RESUMO
Mammalian molar crowns form a module in which measurements of size for individual teeth within a tooth row covary with one another. Molar crown size covariation is proposed to fit the inhibitory cascade model (ICM) or its variant the molar module component (MMC) model, but the inability of the former model to fit across biological scales is a concern in the few cases where it has been tested in Primates. The ICM has thus far failed to explain patterns of intraspecific variation, an intermediate biological scale, even though it explains patterns at both smaller organ-level and larger between-species biological scales. Studies of this topic in a much broader range of taxa are needed, but the properties of a sample appropriate for testing the ICM at the intraspecific level are unclear. Here, we assess intraspecific variation in relative molar sizes of the cotton mouse, Peromyscus gossypinus, to further test the ICM and to develop recommendations for appropriate sampling protocols in future intraspecific studies of molar size variation across Mammalia. To develop these recommendations, we model the sensitivity of estimates of molar ratios to sample size and simulate the use of composite molar rows when complete ones are unavailable. Similar to past studies on primates, our results show that intraspecific variance structure of molar ratios within the rodent P. gossypinus does not meet predictions of the ICM or MMC. When we extend these analyses to include the MMC, one model does not fit observed patterns of variation better than the other. Standing variation in molar size ratios is relatively constant across mammalian samples containing all three molars. In future studies, analyzing average ratio values will require relatively small minimum sample sizes of two or more complete molar rows. Even composite-based estimates from four or more specimens per tooth position can accurately estimate mean molar ratios. Analyzing variance structure will require relatively large sample sizes of at least 40-50 complete specimens, and composite molar rows cannot accurately reconstruct variance structure of ratios in a sample. Based on these results, we propose guidelines for intraspecific studies of molar size covariation. In particular, we note that the suitability of composite specimens for averaging mean molar ratios is promising for the inclusion of isolated molars and incomplete molar rows from the fossil record in future studies of the evolution of molar modules, as long as variance structure is not a key component of such studies.
RESUMO
Plesiadapiformes has long been considered to be an archaic group of Primates. Discovery of a paromomyid plesiadapiform skull and independent analysis of referred postcrania have led investigators to conclude that Plesiadapiformes shares a closer relationship to extant flying lemurs (Dermoptera) than to Primates (= Euprimates of Hoffstetter [1977] Bull Mem Soc Anthropol Paris Ser 13 4:327-346). Despite challenges to this interpretation, the plesiadapiform-dermopteran relationship has gained currency in recent years. Here we show that newly discovered crania of Ignacius graybullianus, preserving previously undocumented portions of the ear, are more similar to primates than to dermopterans. New specimens confirm that paromomyids lacked the petrosal bulla of primates. However, these new specimens also demonstrate that paromomyids likely had: 1) a small promontorial branch of the internal carotid artery; 2) a lateral route for the internal carotid nerves crossing the promontorium; and 3) a ring-like ectotympanic with an annular bridge. This pattern is similar to primitive primates and fundamentally different from dermopterans, which have: 1) no internal carotid artery; 2) internal carotid nerves that take a more medial route; and 3) no annular bridge. Recognition of some primate-like traits, documented here by new evidence, indicates that Paromomyidae is likely to be more closely related to other Paleogene Plesiadapiformes and Eocene Primates than to extant Dermoptera. In view of these findings, a link between paromomyids and extant dermopterans ("Eudermoptera") is not convincingly supported by a single characteristic of the basicranium.
