Mesatirhinus, Osborn 1908

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 8-19

Genus MESATIRHINUS Osborn 1908

Age. Bridgerian.

Subage. Twinbuttean.

Type species. M. megarhinus ( Earle 1891) .

Included species. Genus is monospecific.

Diagnosis. Medium-sized (length P2 to M3 approximately 128–145 mm) dolichorhinine brontothere with no hypocone on M3, well developed suborbital protuberance, and nasals that are moderately flared distally (see Fig. 2 ). Within the Dolichorhininae , Mesatirhinus lacks distinctive generic autapomorphies and is distinguished from other dolichorhinine brontotheres by its plesiomorphic morphology.

Discussion. In 1891, Charles Earle described the skull of a small brontothere from the Washakie Basin, which he named Palaeosyops megarhinus . In 1908, Osborn recognized that this skull was generically distinct from Palaeosyops and gave it the new generic name Mesatirhinus . In the same paper, Osborn also named a second species of Mesatirhinus , M. petersoni , which he distinguished from the type species by its longer skull (especially in the preorbital region) and its longer cheek tooth series. Somewhat cryptically, Osborn added that Mesatirhinus petersoni was further distinguished from M. megarhinus by "an average advance in all the rectigradations (i.e., new morphologic characters)". According to Osborn these evolutionary advances proved that the differences in size and form that he cited between the species were "not merely due to fluctuations of size or differences of sex". In 1929, Osborn (p. 389) restated this observation almost verbatim, but changed it to read "an average advance in the premolar rectigradations" rather than "in all the rectigradations". In this section of the paper Osborn did not elaborate on what these evolutionary advances in premolar morphology were, but elsewhere (p. 393) he stated that M. petersoni was distinguished from M. megarhinus by the former's longer diastema, more pronounced posterior cusp or tritocone (= metacone) on P1, more cingulate P2, and by having a slight rudiment of a protoconule on P2 and P3. According to Osborn, one specimen of M. petersoni (formerly AMNH 1556, specimen now in the British Museum) had a slight elevation of the tetartocone (= hypocone) on P4, a supposed evolutionary advance.

In 1929, Osborn referred the fragmentary holotype of Palaeosyops junius Leidy, 1872 (ANSP 10349), to the genus Mesatirhinus . Leidy had based the species on several small fragments from the right side of the jaw and a sketch of a larger fragment from the left side containing the last premolar and all of the molars. Of this type material, Osborn was only able to locate the right P4 and posterior half of right M 3 in 1906 ( Osborn 1929). The type was reportedly collected near Fort Bridger and Osborn speculated that it had been collected from level B (now part of the Blacks Fork Member) of the Bridger Formation. Because of its small size and presumed early geologic occurrence, Osborn regarded the material as representing a distinct species of Mesatirhinus , M. junius .

Mesatirhinus ( Fig. 2 ) is known from essentially contemporaneous deposits in both the Washakie and Green River Basins. Most specimens used in this study were from the Green River Basin, but because the sample size was relatively small (no more than ten individuals for any given variable) the sample was supplemented by specimens from the Washakie Basin.

The coefficient of variation ( Table 1) for most variables in the combined Mesatirhinus sample falls within the range of 4 to 10 and the average value for all variables (excluding diastema length) is 5.7. Thus, the variation observed in the sample is consistent with the possibility that only a single species is represented. There is nothing about the range or average value of V to suggest that there is any heterogeneity in the sample.

Similarly, cluster analysis of all variables ( Fig. 3 ) shows that all the specimens are closely grouped together and there is no indication that more than a single group is present. If, however, a cluster analysis is run using only the length of the skull, length of the cheek tooth series, and length of the molar series, then two distinct size groups emerge ( Fig. 4 ). Significantly, it was these same variables that Osborn (1908; 1929) used to distinguish Mesatirhinus megarhinus from M. petersoni , and specimens assigned by Osborn to M. megarhinus consistently fall into one size group (the smaller) while specimens that Osborn assigned to M. petersoni tend to fall into the other. A single specimen (AMNH 1651a) that Osborn provisionally referred to M. petersoni , however, groups out separately. This specimen is a large individual and can probably be included in the large-size group but, because of its uncertain association in the cluster dendrogram, it has been excluded from further analysis. T -tests ( Tables 2 and 3) confirm that for many variables the means of both size groups (excluding AMNH 1651a) are significantly different (18 out of 30 variables have probabilities less than.05). Based on the formula given in the Methods section of this paper, there is a 78% chance that at least one of these significant results is in error, but it seems highly unlikely that more than half the results could be attributed to error.

Thus the two size groups recognized by Osborn do exist although they are only evident when certain linear measurements are considered. It must be determined, therefore, whether these size groups represent two species (as Osborn thought) or are members of a single dimorphic species.

