Rapid morphological evolution in placental mammals post-dates the origin of the crown group

Abstract

Resolving the timing and pattern of early placental mammal evolution has been confounded by conflict among divergence date estimates from interpretation of the fossil record and from molecular-clock dating studies. Despite both fossil occurrences and molecular sequences favouring a Cretaceous origin for Placentalia, no unambiguous Cretaceous placental mammal has been discovered. Investigating the differing patterns of evolution in morphological and molecular data reveals a possible explanation for this conflict. Here, we quantified the relationship between morphological and molecular rates of evolution. We show that, independent of divergence dates, morphological rates of evolution were slow relative to molecular evolution during the initial divergence of Placentalia, but substantially increased during the origination of the extant orders. The rapid radiation of placentals into a highly morphologically disparate Cenozoic fauna is thus not associated with the origin of Placentalia, but post-dates superordinal origins. These findings predict that early members of major placental groups may not be easily distinguishable from one another or from stem eutherians on the basis of skeleto-dental morphology. This result supports a Late Cretaceous origin of crown placentals with an ordinal-level adaptive radiation in the early Paleocene, with the high relative rate permitting rapid anatomical change without requiring unreasonably fast molecular evolutionary rates. The lack of definitive Cretaceous placental mammals may be a result of morphological similarity among stem and early crown eutherians, providing an avenue for reconciling the fossil record with molecular divergence estimates for Placentalia.

Mismatch of the morphology model is mostly unproblematic in total-evidence dating: insights from an extensive simulation study

“…Whether these simulations are relevant in reality depends on whether there is any correlation at all between morphological branch lengths and time, or, by proxy, between morphological and molecular branch lengths. Current consensus in the literature (Davies and Savolainen 2006;Halliday, et al 2019;Omland 1997;Ronquist, et al 2012a) ascertains that this is mostly the case (but see Bromham, et al 2002). But of course datasets might differ in that respect, and TED studies should test their morphological partition for clocklikeness before deciding to rely solely on morphological branch lengths for divergence time estimation.…”

Developmental duration as an organizer of the evolving mammalian brain: scaling, adaptations, and exceptions

“…In this case, dunnart longevity is approximately 1.5 to 2 years. Recent evidence suggests an extended parallel divergence of these two crown groups (Halliday et al, 2019). It seems likely that the best account of the dissimilar rates of brain production is that it has been accumulated through the multiple physiological and life-history differences of the two groups.…”

Comparative masticatory myology in anteaters and its implications for interpreting morphological convergence in myrmecophagous placentals

“…The Cretaceous terrestrial revolution and the Cretaceous-Paleogene (K-Pg) mass extinction event are often viewed as milestones in placental mammal evolution ( Meredith et al, 2011 ). These events promoted the opening of terrestrial ecological niches available to placentals, contributing to their morphological diversification ( Romer, 1974 ; Alroy, 1999 ; Halliday et al, 2019 ). The adaptation of the placental feeding apparatus likely contributed to this radiation ( Price et al, 2012 ), as mechanical processing of food items is essential to ensure a better energetic intake ( Hiiemae, 2000 ).…”