Pygmaetitan panxianensis Miao, 1982

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 : 189-196

Pygmaetitan panxianensis Miao, 1982

HOLOTYPE: IVPP V6521 , a left upper tooth row with I3–M3.

TYPE LOCALITY: Shinao Formation, Shinao Basin, Panxian County, Guizhou Province, China.

AGE: Late Eocene? (5 early Oligocene sensu Russell and Zhai [1987]).

REFERRED SPECIMENS: (All from the same locality as the holotype) IVPP V6522 , a left maxillary fragment with P4–M3 ; IVPP V6523 , left P2 and a mandible fragment with right i3, m1 (partial), m2, m3 (unerupted), left i1–i3, and other incisor or canine fragments ; IVPP V6524 , a left mandible fragment with m2 (partial) and m3 ; IVPP V6525 , a partial juvenile mandible with right dc?, dp2–dp4, m1 (unerupted), left di2?, di3?, dp2, dp3, and dp4 (partial) ; IVPP V6526-1 , a fragment of a left juvenile maxilla with DC?, P1 (unerupted), DP2, DP3, and DP4 (partial) ; IVPP V6526-2 , a left mandible fragment with dc (root only), p1 (unerupted), and dp3– dp4 ; IVPP V6527 , a left maxillary and jugal fragment with DP3 and DP4 ; IVPP V6528 , a right maxilla fragment with dp2–dp4 (all damaged) ; IVPP V6529 , a left mandible fragment with dp2 and dp3 (partial) .

DIAGNOSIS: Pygmaetitan panxianensis is intermediate in size between Nanotitanops shanghuangensis and Microtitan mongoliensis . Dentally, Pygmaetitan panxianensis has an enlarged and subcaniniform incisor, a diminished postcanine diastema, a distinct P1 metacone, a distinct P2 metacone, and weak premolar preprotocristae. Hypocones are not seen on P2, P3, or P4. The molars of Pygmaetitan panxianensis 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 upper molar mesostyles are labially expanded. The lower molars have deep valleylike basins and the m3 is elongate.

Pygmaetitan panxianensis shares with Sthenodectes incisivum the unique combination of an enlarged I3, diminished postcanine diastema, and no molar paraconules. However, P. panxianensis is clearly much smaller than S. incisivum and has a more complex P1 and taller lower molar crowns.

DESCRIPTION

UPPER DENTITION: The following description of the adult upper dentition of Pygmaetitan panxianensis is based on the holotype specimen, IVPP V6521 (fig. 89a), with additional observations made on IVPP V6522 (now shown), IVPP V6523 (fig. 89b), IVPP V6526-1 (fig. 92 a, b), and IVPP V6527 (fig. 92c). The holotype, consisting of a left maxillary fragment with a complete cheektooth row, represents a small but remarkably advanced brontothere. The maxillary fragment itself is too poorly preserved to describe any characteristics of the skull. The teeth are only moderately worn, thus facilitating a reasonably complete description of the upper dentition ; however, most of the dental surface is corroded and pitted, thus obscuring some of the finer details.

The only preserved incisor on the holotype, presumably an I3, is greatly enlarged. This incisor is attached to a small fragment of bone that has been glued onto the main maxilla fragment, although these two fragments do not appear to occlude precisely. It is therefore not possible to definitively determine whether the disproportionately large incisor actually belongs to the same individual. However, this large upper incisor is consistent in size with the large lower incisors that characterize this species (see below). Pygmaetitan panxianensis is one of two brontotheres with greatly enlarged incisors. The other example is Sthenodectes incisivum . This upper incisor is plesiomorphic in other respects; the crown is subcaniniform with a strong lingual cingulum. It has developed a broad lingual wear facet the spans the entire height of the crown.

The upper canine is very large as well, with a diameter that exceeds that of the incisor by about a millimeter. The crown of the canine is either worn away or broken off. The specimen lacks a diastema between the incisor and canine, although this could be an artifact of the loss of the bony fragments that would have been positioned between the larger maxillary fragment and the smaller fragment holding the incisor. A minor gap of a few millimeters separates the canine from the P1.