Tables 4 and 5 present the summary statistics for each of the two size groups of Mesatirhinus suggested by the cluster analysis in Figure 3 (AMNH 1651a not included). It will be noted from Table 4 that, although the average value of V for the smaller-size group is typical of extant species (= 4.5), the individual values of V are generally rather low (half are less than 4). The sample size for most variables, however, is probably sufficiently large to show most of the variation that is present. It should be noted that the average value of V is raised somewhat by the high variability of the length of the second upper premolars (values of V over 10), but even if the length of P2 is excluded from the calculation, the average value of V remains more-or-less typical of extant mammalian species (average V = 3.9)

In the larger-size group ( Table 5) the average value of V is rather low (= 3.4) as are the individual values of V (two thirds are less than 4). Some of this low variability, however, may be due to the small sample sizes in the larger-size group. Once again the length of P2 is found to be highly variable with values of V in excess of 10 and, if the length of P2 is excluded from the calculation of the average (as well as diastema length), then the average value drops to 2.8.

Most of the characters used by Osborn (1908; 1929) to separate Mesatirhinus megarhinus from M. petersoni can be attributed to intraspecific variation. This is especially true of skull length and length of the molars. Although differences in molar dimension could indicate a difference in function (and thus a potential taxonomic difference), the overall dimensions of the molars in both groups are actually closely similar. A comparison of Tables 4 and 5 shows that individual molar dimensions in both groups often overlap and, in those cases where there is a size interval between the groups, the difference is no more than 1.5 mm. Similarly, there is a size interval between the length of the molar series in both groups, but the difference is only 3.3 mm.

Osborn's suggestion that skulls of Mesatirhinus petersoni have a longer face than specimens of M. megarhinus is a more convincing argument that the two are separate species, but I have not been able to confirm this observation. Most skulls of Mesatirhinus are very imperfectly preserved, making it impossible to compare facial length to overall skull length. My own impression has been that the preorbital region in both groups is of the same proportional size although, in absolute terms, the face in the larger-size group is naturally longer. It is of interest to note that despite Osborn's claims, the faciocephalic index (see Osborn 1929, p. 255 for definition) that he reported (1929) for the two species was identical (= 48). Thus, by Osborn's own figures, there is no proportional difference in the length of the preorbital region in either group. Even if the differences in facial length suggested by Osborn had been correct, however, the proportional difference could be attributed to allometric changes in a single species rather than to phylogenetic differences.

Contrary to Osborn's statement, specimens that are included in the larger-size group (his Mesatirhinus petersoni ) do not have a longer diastema than specimens belonging to the smaller-size group ( M. megarhinus ). The actual range of diastema lengths recorded for the larger size group is 3.9–6.0 mm (based on three individuals), while in the smaller size group the range is 5.8–8.8 mm (also based on three individuals). A specimen (AMNH 12206) referred to M. megarhinus by Osborn (1929), but not included in the cluster analysis that produced the two size groups ( Fig. 3 ) because of insufficient data, has a diastema length of 12.0 mm. Thus, specimens that Osborn referred to M. megarhinus actually have a diastema that is longer than in any specimen that he referred to M. petersoni .

This observation should not be interpreted to mean, however, that the smaller-size group can be defined as having a longer diastema than in the larger-size group. In all brontotheres the length of the diastema is highly variable (note the values for V in various genera reported in this paper) making it a poor character to use in a systematic assessment. In actuality there is a considerable overlap of diastema length in both size groups, and a cluster analysis ( Fig. 5 ) of diastema length alone shows that specimens belonging to both groups do not cluster separately. A t -test comparison ( Table 3) verifies that the difference between the means of diastema length in both size groups is not statistically significant. a Based, whenever possible, on an average of left and right measurements.

b There is no variance in either of the two groups being compared.

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

b Insufficient data for t -test.

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

b Insufficient data.

c Excluding Diastema Length.

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

b Insufficient data.

c Excluding Diastema Length.

Perhaps Osborn's most convincing argument that the two groups represent two different species, are the supposed differences in premolar morphology. I have concluded from my own study, however, that there is no consistent morphological difference between the size groups. Differences in premolar morphology can be attributed to intraspecific variation and to differences in wear and preservation.

The weight of the evidence suggests that there is only a single species of Mesatirhinus present in the sample and that the two size groups probably represent males and females. That a single species is represented is indicated by the fact that: 1, members of both groups appear to be morphologically identical; 2, the values of V for each of the size groups tend to be rather low while the values of V for all specimens of Mesatirhinus taken together are more typical of extant mammalian species; and 3, although there is a size difference between both groups, in most cases this difference is not appreciable. It would have been desirable to compare canine widths within the two size groups to establish whether there was bimodality (as might be expected if the groups were two distinct species) but, unfortunately, the canines in most of the specimens of Mesatirhinus examined were very poorly preserved. If bimodality of canine size within the groups could be established, I would be willing to regard them as separate species but, without this information, I do not believe that there is enough evidence to justify the recognition of more than a single taxon.

If the alternate approach were to be taken and the two size groups interpreted as representing different species, then there would be a problem in determining which trivial name should be applied to each. Although the type of Mesatirhinus petersoni clearly belongs to the larger-size group, the holotype of the type species of Mesatirhinus , M. megarhinus , is almost too fragmentary to measure. I was only able to obtain estimated measurements for the length of the left M3 (30 mm) and the length of the right M2 (also 30 mm). It will be noted from Tables 4 and 5 that these measurements could apply to either size group. Therefore, contrary to Osborn's conclusion, it is not certain that the type of M. megarhinus belongs to the smaller-size group.

