Protitanops Stock, 1936
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https://doi.org/ 10.26879/944 |
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persistent identifier |
https://treatment.plazi.org/id/039F87B6-FFA6-FFC8-2F12-FE4CFBAE1023 |
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treatment provided by |
Felipe |
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Protitanops Stock, 1936 |
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Genus Protitanops Stock, 1936
Type and only known species. Protitanops curryi Stock, 1936 .
Figures 3–10 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 View FIGURE 10
Diagnosis. Protitanops curryi is a large-bodied brontothere with fronto-nasal horns that shares with other large horned brontotheres, Eubrontotherium clarnoensis , Notiotitanops mississippiensis , and Parvicornus occidentalis , the combination of an upper postcanine diastema and two pairs of upper incisors. P. curryi has large lateral zygomatic swellings that are not present in E. clarnoensis and P. occidentalis . P. curryi has more posteriorly positioned orbits and a less posteriorly extended nasal incision than N. mississippiensis .
Holotype. LACM / CIT 1854 About LACM , a skull with complete dentition and a partial mandible with right m3, left m1 (partial), and m2-m3.
Holotype locality. Lower red beds of the Titus Canyon Formation in canyon east of Thimble Peak, Grapevine Mountains , California (Stock, 1936).
Stratigraphic horizon and age. Late Eocene (late Duchesnean and early Chadronian land mammal “ages”)
Referred specimens. (From the Lower Chisos Formation, Big Bend National Park, Brewster County, Texas) TMM 40932-1, a partial skull with right P2-P3 (partial) and left P2-P4 (referred to Menodus bakeri by Wilson, 1977a); (from the “titanothere quarry”, Lower Chisos Formation, Big Bend National Park, Brewster County, Texas) TMM 41916, a skull with complete dentition except the mesial incisor pair; TMM 41916-8, a partial juvenile skull with partial right dP2-dP4, left dP3 (partial), left dP4, unerupted right and left M1s; TMM 41916- 14, a transversely compressed juvenile skull and mandible in articulation with deciduous upper and lower incisors, canines, left cheek teeth including P1, dP2-dP4, p1, dp3-dp4, and unerupted adult molar crown; TMM 41916-19, a partial skull with right and left P2-M3; (from the Vieja Area, Chambers Tuff, Presidio County, Texas, Porvenir Local Fauna) FMNH PM 138, a dorsoventrally compressed skull with heavily worn and damaged cheek teeth; FMNH PM 157, a complete skull with right P3-M1, left C, P1-P4, M2, M3; FMNH PM 402, edentulous right and left mandibular rami; (from the Vieja Area, Blue Cliff Horizon of the Chambers Tuff, Presidio County Texas, Porvenir Local Fauna) FMNH P 137, a skull with heavily worn right and left cheek teeth; FMNH PM 139 a skull with heavily worn right and left cheek teeth and right and left mandibular rami with broken teeth; FMMH PM 168, mandible with right p3-m3; FMNH PM 205, a partial skull with partial right and left M1-M3; FMNH PM 398, a partial skull, right C, P2-M3, left C, P4-M3; FMNH PM 401, a mandible with complete symphysis, incisor row and damaged cheek teeth; FMNH PM 399, partial mandible with right c, p1-p4, m1- m2 (partial), left p1-m3; (from the From Vieja Area, Big Red Horizon of the Chambers Tuff, Presidio County, Texas, Porvenir local fauna) TMM 45805- 1, a partial mandible with complete symphysis, right I1-I3, C, P1-P4, M1-M2 (partial), left I1-I3, C, P2-M2, M3 (partial); (from the Upper Conglomerate, Prietos Formation, Locality 1, 32 km NW Ojinaga, Chihuahua, Mexico, Rancho Gaitan local fauna) IGM 65-29, a partial skull with right P2-M3; (from the Vieja Area, Chalk Gap Draw, Presidio County, Texas, Little Egypt Local Fauna) TMM 40840-38, a partial mandible with damaged cheek teeth; TMM 40840-39 a partial skull with damaged teeth.
