Sthenodectes incisivum ( Douglass, 1909 )

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 : 85-93

Sthenodectes incisivum ( Douglass, 1909)

HOLOTYPE: CMNH 2398 , a skull lacking the nasal process with right I1, I2–C (crowns broken off), P1–P4 (partial), M1–M3, left I1– C, and P2–M3.

TYPE LOCALITY: Wagonhound Member (Uinta B) of the Uinta Formation, Northeast of Well #2 Uinta County, Utah.

SYNONYMS: Sthenodectes priscus Peterson, 1934 .

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

REFERRED SPECIMENS: (From the Wagonhound Member of the Uinta Formation of Utah) CMNH 9928 , right and left maxillary fragments with right P2–P3 (partial) and left P3–P4 (partial) ; CMNH 11437 (holotype of Sthenodectes priscus ), a partial skull with right C–M3, left P3–M3 (all partial), and an associated mandible with right i1–i2, c–m3, and left i1–m3 ; FMNH P12166 , a mandible with complete dentition ; FMNH P12165 (mistakenly referred to P12168 by Osborn [1929a]), a partial skull with complete dentition; (from the Washakie Formation of Wyoming) YPM 16883, a partial mandible with right i2–p2, p3 (partial), p4, m1(partial), left i1–i2, and c–m3.

DIAGNOSIS: Sthenodectes incisivum is an intermediate-sized hornless brontothere in which the frontal bone intrudes into the nasal bone, thus splitting off a small lateral splint of nasal bone from the main body of the nasal. The nasal splint and overlapping frontal process are small and the suture is not often discernable. The nasal incision extends as far back as the P1. The nasal process is very short, horizontal, unelevated, of relatively constant transverse width, narrow, and with thin and moderately deep lateral walls. The orbits are positioned above M2 and protrude laterally, but as strongly as in Metarhinus . The lateral margin of the premaxillomaxillary rostrum deepens posteriorly and the rostral cavity is not sealed dorsally. Other cranial characteristics include a strongly concave midcranial dorsal surface, a flatter posterior cranial surface, a sagittal crest, strongly curved zygomatic arches, and a ventrally open and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae, infraorbital processes, and a ventral zygomatic flange are all absent.

Dentally, Sthenodectes incisivum has oversized subcaniniform incisors, simple P1 that is surrounded on all sides by cingula, and a distinct P2 metacone. The lingual features of the P2–P4 are tall, and the lingual premolar cingula are thick and continuous. Other dental characteristics include weak premolar preprotocristae, and short lingual crests extending posteriorly from the premolar protocones. Premolar hypocones are absent. The upper molars of S. incisivum have tall, lingually angled ectolophs with weak labial ribs and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. A cingular parastyle shelf is absent. Central molar fossae and anterolingual cingular cusps are present, while paraconules and metalophs are absent. The lower dentition of S. incisivum is characterized by oversized subcaniniform incisors of uniform size, no p1–p2 diastema, a metaconid on p4 but not on p2 or p3, shallow molar basins, and a slender m3.

Only two brontotheres have greatly enlarged incisors, Sthenodectes incisivum and Pygmaetitan panxianensis . P. panxianensis is significantly smaller than Sthenodectes incisivum and it has a more complex p1.

DESCRIPTION

SKULL: The holotype of Sthenodectes incisivum (CMNH 2398) is a nearly complete skull (fig. 34). The only significant missing portion is the nasal process. The skull has been dorsoventrally flattened, thus distorting the shape of the skull from a lateral view. Earlier figures in Douglass (1909) misleadingly depict a seemingly undistorted skull. The shape of the skull is less distorted from a dorsal view (fig. 34b) although the braincase has collapsed inwardly, exaggerating the height of the sagittal crest. Two other skulls of S. incisivum are known. These include CMNH 11437 (holotype of S. priscus ) (fig. 35) and FMNH P12165 (fig. 36a). The skull of CMNH 11437, though less complete, is not dorsoventrally crushed and it offers a more faithful representation of the shape of the skull from a lateral view. The dorsal surface of FMNH P12165 is not preserved, but that skull is uncrushed as well, and its ventral surface is the best of the three skulls.

