered from the Sespe Formation of southern California. Stock (1935) originally assigned this species to the genus Teleodus . However, upon concluding that Teleodus was an invalid genus, Lucas and Schoch (1982) reassigned this species to Duchesneodus . Subsequently, Lucas and Schoch (1989b) considered D. californicus a junior synonym of Duchesneodus uintensis . However, upon examining the fossil material, I have found it to be inconsistent with Duchesneodus uintensis in several ways. There is a short upper postcanine diastema, whereas in skulls of D. uintensis , a postcanine diastema is absent. The p2 trigonid is much longer than the talonid; in mandibles of D. uintensis , the p2 trigonid is only slightly longer than the talonid. Finally, there appears to have been only two lower incisors, whereas D. uintensis has three. It is always possible that a third incisor was present on LACM/CIT 1120 (fig. 171h), but fell out during life, and the alveolus reabsorbed. Nonetheless, the first two differences more compellingly argue against synonymy of D. californicus with D. uintensis . These characters also rule out Megacerops (sensu Mihlbachler et al., 2004b) and Dianotitan lunanensis .

Other species to which Duchesneodus californicus should be compared are Eubrontotherium clarnoensis , Protitanops curryi , and Notiotitanops mississippiensis . Like D. californicus , these species retain an upper postcanine diastema. E. clarnoensis lacks a p3 metaconid and has three lower incisors; it is not a potential synonym of D. californicus . The lower dentitions of Protitanops and Notiotitanops are not known. However, their upper dentitions are consistent with D. californicus except that the holotypes of these species lack distinct anterolingual cingular cusps on the premolars. Such cusps occur frequently on the upper premolars of D. californicus , but they are not present on every specimen. Considering that Protitanops curryi and Notiotitanops mississippiensis are each known only from a single specimen, this seemingly intraspecifically variable character cannot be used to validate Duchesneodus californicus .

To summarize, Duchesneodus californicus is not synonymous with D. uintensis (contra Lucas and Schoch, 1989b). However, the brontothere material referred to D. californicus is simply too fragmentary to conclusively diagnose it as a distinct species or to adequately compare it to the holotypes of Protitanops curryi or Notiotitanops mississippiensis , although it is possible that either of these species is synonymous with D. californicus . Therefore, Duchesneodus californicus is a nomen dubium.

‘‘ Metatelmatherium (?)’’ browni Colbert, 1938

HOLOTYPE: AMNH 20008 , a left upper molar, probably a M3.

TYPE LOCALITY: Pondaung Formation, two miles northeast of Gyat, Myanmar ( Burma).

AGE: Middle Eocene (Sharamurunian land mammal ‘‘age’’).

DETERMINATION: Nomen dubium, possibly a synonym of Sivatitanops birmanicum .

REMARKS

Colbert (1938) based ‘‘ Metatelmatherium (?)’’ browni on a complete left upper molar that is probably an M3 (fig. 172). Colbert felt that this tooth was conspecific with a molar fragment (GSI C341) that had been referred to ‘‘ Metatelmatherium (?)’’ lahirii (another nomen dubium) by Pilgrim (1925), but also felt that these specimens represented a species that was distinct from other specimens that had been referred to ‘‘ Metatelmatherium (?)’’ lahirii by Pilgrim (1925). However, Colbert (1938) gave no reasons for these opinions.

Colbert (1938) found AMNH 20008 to be comparable to Metatelmatherium and listed similarities and differences of the M3s of Metatelmatherium cristatum (5 ultimum ) and AMNH 20008. He found AMNH 20008 comparable to M. cristatum (5 ultimum ) in crown height, absence of an external cingulum, and a slight ridge on the ectoloph of the paracone (probably meaning a labial paracone rib). All of the specimens used in the comparison given by Colbert (1938) are worn teeth, thus comparison of the crown height of these specimens in essentially meaningless. At any rate, crown height is not a diagnostic character of Metatelmatherium . The presence and thickness of external cingulum is variable among most brontothere species, and a paracone rib is common to nearly all brontothere species. Moreover, the deep central molar fossa and prominent anterolingual cingular cusp of AMNH 20008 indicate that this tooth is not Metatelmatherium , but that of a more derived brontothere.

Additionally, Colbert (1938) compared AMNH 20008 to another molar (GSI C330, an M1 or M2) that Pilgrim (1925) had referred to Sivatitanops cotteri (5 S. birmanicum ). AMNH 26008 was distinguished from that specimen by its ‘‘greater breadth index, and entirely different expression of its ectoloph, protocone and hypocone’’ ( Colbert, 1938: 305). The meaning of ‘‘entirely different expression of ectoloph, protocone and hypocone’’ is unclear, but the primary differences between these molars can be attributed to the fact that AMNH 20008 is an M3 while the other is an M1 or M2. The former molar is more heavily worn, thus affecting the appearance of the ectoloph. Moreover, since the teeth are different elements their proportions are not directly comparable.

All of the upper brontothere molars from the Pondaung Formation that have been attributed to Sivatitanops or Metatelmatherium share deep central molar fossae and large anterolingual cingular cusps. Many brontotheres have similar molars; therefore, none of these isolated molars can be specifically identified. The shape of the hypocone of AMNH 20008 is peculiar; it is compressed against the distal cingulum. In this respect, it is similar the distolingual cingular cusp seen in the premolars of Sivatitanops birmanicum . It is possible that ‘‘ Metatelmatherium (?)’’ browni is a junior synonym of S. birmanicum , but at this time ‘‘ Metatelmatherium (?)’’ browni can considered only a nomen dubium.

Additionally, Colbert (1938) referred two partial mandibles to ‘‘ Metatelmatherium (?)’’ browni (AMNH 20016, 20022). One of these (AMNH 20022) is a juvenile that contains a set of unerupted adult incisors, whose crowns are large, conular, with rounded tips and thick lingual cingulids. Tsubamoto et al. (2000) referred additional mandible fragments to ‘‘ Metatelmatherium (?)’’ browni. However, there is no conclusive evidence as to what species of Pondaung brontothere these elements belong.