The P1 of IVPP V6521 is severely worn. Discernable topographical features include a paracone, a straight parastyle, a part of a small lingual heel with a small protoconelike cusp (worn flat), and a small ridge situated anterolingually from the paracone. The posterior portion of the crown is broken off. An intact and unerupted P1 is preserved with IVPP V6526-1 (fig. 92a, b). The P1 of that specimen exhibits two distinct labial cusps, a large paracone and a somewhat smaller metacone. The parastyle is bent slightly lingually while the metastyle is essentially straight. The P1 does not show a well-developed lingual heel, although a thick cingulum extends along the entire lingual margin of the crown. A sharp ridge ascends the lingual side of the metacone and joins the lingual cingulum, forming a small triangular peak. A similar ridge climbs the lingual side of the paracone, although it is less distinct and forms a smaller peak at its junction with the cingulum.

The P2, P3, and P4 are successively larger. These premolars are strongly rectangular in outline with parallel anterior and posterior margins. The P2 of the holotype is incomplete; the labial sheet of enamel has been lost. However, IVPP V6523 includes a complete and unworn P2 (fig. 89b). This specimen exhibits a prominent labial paracone rib, and a much smaller labial metacone rib. The parastyle is straight. The metacone is fully developed, but it is positioned close to the paracone and is strongly shifted lingually. Finally, the metastyle is straight, but strongly angled lingually. These features give the ectoloph a rather arched or rounded appearance. The labial sheet of enamel has been lost on the P3 of the holotype. The P 4 of the holotype has suffered breakage at the midline of the tooth and its width is probably exaggerated ; it may have had a small labial rib, but damage to the surface of the enamel and a minor amount of extraneously concreted material obscures it. The ectolophs of the P3 and P4 are flexed slightly labially in the middle, giving them a sort of rudimentary mesostyle. This mesostyle is more apparent in the P4. The ectoloph of P3 is straight, while that of P4 is incipiently W-shaped due to an anterolabially angled parastyle in addition to the more distinct mesostyle. The lingual heels of P2, P3, and P4 are broad, although only a single lingual cusp (protocone) can be found on each premolar. The protocones of P2 and P3 are tall, slightly anteriorly positioned, and are followed by short lingual crests that arch posterolabially. On P 2 this crest ends at the posterior cingulum. Additionally a faint preprotocrista can be seen on the P2, connecting the anterolabial base of the protocone to the lingual base of the paracone. The P 2 of IVPP V6523 lacks a strong lingual crest, although there is a very small pinpoint of enamel in the hypocone position. The P 3 of the holotype has a small but distinct lingual crest that ends at the junction of the metacone and posterior cingulum where there is a distinct swelling that could be interpreted either as a metaconule, or a labially positioned hypocone. The preprotocrista of P3 is exceedingly faint. A lingual crest can be seen on P4, although it is indistinct. The P 4 lingual crest is more distinct on IVPP V6522 , although it is much less developed than the lingual crest of the P3 of IVPP V6521 . Given the rudimentary nature of the lingual crest, and the hypoconelike and metaconule-like structures on the lingual sides of the premolars, it is probable that these characters were intraspecifically variable, like many other brontotheres .

The labial cingula of P3 and P4 are absent or they were excessively weak. A strong and continuous cingulum stretches around the anterior, lingual, and posterior margins of P2, P3, and P4. On P3 and P4 the anterior cingulum has been partially worn away by interstitial wear.

The left M1 of IVPP V6521 is heavily worn and incomplete; fragments of the posterior side are missing and the tooth has experienced severe interstitial wear, thus distorting its length/width proportion. The M2 is heavily worn as well, although the specimen is complete and has suffered a lesser amount of interstitial wear. The M3 is complete, only lightly worn and most clearly exhibits a full suite of molar characters. Advanced molar characters, best seen in the moderately worn M3, include a narrow anterolabial cingulum that passes below the apex of the parastyle, a relatively tall and lingually angled ectoloph, thin lingual ectoloph enamel, and wedge-shaped lingual margins of the paracone and metacone. There are no discernable labial paracone or metacone ribs, although weak ribs might have been present. Other advanced features include well-developed anterolingual cingular cusps and a round central molar fossa. The molars lack any trace of vestigial paraconules. The mesostyle of M3 is labially expanded. M1 and M2 probably had similarly labially expanded mesostyles, although this structure has been more dramatically worn in these teeth. A small metalophlike ridge can be seen along the anterolabial slope of the hypocones of M1 and M2. The M3 hypocone is well developed but smaller than that of the M2, and it lacks the metalophlike ridge. Labial and lingual molar cingula are thin and discontinuous around the margins of the main cusps.