Sthenodectes incisivum is a medium-sized hornless brontothere. Osborn (1929a: fig. 301) labeled a ‘‘horn’’ and Peterson (1934) discussed ‘‘horn swellings’’, however, there are no hornlike protuberances in the holotype or any other specimen of S. incisivum . In CMNH 2398 there appears to be a thickened ridge of bone in front of the orbit that arches over the maxilla. This feature is not seen in the undistorted specimen, CMNH 11437. Portions of the facial sutures can be seen on CMNH 2398 and, although they are indistinct, they reveal the specialized facial configuration where the frontal bone intrudes into the nasal bone. Part of the right frontonasal suture can be seen on the dorsal surface. The frontal bone projects anteriorly and splits off the posterolateral portion of the nasal bone from the main body of the nasal. From the lateral view of the right side of the skull (not shown) a short lateral nasal splint can be seen dividing the frontal bone and maxilla, although it is indistinct and difficult to see without carefully examining the actual specimen. Note that Douglass’ (1909: fig. 1) original figure of the holotype does not accurately portray the shape of the frontonasal contact.

The nasal incision is very short and most similar to those of Metatelmatherium ultimum and Wickia brevirhinus . This can most easily be seen on CMNH 11437, in which the nasal process is complete. In that specimen the nasal incision extends as far back as the P1. The orbit is situated directly above the M2 and the posterior portion of the M1. The anterolateral root of M1 is positioned below the anterior rim of the orbit. The region of maxillary bone between the orbits and nasal incision forms a shallow concavity.

In the holotype (CMNH 2398), there is a triangular vacuity on either side of the skull just in front of the orbit. Douglass (1909) described these as a feature of Sthenodectes incisivum . However, Gregory (1912) and Osborn (1929a) concluded they were an artifactual result of damage, a conclusion that is borne out by other specimens that are undistorted and lack these vacuities.

CMNH 11437 has a complete nasal process. The nasal process is horizontal, relatively narrow, and it is slightly shorter than the premaxillomaxillary rostrum. The sides form shallow lateral walls. The anterior edge of the nasal process is not preserved.

The premaxillomaxillary rostrum of Sthenodectes incisivum is large and thick. The robust condition of the rostrum relates to the enlarged anterior dentition of this species. The premaxillae are completely fused at the midline. There are no discernable premaxillomaxillary sutures in the holotype. In FMNH P12165 an indistinct premaxillomaxillary suture indicates that the premaxilla terminates about at the posterior notch of the nasal incision. The premaxilla does not contact the nasal. From the lateral view the dorsal margin of the rostrum slopes posterodorsally. From the anterior view it can be seen that the sides of the rostrum diverge posterolaterally and the rostral cavity is dorsally open.

From a lateral view the dorsal surface of the skull is concave midcranially, but it becomes flattened or even slightly convex posteriorly. From a dorsal view the skull is broad between the orbits. The orbits protrude laterally to a degree similar to Fossendorhinus , but not to the extreme degree seen in Metarhinus . The parasagittal ridges converge and form a true sagittal crest, although the height of the sagittal crest of the holotype is exaggerated by the crushed braincase. The zygomatic arches are broad and bowed from the dorsal view. From a lateral view the jugal portion of the zygomatic arch is shallow and horizontal. The squamosal portion is deeper and angled posterodorsally, giving the zygomatic arch a strong curvature. Sthenodectes incisivum lacks an infraorbital process like that seen in Metarhinus . It also lacks an enlarged ventral zygomatic flange as seen in Metatelmatherium .

The nuchal crest is thin and from a dorsal view it is angled anteromedially. The occiput is only slightly tilted backward. From a posterior view, the occiput is not well preserved. However, it seems that the dorsal portion of the occiput is somewhat narrower than the ventral portion. The dorsal margin of the occiput is arched moderately. There are distinct occipital pillars on the posterior surface of the occiput; however, the deeply recessed pit in the center of the occiput between the occipital pillars is exaggerated by distortion.

The ventral surface of the skull of Sthenodectes incisivum is best preserved in FMNH P12165 (fig. 36a). The anterior rim of the posterior nares is positioned anterior to the M3. There is a narrow emargination around the anterior and lateral sides of the posterior nares. The posterior narial canal is elongate and extends posterior to the pterygoid processes, but it does not continue into the sphenoid. A remnant of the thin vomerine septum is preserved at the posterior end of the posterior narial canal of the holotype, but it is not preserved in FMNH P12165. The posterior narial canal of FMNH P12165 is partially filled with sediment. In the holotype (CMNH 2398), the posterior narial canal is mostly cleaned of matrix. In that specimen, the anterior half of the posterior narial canal is lined with a surface of wavy bone with anteroposteriorly directed grooves. It is possible that this wavy layer of bone represents maxilloturbinates continuing into the posterior narial canal, similar to that seen in Dolichorhinus ; however, the dorsoventral crushing of the holotype limits interpretation.