Description
Skull. The type specimen (LACM/CIT 1854) is a complete skull and partial mandible ( Figure 3A View FIGURE 3 ) that is more extensively described and figured elsewhere (Mihlbachler, 2008). The description below emphasizes new information based on the newly referred specimens from Texas and Mexico. Numerous whole and partial skulls have been recovered from the Chisos Formation and the Chambers Tuff of the Vieja area from West Texas. The most complete Chambers Tuff skull (FMNH PM 157) is undistorted and best preserves the overall shape of the cranium ( Figure 4 View FIGURE 4 ). Other Vieja specimens (FMNH PM 137, 138, 139, 205, and 398) are skulls with advanced stages of dental wear. These specimens provide important details on phenotypic variation in adult P. curryi . The Chisos material is taphonomically deformed to a greater extent but finer details of bone and teeth are preserved. The Chisos material includes more immature individuals including several dentally subadult specimens with deciduous teeth (TMM 41916-14, Figure 5 View FIGURE 5 ) (TMM 41916-8, Figure 6 View FIGURE 6 ), other specimens with lightly worn adult dentitions (TMM 41916-6; Figure 7 View FIGURE 7 ), (TMM 41916-19), and one older adult with more heavily worn teeth (TMM 40932-1, Figure 8 View FIGURE 8 ). Additional specimens of P. curryi include the partial adult skull from the Rancho Gaitan local fauna (IGM 65-29, Figure 9 View FIGURE 9 ). This specimen has experienced some dorsoventral flattening, and it is lacking the anterior dentition, the occiput, and much of the left side of the skull.
The frontonasal horns of P. curryi are short knobs, project dorsolaterally and slightly anteriorly, and they are elliptical with the longest axis in the antero-posterior direction. The horns of the adult specimens are observed to range from 80-140 mm in length from the proximal base to the apex with observed circumferences ranging from 220-300 mm ( Table 1). As with other brontotheres, horn size appears related to ontogenetic age. In the subadult skulls (TMM 41916-14, 41916-8) ( Figures 5 View FIGURE 5 , 6 View FIGURE 6 ) horns are in evidence but they are very small protrusions with between 10-20 mm of relief. In those specimens, the proximal base of the horn, formed by the nasal bone, is unfused to the horn apex, which is formed by a thickened anterior projection of the frontal bone. One Chisos skull (TMM 41916- 6) is an ontogenetically younger specimen with erupted but unworn M3s. This specimen has small incompletely co-ossified frontonasal horns. The proximal base of the horn, formed by the nasal bone, is unfused to the horn apex which is formed by a thickened anterior projection of the frontal bone. In more dentally mature specimens (Figures
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3, 4, 8) the frontonasal horns are fully co-ossified, and the frontonasal interface is no longer evident. The variation seen between young and older specimens here is a typical ontogenetic trajectory observed in other species of horned brontotheres (Mihlbachler, 2007b, 2008, 2011; Mihlbachler and Deméré 2009). Most of the specimens referred to Protitanops curryi are larger, more robust “malelike” specimens (FMNH PM 137, FMNH PM 138, FMNH PM 139, FMNH PM 157; IGM 65-29; TMM 40932-1). A smaller number of individuals (FMNH PM 205, FMNH PM 398) have smaller, shorter horns, although in both of these specimens the horns are insufficiently preserved for accurate measurement.
The anterior margin of the nasal process sometimes has a median notch where the right and left nasal bones are incompletely fused. In more
MIHLBACHLER & PROTHERO: TEXAS BRONTOTHERIIDAE dentally advanced adults this notch is absent and the nasals are more fully co-ossified.
The orbit of the type specimen is positioned directly above the posterolateral root of M1 and the anterolateral root of M2. In the Texas specimens, the orbit is similarly positioned, but can be slightly more anteriorly positioned (TMM 41916-6) but not to the extent that it alters how phylogenetic character 13 (related to the position of the orbit) is coded. Similarly, minor amounts of intraspecific variability in the position of the orbit with respect to the molars is observed in Duchesneodus uintensis and Parvicornus occidentalis (Mihlbachler, 2008; Mihlbachler and Deméré, 2009).
The type specimen from California has the most deeply concave dorsal profile of all the skulls. In Texas specimens (TMM 41916-6, FMNH PM 157), the dorsal surface of the skull is less deep but still convex (saddle shaped) from above the orbits to the dorsal rim of the occiput. Taphonomic dorsoventral flattening may distort the apparent depth of the concave dorsal profiles of some of these skulls.
The degree to which the parasagittal ridges converge posteromedially and constrict the posterodorsal surface of the skull is intraspecifically variable in Protitanops curryi and other brontotheres (Mihlbachler, 2011). The degree of constriction is most pronounced in the type specimen and Chisos specimens ( TMM 41916-6 View Materials , Figure 7 View FIGURE 7 ; TMM 40932-1 View Materials , Figure 8 View FIGURE 8 ). The parasagittal ridges are more widely spaced in the subadult Chisos specimen ( TMM 41916-8 View Materials , Figure 6 View FIGURE 6 ), other adult Vieja specimens ( FMNH PM 137 ; FMNH PM 157 , Figure 4 View FIGURE 4 ; FMNH PM 205 ; FMNH PM 398 ), and the Prietos skull ( IGM 65-29 View Materials , Figure 9 View FIGURE 9 ) .
