Scott et al.—New diminutive species of Catopsalis from Alberta
at the bases of successive cusps (Fig. 5). In contrast, the cusps in the middle row inC. kakwa n. sp. are generally subquadrate with near-flat surfaces, and appear columnar in labial or lingual view, erect, and are fully separated from one another, with no evidence of overlap of the bases of successive cusps. Further, several of the cusps in the middle row on M1 in Valenopsalis and C. alexanderi exhibit strong labial and lingual accessory crests, in contrast to C. kakwa n. sp., in which accessory crests are undeveloped. Differences are also seen in the cusps of the labial row of M1. In Valenopsalis and C. alexanderi, the labial sides of the labial cusps are anteroposteriorly compressed, imparting a subtriangular horizontal section to the cusps, and the cusps are separated from one another by wide valleys. In contrast, the labial sides of the labial cusps in C. kakwa n. sp. are smoothly rounded and somewhat swollen, imparting a subcircular cross section to the cusps, and a narrow valley separates successive cusps. Differences in the structure of m1 are equally apparent, e.g., the anterior cusps of the labial row in Valenopsalis and C. alexanderi are subconical to subquadrate anteriorly but become longer at their bases and increasingly crescentic posteriorly, with the posterior sides strongly concave. The labial cusps become increasingly reclined (i.e., less erect) posteriorly, and there is significant overlap at the bases of successive cusps. In contrast, the labial cusps in C. kakwa n. sp. are subconical to subquadrate, columnar, and are vertically oriented; the posterior surfaces of successive cusps are flat, and there is no overlap at the bases of successive cusps. The comparative material for Catopsalis waddleae and
C. foliatus is limited, but in the parts that can be compared adequately, C. kakwa n. sp. resembles these species more so than either Valenopsalis and C. alexanderi, e.g., the labial cusps on m1 of C. waddleae and C. foliatus are strongly quadrate anteriorly, similar to those in C. kakwa n. sp., but differ in that they become weakly crescentic posteriorly. The more posterior labial cusps in both C. waddleae and C. foliatus exhibit a faint recline posteriorly, in contrast with the more upright, columnar cusps in C. kakwa n. sp., but the m1s of all three taxa are gen- erally similar in having the posterior surfaces of the labial cusps essentially flat, showing no evidence of cusp interlock. The dentition of Catopsalis kakwa n. sp. differs from those
of the younger species of Catopsalis, C. fissidens and C. cal- gariensis, primarily in their smaller size and in having fewer molar cusps, but despite these differences, the three species share a number of features, e.g., the p4 of C. kakwa n. sp. and C. fissidens are tall relative to length, and the posterolabial shelf and associated accessory cusps are weak or undeveloped. The molar cusps in each of the three species are essentially erect, columnar, and exhibit little development of the cusp interlock- ing that occurs with the more crescentic cusps in Valenopsalis and the older species of Catopsalis (Fig. 4). The cusps in the middle row on M1 are subquadrate in horizontal section, and the labial and lingual accessory crests, strongly developed in Valenopsalis and C. alexanderi, are only weakly developed in C. kakwa n. sp., C. fissidens, and C. calgariensis. The middle cusp row diverges lingually toward the anterior end of the crown in C. kakwa n. sp., C. fissidens, and C. calgariensis, as opposed to the subparallel labial and middle cusp rows in Valenopsalis and C. alexanderi. Additional similarities can be seen in the lower molars, e.g., the cusps of both rows are nearly vertical in
907
their orientation, with little tendency to recline posteriorly; the spaces between successive labial cusps are narrow and slit-like; cusp interlock is undeveloped; and a prominent labial cingulid is developed on m1 in each of the three younger species. Although C. kakwa n. sp. shares several features with C. fissidens and C. calgariensis, it differs in significant ways, e.g., the ratio of M1 length to M2 length is higher in C. fissidens (1.66, data from Lucas et al., 1997) and C. calgariensis (1.81, data from Middleton, 1982) when compared to that of C. kakwa n. sp. (1.29), and the ratio of m1 length to m2 length is accordingly lower (1.7, 1.6, and 1.4, respectively). The lingual cusp row on M1 of C. kakwa n. sp. is smoothly arcuate, with the cusps rapidly decreasing in size anterior to the crown midpoint and imparting a roughly teardrop shaped outline to the crown, whereas the lingual row is essentially straight in both C. fissidens and C. calgariensis, with the cusps remaining rela- tively large and imparting a weakly subquadrate (C. fissidens)to quadrate (C. calgariensis) outline to the crown. Enamel infold- ing on the upper and lower first molars is weak or undeveloped in C. kakwa n. sp., as opposed to the strong infolding seen in the C. fissidens and C. calgariensis, and the enamel is smooth, rather than wrinkled. Body mass.—The body mass of North American taenio-
labidoid species is of particular interest because the clade includes the largest multituberculates so far discovered. Exactly how large these mammals were, however, is a matter of some uncertainty, because different equations used in calculating the estimate give widely varying results. By way of example, Williamson et al. (2015, p. 20) presented three estimates of body mass for each taeniolabidoid species, based in whole or in part on calculations of occlusal surface area of m1. The use of m1 in body mass estimates became entrenched in paleontology fol- lowing Gingerich et al. (1982), who established that this tooth locus exhibited the least variation in relation to body mass. A considerably more recent body of work (e.g., Hopkins, 2008;
Millien, 2008; Freudenthal and Martin-Suarez, 2013) has con- firmed that, particularly in the case of rodents, the total length of the lower cheek tooth row is a more reliable predictor of body mass. Body mass estimates of Multituberculata commonly make reference to equations derived from extant Rodentia (e.g., Wilson et al., 2012; Williamson et al., 2015): multituberculates are thought to have occupied a large part of the range of body size of extant rodents, and the cimolodontan tooth row is func- tionally similar to that of rodents when considered in its entirety, and particularly in light of the largely unidirectional power stroke in mastication, although the direction of the power stroke differs between the two (Gingerich, 1973; Krause, 1982). Despite these similarities, there are two principal differences in the cheek teeth between the two clades. First, most cimolo- dontans possess a distinctive blade-like p4 at the anterior extent of the lower cheek tooth row, although most of the surface of this tooth does not occlude with the upper teeth (e.g., Krause, 1982). Although this is true of a majority of cimolodontans, the p4 of taeniolabidoids differs in being considerably smaller, lower, and somewhat triangular in lateral outline, and com- monly exhibits occlusal wear on most of the crown, resembling in these respects the p4 of many extant rodents. The second difference, of greater importance, is that the identities of the teeth comprising the cheek tooth row differ between the two
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