Pseudodiplacodon, Mader 2000

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 66-70

Genus PSEUDODIPLACODON Mader 2000

Age. Uintan.

Subage. Early? and Late Uintan.

Type species. P. progressum ( Peterson 1934) .

Included species. Genus is monospecific.

Diagnosis. Large-sized (length P2 to M3 approximately 207–234 mm) plesiomorphic diplacodont brontothere with bulbous horns that are roughly circular in cross-section; robust nasals that are rounded distally, curved ventrally, and shorter than in Eotitanotherium ; small canines, and two poorly separated lingual cusps on P4. In general, the length of the cheek tooth series is usually shorter than in specimens of Eotitanotherium , but the skull is larger in size.

Discussion. The first primitive horned brontothere (diplacodont) material to be discovered was described by Marsh in 1875 and given the name Diplacodon elatus . In 1934 Petersen described a new species, which he referred to this genus, and named D. progressum . Mader (1989; 1991; 1998), however, found that “ Diplacodon ” progressum is not referable to Diplacodon Marsh , which he regarded as a nomen dubium or as a possible senior synonym of Eotitanotherium . Accordingly, Mader (2000) assigned the new generic name Pseudodiplacodon (literally “false Diplacodon ”) to this species.

Mihlbachler (2005), however, disagreed with Mader and synonymized Diplacodon , Pseudodiplacodon , and Eotitanotherium . Furthermore, Mihlbachler referred all specimens belonging to these taxa to a single species, Diplacodon elatus . According to Mader (1989; 1998; 2000) Pseudodiplacodon and Eotitanotherium are distinguished from each other by their horn morphology and the morphology of the cusps on the posterior premolars. Mihlbachler, however, asserted that there is a continuum of horn sizes between the specimens assigned to both genera and that the premolar morphology is variable. Mihlbachler further suggested that the difference in horn size between Pseudodiplacodon and Eotitanotherium is generally consistent with the pattern of cranial variation found in other brontotheres, including Megacerops as well as a monospecific quarry sample of Duchesneodus . Based on my own observations, however, the variation in horn size and morphology in the Duchesneodus quarry sample is minor compared to the difference between the horns in Pseudodiplacodon and Eotitanotherium and, in my opinion, Mihlbachler (see Mihlbachler et al. 2004) has incorporated several distinct genera under the name Megacerops .

With regard to horn morphology, it should be noted that it is not only the size of the horn that distinguishes Pseudodiplacodon from Eotitanotherium , but also the shape. In Pseudodiplacodon the horn is round in cross-section and tends to be rather bulbous, although the length of the horn may vary. In Eotitanotherium , however, the horn has a distinct elliptical cross-section and, in this sense, it is much more similar to the horn of Protitanotherium , but is not as robust.

Regarding the premolars, it should be noted that in 1989 and 1998 Mader stated that Pseudodiplacodon is characterized by poorly separated lingual cusps on P3 and P4 (whereas they are well separated in Eotitanotherium ), but in 2000 he restricted this observation to P4. The term “poorly separated” was meant to indicate that the cusps are more-or-less connected to each other as opposed to meaning that the cusps are located close together. Although Mihlbachler has stated that specimens assigned to Pseudodiplacodon by Mader distinctly vary in the degree to which the lingual cusps are “separated,” his own description of several specimens indicates that the protocone and hypocone on the posterior premolars are connected by a crest (i.e., they are poorly separated from each other in the sense of Mader). Although Mihlbachler stated that in the holotype of Pseudodiplacodon progressum there is no such crest on the left P4 (but there is one on the right), my own notes concerning the specimen reflect that a thin connecting ridge is present on both sides.

As a final consideration, I would like to point out that several of the diagnostic characters used by Mihlbachler to distinguish Diplacodon (including both Pseudodiplacodon and Eotitanotherium ) from Protitanotherium (as defined by Mihlbachler) are not valid. According to Mihlbachler, in Diplacodon the nasal process has upturned sides, elevated and widely separated horns, and a dorsoventrally deep nasal incision where the posterior notch of the incision rises much higher than the orbit. There is a specimen (AMNH 117163) referable to Protitanotherium (sensu Mihlbachler) , however, which has each of these characters, demonstrating that they are not unique.

