908


Journal of Paleontology 92(5):896–910


clades. The cimolodontan lower cheek tooth row consists of p4 and m1–2, with m1 being enlarged relative to m2 in many derived species, including taeniolabidoids (in many cimolo- dontan species, a small peg-like p3 occurs anterior to the enlarged p4, nestled in a concavity near the base of the latter; the p3 might have had a buttressing function [Gidley, 1909; Simpson, 1937], but it does not otherwise participate in occlu- sion with any of the upper teeth [Krause, 1982]). In contrast, the lower cheek tooth row in most extant rodents consists of m1–3 (with the three molars generally being of nearly equal length) and variably zero, one, or two premolars. Because the m1 is elon- gated relative to the total length of the tooth row in cimolo- dontans, it does not have the same relationship to body mass as those established by Legendre (1986) using the m1 occlusal area for either extant Rodentia or Mammalia more generally. How- ever, because it has now been demonstrated that the length of the entire cheek tooth row is strongly correlated with body mass in extant rodents (Hopkins, 2008), using this measure eliminates bias stemming from the increased relative length of m1 in taeniolabidoids specifically, and cimolodontans more generally. Because there are no specimens of Catopsalis kakwa n. sp.


at hand that preserve the lower tooth row in its entirety, we took a composite lower cheek tooth row measurement using the mean lengths of p4 (2.4mm),m1 (5.5mm), and m2 (4.0mm).We then estimated the body mass of C. kakwa n. sp. using regressions for all rodents, rodents with premolars, and rodents without pre- molars from Freudenthal and Martin-Suarez (2013) and using the m1 occlusal area regressions for all mammals, small mam- mals, and rodents fromLegendre (1986). In addition to C. kakwa n. sp., we also estimated body mass for all but one of the North American taeniolabidoids included by Williamson et al. (2015; table 4) using both protocols (Table 2) (Kimbetopsalis was not included here, because the lower dentition remains unknown). Using the regressions of Freudenthal and Martin-Suarez (2013), C. kakwa n. sp. was estimated to have had a body mass between 0.4138 kg (all rodents regression) and 0.6645 kg (rodents with premolars regression), a range comparable to that of eastern gray squirrels (Nowak, 1999). Calculations using the regressions of Legendre (1986) resulted in body mass estimates that were uni- formly higher than those using the lower tooth row regressions of Freudenthal and Martin-Suarez (2013), with results ranging from 0.5313 kg (small mammal regression) to 1.1710 kg (rodent regression). Although the results using the lower tooth row regressions varied for C. kakwa n. sp., the estimated body mass range is comparable to that of single extant rodent species and should not be viewed as seriously discordant. Individuals of Ondatra zibethicus (Linnaeus, 1766) (muskrats), for instance, have a within-species body mass ranging from 0.681 kg to 1.820 kg, a mass that is seasonally variable, as well as encom- passing sexual dimorphism (Nowak, 1999); discordant estimates are more of a conceptual problem in estimating the body mass of larger animals (Millien, 2008; Wilson et al., 2012). Our calcu- lations confirm that C. kakwa n. sp. is the smallest taeniolabidoid so far discovered, with a body mass estimated to have been approximately one-half to two-thirds that of Valenopsalis joyneri, the next largest taeniolabidoid. Further, our body mass estimates for North American taeniolabidoids more generally using lower tooth row regressions are lower than those using m1 occlusal area (Williamson et al., 2015). These are strikingly


lower for larger species, e.g., body masses of 103.09 kg and 107.55 kg using m1 area were estimated for Taeniolabis taoensis using the all mammals and rodent regressions of Legendre (1986), suggesting a body size that would have easily exceeded that of Hydrochoerus hydrochaeris (Linnaeus, 1766) (capybara, with maximum body mass of ~79 kg; Nowak, 1999), the largest extant rodent. Our body mass estimates for T. taoensis range from 7.6916 kg (rodents without p4 regression) to 19.4402 kg (rodents with p4 regression), suggesting a body size more com- parable to that of the American beaver (Castor canadensis Kuhl, 1820; typical body mass ~12–20 kg; Nowak, 1999). Although tooth row length could be better correlated with body size in extant rodents than m1 occlusal area, its utility in estimating body mass in Multituberculata should nonetheless be viewed with some caution, because there might be profound metabolic differences between rodents and multituberculates that likely affect the relationship between how much food can be processed and how nutrients are converted into body tissues. Given this caveat, our estimates should be viewed as preliminary.


Conclusion


Taeniolabidoidea is one of several groups of North American multituberculates that underwent a significant radiation in the first few hundred thousand years following the extinction of


nonavian dinosaurs some 66 million years ago (Weil and Krause, 2008; Wilson et al., 2012). The discovery of a diminutive species of taeniolabidoid multituberculate in rocks of late early Paleocene age is unexpected, least of all because the general overall trend in taeniolabidoid evolution appears to have been one of increasing body size, but also increasing size in concert with larger molar size, greater cusp number, and increased dental complexity and morphological disparity (Middleton, 1982; Wilson et al., 2012; Williamson et al., 2015). Several recent phylogenetic analyses of multituberculates have included taeniolabidoids (Xu et al., 2015; Williamson et al., 2015; Mao et al., 2016), with the phylogeny of Williamson et al. (2015) being the most comprehensive with respect to Taeniolabidoidea. The results of Williamson et al. (2015) suggest that Valenopsalis and Catopsalis alexanderi, previously the two smallest North American taeniolabidoids, are the most basal, with larger body size generally being more characteristic of geologically younger, derived species, including C. calgariensis, the youngest and one of the largest known species of Catopsalis. The relatively late occurrence of C. kakwa n. sp., from rocks that are nearly contemporaneous with those that preserve C. calgariensis, implies either a significant ghost lineage during most of the early Paleocene, or reversal of several characters, including body size, during the latter part of the early Paleocene. Pending the results of our analysis of taeniolabidoid systematics, the more likely alternative must necessarily remain an open question.


Acknowledgments


We extend thanks to field parties fromTMPandUALVP for their efforts in collectingmicrofossils fromthe localities included in this paper. We thank M. Tanner (WinSport Canada) for continued access to Canada Olympic Park, and the City of Calgary for


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