Testing a developmental model in the fossil record: molar proportions in South American ungulates

Abstract

Abstract.-A developmental model, based upon murine rodents, has been proposed by Kavanagh et al. (2007) to explain lower molar proportions in mammals. We produce a clade-wide macroevolutionary test of the model using the dental evolutionary trends in a unique radiation of extinct mammals endemic to South America (''Meridiungulata'') that comprise a diverse array of molar morphologies. All of the South American ungulate groups examined follow the inhibitory cascade model with the exception of two groups: Interatheriidae (Notoungulata) and Astrapotheria. For most taxa studied, ratios between lower molar areas are greater than 1.0, indicating a weak inhibition by m1 on the subsequent molars in the tooth row, and a trend to greater absolute size of the posterior molars. Comparisons of mean ratios between clades indicate that a significant phylogenetic signal can be detected, particularly between the two groups within Notoungulata-Typotheria and Toxodontia. Body mass estimates were found to be significantly correlated with both m3/m1 and m2/m1 ratios, suggesting that the larger body size achieved the weaker inhibition between the lower molars. Molar ratio patterns are examined and discussed in relation to the independent and numerous acquisitions of hypsodonty that are characteristic of dental evolution in ''Meridiungulata.''

Testing the inhibitory cascade model in Mesozoic and Cenozoic mammaliaforms

“…For example, our data show that both arvicolines [14] and murines [8], while distinct in relative molar size from one another, fall within the range of observed variation for mammals as a whole, as well as within the region of morphospace that the IC Model predicts. While variation in the regression parameters of subgroups is high [13], the IC Model is generally consistent with the higher-level pattern observed across Mammalia. That the second prediction of the IC Model is not upheld, with the second molar being slightly, albeit significantly larger than would be expected, is not unsurprising given that the majority of taxa which fall outside of the predicted area do so with m2 as the largest tooth.…”

“…In each case, however, the observed data fell within the region of morphospace consistent with the predictions of an inhibitory cascade, even if the parameters of the regression line differed. High variability has also been noted in South American ungulates [13], with two groups (Astrapotheria and Interatheriidae) deviating significantly from the IC Model, as well as falling outside of the predicted region of morphospace.…”

“…Area was estimated for each tooth as the product of length and width, following the method of both Polly [10] and Wilson [13]. For specimens that were measured directly, length and width were obtained from occlusal-view photographs in ImageJ v1.45s [44].…”

“…The predictions of the IC Model have been found to, for the most part, be applicable to Rodentia [12] as a whole, and South American ungulates [13], although in each case, several taxa fell outside of the expected area. Thus far, the largest deviation from the predictions of the IC Model has been found in canids (dogs and their kin) [11], but also in arvicoline rodents (voles and lemmings) [14], leading the latter authors to conclude that the IC Model might not be generalisable even across rodents.…”

Theoretical Modelling of the Molluscan Shell: What has been Learned From the Comparison Among Molluscan Taxa?

“…Besides the briefly dis-cussed Buckman's laws of covaration, well-known examples of macro-evolutionary patterns come from the studies of the tetrapod limb (Oster et al 1988;Shubin et al 2009;Zhu et al 2010) and the molar proportions in mammals (Polly 2007;Kavanagh et al 2007;Renvoisé et al 2009;Salazar-Ciudad and Jernvall 2010;Wilson et al 2012). Reciprocally, the observation of these macroevolutionary patterns often act as a source of inspiration to investigate the underlying rules of development, because, at the end, they are the patterns that the Neo-Darwinian theory was unable to account for.…”