The zygomatic arches of P. curryi are expanded laterally with swellings at the junction of the jugal and squamosal. The swellings range from 40–98 mm in thickness. Protitanops curryi was previously described as having a straight zygomatic arch (Mihlbachler, 2008). This assessment was based on the left zygomatic arch of the type specimen. However, the left zygomatic arch of the type is damaged and has been partly reconstructed with plaster. On the right side of the type specimen, and in Texas specimens with whole, undistorted zygomatic arches, the temporal part of the zygomatic arch is angled posterodorsally, giving the zygomatic arch a curved appearance from lateral views of these skulls. The degree of curvature is variable, and seems more pronounced in the more robust specimens suggesting it could be an aspect of sexual dimorphism.
From a dorsal view, the nuchal crest (the postero-dorsal edge of the skull) is thicker and more rugose in some specimens ( Figure 4 View FIGURE 4 ), and thinner and smoother in others. The nuchal crests of all of the Texas skulls have a broadly concave curvature when viewed from a dorsal perspective. The nuchal crest of the type specimen is more sharply notched mesially, although this difference may be a consequence of damage; parts of the bone surface appear to have been broken away along the rim of the nuchal crest. From a posterior view, the occiput of FMNH PM 157 ( Figure 4E View FIGURE 4 ) closely resembles the type; the dorsal portion of the occiput is similar in width to the ventral portion, and it is slightly constricted in the middle. The occipital pillars are similarly large with a deep central depression. The occiput of TMM 41916-6 View Materials ( Figure 7D View FIGURE 7 ) is shaped differently; the dorsal portion of the occiput is narrower than the ventral portion and it is less constricted in the middle .
In the type specimen, the posterior edge of the palatine bone (which forms the anterior margin of the posterior nares) is between the right and left M3s, just slightly posterior to the protocones. This same margin in the Texas specimens is consistently more anteriorly positioned, and there is some variation in its exact position with respect to the dentition. Some of this variation may be ontogenetic and relate to remodeling of the skull in association with eruption and wear of the dentition throughout life. The type specimen, in which the posterior nares are the most posteriorly situated among all known P. curryi skulls, is also one of the most ontogenetically advanced specimens, based on the state of dental wear. TMM 41916-19 View Materials (unfigured) is ontogenetically the youngest individual in the sample with a completely erupted set of adult dentition, with an unworn left M3 and barely worn right M3. In this specimen, the anterior margin of the posterior nares is positioned anterior to the M3s. In other adult Texas specimens, including the young adult TMM 41916-6 View Materials ( Figure 7C View FIGURE 7 ) and more advanced adults with more substantial dental wear, the anterior margin of the posterior nares is between the M3 protocones or slightly anterior to the protocones .
The elongate posterior narial canal, a feature associated with the elongated postorbital cranium of brontotheres, is divided by an elongate thin vomer. The vomer is damaged or unpreserved in most specimens, but can be readily seen in the ventral view of TMM 41916-6 ( Figure 7C View FIGURE 7 ). In that specimen, sediment still fills the roof of the posterior narial canal but it has been fully cleared of sediment at its posterior end. The vomer continues in the posterior narial canal and terminates posteriorly at the midline of the skull between the foramina ovale of the basicranium.
The tube-shaped external auditory pseudomeatus is visible in all the large adult specimens. This shape is created by the mastoid process, which curves anteriorly and contacts the postglenoid process, forming an enclosed canal.
Upper dentition. The upper dental formula of Protitanops curryi is 2/1/4/3. The type of P. curryi has two pairs of very small amorphous incisors separated by a brief mesial diastema. The dentition of TMM 41916-6 is figured in closeup ( Figures 7 View FIGURE 7 F- G). Several Texas specimens have incompletely preserved incisors and or incisor alveoli that indicate the same dental formula with two very small incisor pairs separated by a mesial diastema and positioned along the anterior edge of the canines in a straight row. The upper canine crowns are 24-30 mm diameter. The canine diameter of the type specimens as at the lower end of this range (23 mm). A short postcanine diastema is consistently present and ranges from 5-11mm. The persistence of a postcanine diastema differentiates P. curryi from Dianotitan lunanensis , Duchesneodus uintensis , Megacerops coloradensis , and M. kuwagatarhinus , which all lack an upper postcanine diastema.