In the basicranium of the holotype skull (CMNH 2398) the mastoid process contacts the posterior wall of the postglenoid process, thus forming a tubelike external auditory pseudomeatus. Gregory (1912) and Osborn (1929a) noted this characteristic; however, the ventrally closed external auditory pseudomeatus of the holotype appears to be an artifact of distortion. In the less distorted specimen, FMNH P12165 (fig. 36a), the postglenoid and mastoid processes do not contact each other, thus leaving a ventrally open external auditory pseudomeatus. In other respects, the basicranium of Sthenodectes incisivum is typical of brontotheres with a widely separated foramen ovale and foramen lacerum.

UPPER DENTITION: The upper dentition of the holotype (CMNH 2398) and that of FMNH P12165 are essentially the same although those of the later are more complete and less worn (fig. 36). Dentally, Sthenodectes incisivum is most easily distinguished by its large teeth, particularly the oversized incisors and canine that rival the premolars in size. The incisor row forms a semicircular arch that is positioned anterior to the canines. The anterior teeth are positioned closely together and there are no diastemata in the entire dental battery. The crowns of the I1s converge medially and make contact at the midline. The incisors progressively increase in size laterally. The apices of I1 and I2 are worn slightly, but all three incisors appear to have been tall and essentially subcaniniform in shape. All three upper incisors display a broad lingual cingulum. The lingual heel of I1 is thick and tall, forming a large fossa in the center of the crown between the main cusp and the lingual heel. In more distal incisors the central incisor fossa is less prominent. The canine is extremely tall and pointed and it has a distinct posterolingual cingulum. The canine of CMNH 11437 is smaller than the canines of FMNH P12165 or CMNH 2398.

There is no postcanine diastema in FMNH P12165 or in CMNH 2398 although the P1s are placed somewhat medially to the canines. The lateral view of CMNH 11437 suggests a short postcanine diastema. This is partially related to the medial placement of the P1. Additionally, the canine appears to be displaced slightly in this specimen.

The P1 is poorly preserved in the holotype, but that of FMNH P12165 is intact. The P1 is a small tooth with a single cusp and a posterior heel. However, the P1 is unusual in that the crown is completely encircled by a cingulum. The same characteristic morphology is seen in CMNH 11437. The P2–P4 are essentially rectangular in outline although the lingual margin of P2 is rounded. The parastyle of P2 is directed anteriorly while the parastyles of P3 and P4 are directed slightly labially. The metacone of P2 is positioned directly posteriorly from the paracone. The metastyle of P2 arches somewhat lingually while those of P3 and P4 are nearly straight. There are distinct labial paracone ribs on the P2–P4 that become progressively smaller in more posterior premolars.

The lingual features of the P2–P4 are unusually tall, although photos of the occlusal surfaces do not clearly reveal this character. On each premolar there is a tall protocone, but hypocones are not present. On the P2 there is both a small preprotocrista as well as a short lingual crest extending posteriorly from the protocone. On the P3 these crests are less distinct and in P4 they are essentially absent. This lingual premolar morphology of the holotype differs slightly. In that specimen the lingual side of the P2 has a single ovoid loph that arches around the lingual side of the crown. On the P3 there is a similar structure, but it could best be described as an ovoid protocone. The maxillary fragments of CMNH 9928 offer a glimpse at the unworn lingual morphology of the P3 and P4. The lingual morphology is similar to that of the other specimens, but a very short thin lingual crest can be seen extending posteriorly from the P4 protocone. This crest would be quickly obliterated by wear and could have originally been present in premolars of specimens with more dental wear. The labial cingula of P2–P4 tend to be continuous or slightly discontinuous. The lingual cingula, on the other hand, are always thick and continuous.

The molars of Sthenodectes incisivum exhibit numerous brontotheriine apomorphies, including tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in molars that are not heavily worn. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. S. incisivum molars exhibit both small anterolingual cingular peaks and shallow central fossae. There are no traces of paraconules of metalophs, nor is there a hypocone on the M3, although the posterolingual cingulum of the M3 can be very thick. The labial cingulum is distinct, but it is discontinuous around the mesostyles. Unlike the lingual premolar cingula, the lingual molar cingula are weak.

MANDIBLE AND LOWER DENTITION: Fortunately, one skull of Sthenodectes incisivum (CMNH 11437) is associated with a mandible (fig. 37a, b). That specimen reveals that the lower incisors, like the uppers incisors, are greatly enlarged. Other mandibles are referable to S. incisivum due to their large incisors, including YPM 16883, a partial mandible with very lightly worn lower dentition (fig. 37c–f).

