Morphological evolution in therocephalians breaks the hypercarnivore ratchet

Abstract

Large carnivorous mammals have been suggested to show a ratchet-like mode of morphological evolution. A limited number of specializations for hypercarnivory evolve repeatedly in multiple clades, with those lineages evolving such specialities being unable to retreat back along their evolutionary trajectory or jump between adaptive peaks. While it has been hypothesized that such mechanisms should have applied to the evolution of other terrestrial carnivores, the non-mammalian synapsid clade Therocephalia appears to defy this expectation. The earliest, basalmost members of this clade are large macropredators, and it is later that small carnivores appear, seemingly evolving from top-predator ancestors. In order to test this reading of therocephalian evolution, variation in rates of body size evolution were tested for and incorporated into an ancestral reconstruction. Similar studies were made of the evolution of discrete characters related to carnivory. All analyses indicate the ancestral therocephalian was a large macro-predator, with serrated teeth, elongated canines and robust lower jaws. Small sizes apparently evolve later. It is therefore suggested that the hypercarnivore ratchet is a feature of mammalian evolution.

A Carboniferous synapsid with caniniform teeth and a reappraisal of mandibular size-shape heterodonty in the origin of mammals

“…Though the dentitions of Carboniferous taxa appear to have relatively smooth mesiodistal edges, the canine and antecanine dentition became modified at a later stage with serrations and denticles, forming a ‘ziphodont’ dentition. Ziphodonty in some large-bodied Dimetrodon species [ 21 ] and numerous therapsids [ 61 ] might indicate more efficient processing of protein in active terrestrial animals that had relatively greater metabolic requirements than the earliest sphenacodontians. Prior to this, the emphasis on prey capture may reflect increasing environmental heterogeneity as the Carboniferous drew to a close [ 62 , 63 ], with seasonal fluvial palaeoenvironments like that of the Birthday Bonebed [ 35 ] providing an ideal mix of terrestrial and semi-aquatic prey that allowed basal synapsid faunivores to experiment with varying their exposure to aquatic resources, enabling their trophic ecologies to become more firmly planted in the terrestrial realm.…”

Diving into a dead-end: asymmetric evolution of diving drives diversity and disparity shifts in waterbirds

“…repeated evolution towards a specialism from which reversion is not predicted). These ratchet patterns have been repeatedly found in the evolution of mammalian carnivores [11,12,45,46], but here we develop one of the first large-scale examples in birds, using the case of diving in waterbirds.…”

“…This dichotomy is a key theme in macroevolution. Dollo's law of irreversibility [9], Cope's rule on body size [10] and ratchet mechanisms in macroevolution [11][12][13] are all examples of asymmetry in evolution, where the trajectories of trait evolution appear irreversible and directional over various time scales, from generations to epochs.…”

Conical and sabertoothed cats as an exception to craniofacial evolutionary allometry

“…Felids epitomize a meaningful case study for research on evolutionary allometry in mammals due to their specialised cranial morphology, paired with the hypercarnivorous diet shared by the entire clade. They also represent a case study for clades that are putatively affected by ratchet-like mechanisms in morphological evolution (i.e., constant and directional evolution occurring in specific clades due to their impossibility to retreat back along their evolutionary trajectory and leading them to extreme and suboptimal trait values, by analogy with the so-called “Muller’s ratchet” mechanism described in molecular evolution 90 , 91 , as suggested for several groups of hypercarnivorous mammals, non-mammalian synapsids, and dinosaurs 58 , 92 , 93 . Our results provided support to the validity of CREA at the family level.…”

“…The acquisition of extreme features concerning any of these evolutionary factors (e.g., adapting biomechanically demanding structures such as sabertoothed upper canines; occupying extremely narrow and specialised ecological niches) is likely to be associated with peculiar patterns of morphological evolution, determining potential exceptions to common biological rules such as CREA, as observed in many hyper-specialised extinct lineages of nonmammalian synapsids often characterised by higher extinction rates due to their vulnerability to sudden ecological changes 45 (see also Piras et al 64 and Machado 125 for analogous considerations in sabertoothed cats and canids, respectively). This vulnerability is possibly resulting from ratchet-like mechanisms of morphological evolution, that are known to force many hypercarnivorous clades to evolve gradually more and more extreme adaptations related to their predatory strategy 58 , 92 . The results emerging from our study are consistent with the findings of Lamsdell 126 , who hypothesised, by studying the body trait variations of horseshoe crabs in response to aquatic-terrestrial transitions, a strong role of heterochronic changes in shaping their morphological evolution in presence of ratchet-like mechanisms.…”