LOWER DENTITION: IVPP V6523 includes a number of isolated lower incisors, including a complete left incisor row (i1–13) (fig. 90). The lower incisor crowns are rather large. They are all about the same size, although they differ in shape. The i1 and i2 have a semispatulate shape with rounded apices. The i3, on the other hand, is more subcaniniform with a pointed apex. From a lingual view, the i1 and i2 crowns are slightly asymmetrical, while the i3 is mesiodistally more elongate and more severely asymmetrical. Thin lingual cingulids are present in all three lower incisors. Labial cingulids are absent.

IVPP V6524 includes a partial m2 and a complete m3 (fig. 91). IVPP V6525 contains an exposed but unerupted m1 (fig. 93). The lower molars of Pygmaetitan panxianensis have relatively thin enamel. The talonid and trigonid basins are relatively deep and valley-, or V-, shaped. Weak lingual protoconid and hypoconid ribs are often seen in the lower molars of brontotheres, but they are not evident in these specimens. The m3 is elongate. Labial molar cingulids are exceedingly faint, but this cingulid can be seen wrapping around the posterior margin of the hypoconulid. Lingual cingulids are absent.

DECIDUOUS UPPER DENTITION: Most of the known specimens of Pygmaetitan panxianensis are juveniles. The best juvenile specimen with upper deciduous dentition is IVPP V6526-1 (fig. 92a, b). The anteriormost tooth of IVPP V6525-1 is probably a deciduous canine. The unerupted tooth just behind it is a P1 and is described above with the adult dentition. The remaining teeth are DP2, DP3, and a portion of DP4. An additional specimen, IVPP V6527 (fig. 92c), has a complete DP4. The deciduous premolars are more molarized than the adult premolars. The deciduous premolars show a trend of being progressively ‘‘derived’’ posteriorly. In other words, the anterior deciduous premolars are the least molariform and retain a number of plesiomorphic states. Moving posteriorly, the deciduous premolars increasingly show a number of apomorphic conditions that resemble the adult molars.

All of the deciduous premolars share with the molars the following conditions: relatively tall ectolophs, thin enamel on the lingual band of the ectoloph, angled lingual margins of the paracone and metacone, and relatively weak labial ribs. The DP2 is more asymmetrical than typical adult molars. The anterior portion of the ectoloph is more elongate than the posterior portion. The DP3 and DP4, on the other hand, are fully molariform in terms of their overall shape. The mesostyle of the DP2 is small and angled posterolabially. The parastyle is less strongly deflected lingually. Because of the smaller mesostyle and less labially directed parastyle, the labial margins of the cusps are not as deeply concave as those of the adult molars. The DP3 and DP4 mesostyles are well developed; however, the DP3 and DP4 mesostyles are less labially expanded that those of the adult molars. A small but distinct preprotocrista is present on the DP2, although a distinct paraconule is not embedded in this crest. DP3 lacks a preprotocrista, although it retains a small paraconule. DP4, like the adult molars, entirely lacks a preprotocrista and paraconule. A deep central molar fossa is not seen on DP2, although, like the molars, DP3 and DP4 express this condition. The lingual cingulum of DP2 climbs to the peak of the parastyle; this character resembles the most basal brontotheres such as Eotitanops and Palaeosyops , although the cingulum does not form a thickened parastylar shelf as seen in these taxa. In adult molars of more derived brontotheres, the anterior cingulum wraps around the anterolabial margin of the tooth in a position proximal to the peak of the parastyle; the DP3 and DP4 of Pygmaetitan panxianensis show this condition. DP4 shares with the adult molars a strong anterolingual cingular cusp, although this structure is absent on DP2 as well as DP3. Finally, the lingual cingulum of DP2 is thick and continuous with the anterior and posterior cingula; the lingual cingula of DP3 and DP4 are very weak and more closely match the adult condition.