The inferior margin of the symphysis is steep ($ 45 °). The position of the posterior margin of the symphysis fluctuates between the midpoint of the p3 and the posterior margin of p3. The lower incisors form a semicircular arch anterior to the canines. The crowns are distinctly tall and pointed with very thick lingual cingulids. The incisors are all of similar size although the i3 is shorter and more mesiodistally elongate than the other incisors. The canine is very tall with a thick lingual cingulid.

There are no diastemata throughout the entire lower dentition of YPM 16883. However, in other specimens, FMNH P12166 and CMNH 11437l, there are short postcanine diastemata. This is perhaps related to the fact that the canines of these specimens are smaller that those of YPM 16883.

The p1 is a small single-cusped tooth with a slightly elongate talonid heel. The trigonid of p2 is nearly twice as long as the talonid. The p3 trigonid is somewhat longer than the talonid, while the p4 trigonid is similar in length to the talonid. The talonid and trigonid are of similar width in p2, while in p3 and p4 the talonid is broader than the trigonid. The paralophid of p2 is incomplete, but other specimens indicate that it arches slightly lingually, resulting in a small lingual trigonid notch. The protolophid is straight but positioned lingually. The trigonid of p3 is similar to that of the p2, but the lingual notch is somewhat larger and the protolophid is more lingually directed. The paralophid and protolophid of p4 arch 90 ° lingually. The protolophid of p4 ends in a large metaconid, while metaconids are absent on p2 and p3. The talonid of p2 has a well-developed cristid obliqua, a short hypolophid, and a weakly developed lingual notch. The talonids of p3 and p4 are more strongly developed with longer hypolophids, longer cristids obliqua, and more molariform basins. The p1 and p2 have distinct lingual and labial cingulids. The p3 and p4 lack lingual cingulids but retain labial cingulids.

The molars are typical advanced brontothere molars with thin lingual enamel, weak lingual ribs, an elongate m3, and a widened m3 hypoconulid. The labial molar cingulid is thick but variably continuous and discontinuous around the paracone and hypocone.

REMARKS

When Douglass (1909) named Sthenodectes incisivum he questionably referred it to the genus Telmatherium , although he commented that it probably was a different genus. Douglass’ (1909) description of ‘‘ Telmatherium ?’’ incisivum was brief and insuffi-

Fig. 37. Selected views of mandibles referred to Sthenodectes incisivum . (A) Left view and (B) dorsal view of CMNH 11437. (C) Left molars, (D) left premolars, (E) labial view of right incisors and canine, and (F) lingual view of right incisors and canine of YPM 16883. (º 2005 Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. All rights reserved.)

cient. Gregory (1912) made a more thorough comparison of the holotype with other brontotheres and concluded that it represent- ed a new genus, and, thus, revised the name to S. incisivum . Gregory (1912) distinguished S. incisivum from Metatelmatherium ultimum by (1) the enlarged incisors, (2) the absence of a postcanine diastema, (3) thickened premolar cingula, (4) the ventrally enclosed external auditory pseudomeatus, (5) tapering nasals, and a few other less significant details. Gregory’s (1912) observations generally distinguished S. incisivum from all known brontotheres, except for character four, an error related to taphonomic distortion of the holotype skull. In the same year, Riggs (1912) referred two additional specimens to S. incisivum , a skull (FMNH P12165, mistakenly referred to as P 12168 by Osborn [1929a]) and a mandible (FMNH P12166).

Peterson (1934) named another species, Sthenodectes priscus , based on CMNH 11437. Peterson distinguished this species from S. incisivum based on the rounder shape and thinner cingulum of the canine, less hypsodont dentition, less developed cingula, and a short diastema. Each of these characteristics is refutable. The rounder shape of the canine and its thinner cingulum are related to the smaller size of the canine. Canine size is variable within many brontothere species and, thus, does not warrant a distinct species. Although the premolar cingula of S. incisivum tend to be thick in comparison to other species, the minor differences in the thickness of the lingual premolar cingulum between CMNH 11437 and CMNH 2389 are consistent with a pattern of intraspecific variability in cingula thickness seen in most brontothere species. Finally, the apparently less hypsodont dentition has to do with the fact that the teeth of CMNH 11437 are more worn than are those of CMNH 2398. Therefore, S. priscus is considered a junior synonym of S. incisivum .