A further potential complication is presented by the holotype of Palaeosyops junius Leidy , which Osborn (1929) referred to the genus Mesatirhinus . Because the holotype consists of only two isolated lower cheek teeth, it is difficult to be certain which genus it actually represents. Of the three recognized genera of Bridgerian aged brontotheres ( Palaeosyops , Mesatirhinus , and Telmatherium ), the small size of the type is most consistent with Mesatirhinus although the teeth of Mesatirhinus are typically a little larger.

Based on the locality information available, Osborn (1929) suggested that the type of Mesatirhinus junius was from Bridger B (Blacks Fork Member) but, the only brontothere clearly documented from this level is Palaeosyops . The type of M. junius is much smaller than any specimen of Palaeosyops , however, and the occlussal surfaces of the teeth lack the circular wear facets typical of that genus.

The small size of the holotype of Mesatirhinus junius is not, by itself, a sufficient argument for accepting M. junius as a distinct species. Osborn (1929) provisionally referred one other specimen (AMNH 12686, a right M1 and M3) to the species M. junius based on its small size, but I have included this same specimen in the small-size group of M. megarhinus . Although AMNH 12686 is a diminutive individual, it will be noted from Table 1 that, for the most part, the values of V for the length and width of right M1 and M3 were not unusually high (although the value of V for length of RM1 is 7.8). Thus, the teeth of AMNH 12686 and those of the other specimens of Mesatirhinus analyzed, all generally fall within the range of size variation typically encountered in a single extant mammalian species. In my opinion, AMNH 12686 probably represents a small Mesatirhinus megarhinus , not a separate taxon.

If it could be demonstrated that the holotype of Mesatirhinus junius is actually from the Blacks Fork Member of the Bridger Formation, then the possibility exists that it might represent a smaller species from that level. Before this conclusion could be reached, however, it would be necessary to obtain a sample of Mesatirhinus from the Blacks Fork Member so that it could be statistically compared against the sample already documented from the Twin Buttes Member. In the absence of such a sample it is important to note that no other specimen of Mesatirhinus has ever been collected from the Blacks Fork Member and that the stratigraphic placement of the holotype of Mesatirhinus junius in the Blacks Fork Member is doubtful at best. The only locality information that is available for the specimen is that it was collected "near Fort Bridger". The expression "near" is obviously a subjective term that could refer to several feet or several miles. Although it is true that only outcrops of the Blacks Fork Member occur in the immediate vicinity of Fort Bridger, outcrops of the Twin Buttes Member begin to occur approximately twelve miles (19 km) to the east. In my opinion the holotype of Mesatirhinus junius is probably from the Twin Buttes Member.

If the holotype of Mesatirhinus junius is a specimen of Mesatirhinus , it almost certainly belongs to the small-size group of Mesatirhinus discussed above. If the two size groups of Mesatirhinus were to be interpreted as representing two species, then M. junius would have nomenclatural priority among members of the small-size group. Thus, if Osborn were correct in referring M. megarhinus (the type species) to this size group, then M. megarhinus is invalid because it is a junior synonym of M. junius . Alternatively, the holotype of M. megarhinus could belong to the larger size group, in which case M. megarhinus would remain valid and be a senior synonym of M. petersoni while M. junius would continue to be the valid name for the small size group.

My present conclusion, however, is that there is only a single species of Mesatirhinus represented by all of the Mesatirhinus specimens collected from the Green River and Washakie Basins, a conclusion supported by the recent work of Mihlbachler (2005). If the generic assignment of Leidy's Palaeosyops junius to the genus Mesatirhinus is correct, therefore, the type species of Mesatirhinus ( M. megarhinus ) is invalid because it is a junior synonym of M. junius . Mihlbachler (2005) accepted this synonymy, but, for the present, I prefer to treat M. junius as a nomen dubium because of the meagerness of the type material and its questionable generic identity. Gunnell and Yarborough (2000) regarded Palaeosyops junius as a junior synonym of Palaeosyops paludosus , the type species of Palaeosyops , which is a genus that is clearly distinct from Mesatirhinus .

Figures

FIGURE 2. Skull of Mesatirhinus megarhinus in A, anterior; B, dorsal; C, ventral; and D, lateral view. After Osborn. 1929.

FIGURE 3. Cluster dendrogram for specimens of Mesatirhinus from the Green River and Washakie Basins resulting from a cluster analysis of all variables listed in Table 1. a, holotype of Mesatirhinus petersoni.

FIGURE 4. Cluster dendrogram for specimens of Mesatirhinus from the Green River and Washakie Basins resulting from a cluster analysis of basilar skull length, length of cheek tooth series, and length of molar series.

FIGURE 5. Cluster dendrogram for specimens of Mesatirhinus from the Green River and Washakie Basins resulting from a cluster analysis of diastema length. SMALL, individual belonging to small-size group; LARGE, individual belonging to large-size group; UNKNOWN, size-group to which individual belongs was not determined.