The premolars of the holotype lack some character information due to wear. The adult premolars of TMM 41916-6 are less worn ( Figure 7E View FIGURE 7 ). The P1 is smaller than more posterior premolars, with a more obliquely angled mesial edge. Two labial cusps (paracone and metacone) are present. The lingual side of the P1 forms a small lingual heel with two small but distinct cusps, protocone and hypocone, that are connected by a short crest. The P2, P3, and P4 are more rectangular in outline with more nearly parallel anterior and posterior sides. The parastyle of P2 extends anteriorly while the parastyles of P3 and P4 are deflected progressively more labially. The premolar metastyles are consistently directed posteriorly. A P4 mesostyle, which is variably present and absent in some species (e.g., Eubrontotherium clarnoensis , Duchesneodus uintensis , Parvicornus occidentalis ) does not occur on any of the specimens referred to Protitanops curryi . Small preprotocristae are discernable on the P2-P4. On P3 and P4, the preprotocristae are visible as small crests or beads of enamel that connect the protocone to the lingual base of the protocone. The lingual cusps of the P2 of TMM 41916-6 are connected by a thin, poorly developed crest. The lingual cusps of the P3 and P4 of the same specimen are fully separated, and no lingual connecting crests are seen. In other specimens (FMNH PM 157, TMM 40932-1, Figure 7D View FIGURE 7 ; TMM 41916-19) the lingual cusps are more strongly connected by a connecting crest. This crest tends to be tallest on the P2 and becomes progressively shorter in more posterior premolars. The variable morphology of the lingual sides of the premolars is consistent with patterns of premolar variation seen within many other brontothere species (Mihlbachler, 2008).
The molars of the type are heavily worn, although the lesser worn molars of TMM 41916-6 ( Figure 6G View FIGURE 6 ) and other specimens more fully reveal a complete suite of molar traits. The molars of Protitanops curryi lack diagnostic autapomorphies and show a suite of conserved characteristics that characterize members of the subfamily Brontotheriinae , including tall, lingually angled ectolophs, very weak labial ribs, and thin lingual ectoloph enamel. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. The lingual margins of the paracone and metacone are wedge shaped. Distinct central molar fossae and anterolingual cingular cusps are seen on all three molars. As in many other brontotheres, the hypocone of the M3 is variable. In the type, there is no distinct hypocone on M3, although a crest extends from the distolingual corner of the M3 to the lingual base of the metacone. The M3 of TMM 41916-6 has a distinct hypocone that is smaller than the hypocones of more anterior molars. FMNH PM 157 ( Figure 4C View FIGURE 4 ) has a larger, more molariform hypocone. Most other Texas specimens have intermediately sized hypocones.
Mandible and lower dentition. The type of P. curryi includes a poorly preserved mandible with heavily worn and incompletely preserved dentition. FMNH PM 139 includes an associated skull and right and left mandibular rami. However, these rami are incomplete, edentulous and, on the whole, not very informative. However, a number of mandibles from the Vieja area (FMNH PM 168, 399, 401, 402; TMM 40840-38, 45805-1) have been recovered, and although these are unassociated with cranial remains, the dimensions of these specimens are above the size range of Duchesneodus uintensis , the only known brontothere whose stratigraphic ranges overlap. They are similar in size and morphology to the type mandible of P. curryi and are therefore referred to that species.
The best preserved and most character-rich mandible referred to Protitanops curryi is TMM 45805-1 ( Figure 9 View FIGURE 9 ), which consists of a complete mandibular ramus with right and left anterior dentition and premolars. The transverse width of this mandible at the canines is similar to the width of the mandible at the P2. In lateral view, the inferior margin of the symphysis is angled slightly less than 45 degrees from the horizontal and its slightly convex shape transitions smoothly into the inferior ventral surface of the horizontal ramus. Other specimens have similarly shaped and proportioned mandibles but they are less finely preserved. The posterior margin of the mandibular symphysis of TMM 45805-1 is positioned between the posterior edges of P4. In other mandibles (FMNH PM 399, 401), the posterior margin of the symphysis is just slightly anterior to the posterior edge of P4. The observed fluctuation in the length and position of the symphysis with respect to the dentition is minor and easily fits within a single character state (Character 71, state 3). Similar variation is seen in other brontothere species and may relate to remodeling of the mandible in association with eruption and wear of the dentition throughout life.