I continue to regard Pseudodiplacodon and Eotitanotherium as distinct forms. In addition to the reasons cited above, this distinction is further suggested by skull size and proportional differences in the dentition. I estimate that the holotype skull of Pseudodiplacodon progressum is 15 to 20% larger than the holotype of Eotitanotherium osborni (determined by digitally scaling images of the skulls). Despite the fact that Pseudodiplacodon is larger than Eotitanotherium , however, the tooth row tends to be shorter (although the dental size ranges actually overlap). This suggests to me a possible adaptive difference between the forms, implying that they are different taxa.

All of the known specimens of Pseudodiplacodon ( Fig. 22 ) are from the Uinta Basin of Utah and all but one are recorded from the Myton Member of the Uinta Formation. CM 2858 was originally reported ( Peterson 1914a) from the upper part of Uinta B (Wagonhound Member), but in 1934 Peterson stated that this level might be at the base of Uinta C (Myton Member). Regardless of the correct stratigraphic level, there does not appear to be any morphological difference between CM 2858 and the specimens of Pseudodiplacodon clearly documented from the Myton Member. Furthermore, although CM 2858 is rather poorly preserved and cannot be thoroughly measured, it appears to fall within the size range of the Myton specimens as well. Thus, there is no reason at present to separate CM 2858 from the other specimens for the purpose of statistical analysis.

Table 17 shows that the individual and average values of V for the sample of Pseudodiplacodon are generally within the parameters of a single extant mammalian species. Almost three quarters of the individual values of V are within the range of 4 to 10 and, of those that are not, five out of nine are less than 4, suggesting that the samples from which they are derived are too small to show all of the variation originally present. Two of the values of V that are greater than 10 are based on canine size, which might suggest sexual dimorphism. Diastema length is another variable for which the value of V is greater than 10, but the value of V for diastema length is very high in all brontothere genera analyzed for this study. Only a single value of V greater than 10 (width of left P3) cannot be attributed to high intraspecific variation or to sexual dimorphism. The values of V (rounded to the nearest whole number) for the width of the right P3 and the length of both P3's is within the range of 4 to 10, however, so this single high value can probably be discounted. Thus, an analysis of the coefficient of variation for the sample suggests that the sample is homogeneous and gives no indication that more than a single taxon is represented.

Cluster analysis ( Fig. 23 ) also generally suggests homogeneity in the sample, although there is a single specimen (FMNH P 14632) that groups out separately. This is a very large individual that is well outside of the size range for most Pseudodiplacodon specimens and is within the size range of Eotitanotherium . Although the large size of this individual could suggest that it represents a distinct and as yet unnamed species, I am not inclined to recognize a new taxon at this time. I do not believe that the finding of a single discontinuously large individual is sufficient grounds for the recognition of a new species, especially considering that the coefficient of variation for the majority of variables in the total Pseudodiplacodon sample is well within the limits established for a single species. In my opinion, in order to demonstrate that a second taxon is present, a sample of larger individuals would have to be obtained that can be compared statistically against the specimens of more typical size already analyzed.

a Based, whenever possible, on an average of left and right measurements.

b Excluding Diastema Length.

As indicated above, two of the coefficients of variation for canine size (buccal-lingual width of the left and right canines) are greater than 10 and may indicate that canine size is sexually dimorphic. Unfortunately, the samples upon which these figures are based are too small to clearly establish whether canine size is bimo- dally distributed. It should be noted, however, that the high coefficient of variation for these two variables is due entirely to the single large individual (FMNH P 14632) discussed above. If this individual is dropped from the analysis and the coefficient of variation recalculated with the remaining two individuals in the sample, the values for V become 3.8 and 3.1 for the left and right canine respectively. Thus, if the large individual represents a distinct taxon, there is no evidence at present for sexual dimorphism in canine size for the smaller members of the sample. Alternatively, the single large individual could represent a male while the other seven individuals represent females.

As pointed out by Mader (2000), the low sample size in this statistical study limits its reliability. It should be repeated, therefore, if additional specimens of Pseudodiplacodon become available.

Figures

FIGURE 22. Holotype skull of Pseudodiplacodon progressum (CM 11879A) in lateral view.

FIGURE 23. Cluster dendrogram for specimens of Pseudodiplacodon resulting from a cluster analysis of all variables listed in Table 17. a, type of Pseudodiplacodon progressum.