JUVENILE MANDIBLE AND DECIDUOUS LOWER DENTITION: IVPP V6525 is a nearly complete juvenile mandible with deciduous dentition (fig. 93a, b). IVPP V6526-2 contains an additional set of deciduous premolars that are somewhat better preserved (fig. 92c, d, e). The mandibular symphysis of IVPP V6525 is short and very broad and the inferior angle of the symphysis is nearly vertical. Although there were undoubtedly ontogenetic changes in the overall shape of the mandible, it is likely that the adult mandible possessed similar peculiarities, even if not as extreme. Unfortunately, the crowns of the right di2, di3, and left dc are not preserved. A dp1 or p1 is not present in IVPP V6525, and there is a short gap between the dc and dp2. This gap is filled by an erupting P 1 in IVPP V6526-2, thus indicating the absence of a postcanine diastema, at least at this ontogenetic stage. The morphology of the dp2 generally resembles a typical adult brontothere p2, although a large metaconid is present; metaconids are not typically found in the adult p2s of brontotheres. The dp2 metaconid is situated very close to the protoconid and is positioned posteriorly and slightly lingually from it. The dp2 contrasts with the molariform condition in the following ways: the trigonid is relatively longer, the paralophid and protolophid are straight rather than lingually arched, the protoconid is positioned medially rather than labially, and the talonid of dp2 is essentially molariform, with lingually arching cristid obliqua and hypolophid forming a U-shaped crest that is remarkably deep in comparison to typical brontothere adult premolars or molars. The dp3 and dp4 contrast with typical adult premolars in their possession of a pronounced entoconid, and they are essentially molariform, although they have somewhat more elongate proportions than typical adult molars, and also have remarkably deep U-shaped crests. As noted above, the lower adult molars are remarkably tall in comparison to typical brontotheres, with deep valley-shaped trigonid and talonid basins. The lateral views of IVPP V6525 and IVPP V6526-2 reveal that the deciduous premolars increase in crown height posteriorly. In IVPP V6525, the dp4 approaches the remarkable crown height of the unerupted m1.

REMARKS

Pygmaetitan panxianensis Miao, 1982 , is based on a maxillary fragment with a nearly complete set of adult upper dentition. Miao (1982) noted that P. panxianensis is easily differentiated from small North American brontotheres, and except for its enlarged incisors and canine, is similar in many ways to derived brontotheres from the late Eocene. Miao (1982) briefly compared P. panxianensis with Eotitanops and Microtitan and more detailed comparisons with small Asian and North American brontotheres are made here.

Pygmaetitan panxianensis is substantially larger than the minuscule Nanotitanops shanghuangensis , with cheek-tooth dimensions that are more than double. Additionally, N. shanghuangensis molars retain small paraconules, whereas P. panxianensis molars do not. The cheek teeth dimensions of the holotype specimen of P. panxianensis just slightly exceed the largest specimens of Eotitanops , and are similar in size to the smallest Palaeosyops specimens. Nonetheless, the molars of Pygmaetitan panxianensis are clearly more advanced than Eotitanops and Palaeosyops , and clearly establish it as a member of the Brontotheriinae . Other small brontotheres, Microtitan mongoliensis , Mesatirhinus junius , Metarhinus fluviatilis , Metarhinus abbotti , and Fossendorhinus diploconus exceed Pygmaetitan in size by a substantial margin. Moreover, none of these species has extremely enlarged incisors or a P1 metacone, nor do they lack a postcanine diastema. Size comparison with Acrotitan ulanshirehensis is complicated by the lack of comparable dental material, although Acrotitan seems somewhat larger. However, the narrow mandibular symphysis, small incisor alveoli, and long p1–p2 diastema of Acrotitan ulanshirehensis are obviously out of character with the enlarged upper incisor and reduced upper postcanine diastema of Pygmaetitan panxianensis .

The extremely enlarged incisor of Pygmaetitan panxianensis is a characteristic shared with Sthenodectes incisivum , a much larger North American hornless Uintan brontothere. The deep, valley-shaped trigonid and talonid basins seen in the adult lower molars closely resemble the gigantic late Eocene Asian brontothere, Embolotherium . The very labially expanded mesostyle that is best seen in the M3 of IVPP V6521 is perhaps an autapomorphic condition, although it is difficult to compare mesostyle proportions to species that are only known from specimens with heavily worn teeth.

Many juvenile specimens were found with adult specimens of Pygmaetitan panxianensis , and were assigned to this species by Miao (1982). As Butler (1952) noted, the upper deciduous premolars of brontotheres tend to be molariform, although the more anterior deciduous premolars tend to lag behind in the accumulation of typical molar apomorphies. This appears to be the case in Pygmaetitan . In terms of adult molar characters, the DP2 retains the greatest number of plesiomorphic character states; the DP3 has an intermediate number of plesiomorphic states, and the DP4 has the least. Despite the more ‘‘primitive’’ condition of the deciduous dentition in comparison to the adult molars, the deciduous premolars of Pygmaetitan are quite molariform in comparison to other brontothere species where the DP2 is substantially more elongated ( Butler, 1952). However, because of the lack of comparable material for most other brontothere species, the phylogenetic or taxonomic value of the deciduous teeth of Pygmaetitan panxianensis is uncertain. Since Butler’s (1952) analysis of the deciduous dentition of perissodactyls, deciduous dental material has been collected for several more brontothere species, although at present there are no descriptions of this material.