The mandibular dental formula of Protitanops curryi is 3/1/4/3. Although the incisors and canines are not preserved in the type, two of the Texas specimens (FMNH PM 401, TMM 45805-1) retain five lower incisors (two right and three left). In both specimens the right i3 appears to have been lost in life with the alveolus having been lost due to subsequent bone remodeling ( Figure 10C View FIGURE 10 ). The incisor row forms a subtle arch between the canines. The incisor row is less arched than that of Parvicornus occidentalis and more similar to the flatter incisor row of Eubrontotherium clarnoensis . There are no diastemata anterior to the canines and even the space that the missing right i3 would have been occupied has been closed. The i1 and i3 are small and of similar size, while the i2 is larger. The apices of the incisor crowns are worn flat; however, the labial edges of the incisor crowns have a wedge-shaped appearance. Each incisor has a distinct lingual heel. Similarly, the canines of TMM 45805-1 are worn flat. Lower canine size, which is likely to be a sexually dimorphic trait in brontotheres (Mihlbachler 2011), is variable, with crown diameter ranging from 19-36 mm. All mandibles retain a postcanine diastema that is roughly similar in length or slightly longer than the antero-posterior length of the p2. A diastema between the p1 and p2 is not present.
The mandibles referred to Protitanops curryi all have poorly preserved or heavily worn cheek teeth. Nonetheless, the overall morphology and proportions of the cheek teeth appear to be undifferentiated from Eubrontotherium clarnoensis , Parvicornus occidentalis , and Duchesneodus uintensis . The p1 of TMM 45805-1 (preserved on the left side) is small with a single cusp and a small broad talonid heel and is rooted in the mandible vertically. The p2 trigonid is anteroposteriorly longer than the talonid, but not more than twice as long. All of the occlusal features of the p3 are worn away. The trigonid and talonid of p2 have similar bucco-lingual widths. A cristid obliqua is seen on the left p2, but is more worn on the right p2. The notch on the labial side of the p2 formed by the cristid obliqua is relatively broad in this respect resembles other closely related species, and differs from some large Asian species that have a much deeper and narrower labial notch. The p4 trigonid and talonid are similar in length and width, while other features of the p4 have been worn away. The teeth are too worn to make determinations about the presence or absence of metaconids on the premolars. Other specimens (FMNH PM 399, TMM 40840-38) have more completely preserved molars. The molars of these specimens, like many brontotheres, are conserved; they lack autapomorphous features or species-specific combinations of characters. Typical characteristics include shallow occlusal basins, thinned lingual enamel, and an elongate m3.
Stratigraphic horizon and age. Late Eocene (Duchesnean mammal “age.”
Referred specimens. (From the Skyline Channels, Devil’s Graveyard Formation, Aqua Fria-Green Valley Area of Trans Pecos Texas) TMM 41853-17, a palate with complete dentition; TMM 41850-30, a rostral fragment with complete incisor row and right canine; TMM 41715-6, left mandibular ramus with p2 (partial), p3-m3; TMM 42254-2, a right ramus with p2-m3.
Description
The two cranial specimens, TMM 41853-17 ( Figure 11 View FIGURE 11 A-C) and 41850-30 ( Figure 11 View FIGURE 11 D-F) have complete incisor rows with two small incisor crowns per side, slightly arched incisor rows and a distinct postcanine diastema. The incisors have a short pointed apex, blunted somewhat by apical wear that is more prominent in the lateral incisor giving it a subtle caniniform appearance. The mandibular symphyses of TMM 41715-6 ( Figure 11G View FIGURE 11 ) and TMM 42254-2 ( Figure 11H View FIGURE 11 ) extend posteriorly as far as the trigon of the P3. The weakly arched shape of the incisor row and the position of the posterior margin of the mandibular symphysis (anterior to p4) is most consistent with Parvicornus occidentalis . One mandible (TMM 41715-6) has a postero-lingually angled cristid obliqua and p3 metaconid. Metaconids are variably present and absent in P. occidentalis , therefore this character would not contradict an assignment to cf. P. occidentalis .
The early Duchesnean specimens from the Skyline Channels tentatively referred to cf. Parvicornus occidentalis were previously referred to? Duchesneodus cf. uintensis primarily on the basis of size by Wilson and Stevens (1986). However, unlike specimens described here, Duchesneodus uintensis lacks an upper postcanine diastema and has a more posteriorly situated mandibular symphysis. Other species can also be ruled out with some certainty: Parabrontops gobiensis retains three upper incisors; Dianotitan lunanensis lacks a postcanine diastemata; Protitanops curryi , Notiotitanops mississippiensis , and Eubrontotherium clarnoensis also have more posteriorly extended mandibular symphyses.
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