Abstract:Axial morphology was dramatically transformed during the transition from terrestrial to aquatic environments by archaeocete cetaceans, and again during the subsequent odontocete radiation. Here, we reconstruct the sequence of developmental events that underlie these phenotypic transitions. Archaeocete innovations include the loss of primaxial/abaxial interaction at the sacral/pelvic articulation and the modular dissociation of the fluke from the remainder of the tail. Odontocetes subsequently integrated lumbar… Show more
“…One interesting caveat to this pattern is that it likely only applies to terrestrial mammals. Secondarily aquatic mammals such as whales and sirenians have highly uniform vertebrae, accompanied by loss of observable regions in some cases, associated with their derived ecology and mode of locomotion 34,35 . Although this study focuses on terrestrial mammals to provide a relevant comparison to non-mammalian synapsids, exploring the relationship between complexity and ecology within aquatic mammals will provide an interesting line of future inquiry.Fig.…”
A fundamental concept in evolutionary biology is that life tends to become more complex through geologic time, but empirical examples of this phenomenon are controversial. One debate is whether increasing complexity is the result of random variations, or if there are evolutionary processes which actively drive its acquisition, and if these processes act uniformly across clades. The mammalian vertebral column provides an opportunity to test these hypotheses because it is composed of serially-repeating vertebrae for which complexity can be readily measured. Here we test seven competing hypotheses for the evolution of vertebral complexity by incorporating fossil data from the mammal stem lineage into evolutionary models. Based on these data, we reject Brownian motion (a random walk) and uniform increasing trends in favor of stepwise shifts for explaining increasing complexity. We hypothesize that increased aerobic capacity in non-mammalian cynodonts may have provided impetus for increasing vertebral complexity in mammals.
“…One interesting caveat to this pattern is that it likely only applies to terrestrial mammals. Secondarily aquatic mammals such as whales and sirenians have highly uniform vertebrae, accompanied by loss of observable regions in some cases, associated with their derived ecology and mode of locomotion 34,35 . Although this study focuses on terrestrial mammals to provide a relevant comparison to non-mammalian synapsids, exploring the relationship between complexity and ecology within aquatic mammals will provide an interesting line of future inquiry.Fig.…”
A fundamental concept in evolutionary biology is that life tends to become more complex through geologic time, but empirical examples of this phenomenon are controversial. One debate is whether increasing complexity is the result of random variations, or if there are evolutionary processes which actively drive its acquisition, and if these processes act uniformly across clades. The mammalian vertebral column provides an opportunity to test these hypotheses because it is composed of serially-repeating vertebrae for which complexity can be readily measured. Here we test seven competing hypotheses for the evolution of vertebral complexity by incorporating fossil data from the mammal stem lineage into evolutionary models. Based on these data, we reject Brownian motion (a random walk) and uniform increasing trends in favor of stepwise shifts for explaining increasing complexity. We hypothesize that increased aerobic capacity in non-mammalian cynodonts may have provided impetus for increasing vertebral complexity in mammals.
“…corresponding to fast-swimming pelagic dolphins [13,16,17]. On the other hand, Viglino et al [18] did not find a correlation between vertebral morphology and habitat when using phylogenetic comparative methods on seven dolphin species.…”
Cetaceans represent the most diverse clade of extant marine tetrapods. Although the restructuring of oceans could have contributed to their diversity, other factors might also be involved. Similar to ichthyosaurs and sharks, variation of morphological traits could have promoted the colonization of new ecological niches and supported their diversification. By combining morphological data describing the axial skeleton of 73 cetacean species with phylogenetic comparative methods, we demonstrate that the vertebral morphology of cetaceans is associated with their habitat. All riverine and coastal species possess a small body size, lengthened vertebrae and a low vertebral count compared with open ocean species. Extant cetaceans have followed two distinct evolutionary pathways relative to their ecology. Whereas most offshore species such as baleen whales evolved towards an increased body size while retaining a low vertebral count, small oceanic dolphins underwent deep modifications of their axial skeleton with an extremely high number of short vertebrae. Our comparative analyses provide evidence these vertebral modifications have potentially operated as key innovations. These novelties contributed to their explosive radiation, resulting in an efficient swimming style that provides energetic advantages to small-sized species.
“…Although best‐developed in mammals, these regions can be morphometrically recovered even within groups showing more subtle variation along the vertebral column (e.g. snakes: Head & Polly, 2015; odontocete cetaceans: Buchholtz & Gee, 2017); this is referred to as ‘regionalized but de‐differentiated’ (Buchholtz and Gee, 2017). In contrast, the vertebral column of actinopterygian fishes has been divided into only two general areas: an abdominal and a caudal region, distinguished by the presence of haemal spines in the latter (e.g.…”
The postcranial axial skeleton of actinopterygian fishes is typically divided into three regions: (1) an anterior abdominal region, (2) a posterior caudal region and (3) those vertebrae supporting the caudal fin. However, in some actinopterygians, the axial skeleton is more finely subdivided, with up to six morphologically distinct sub-regions recognized. Phylogenetic continuity and homology of structures across these sub-regions have not been investigated in detail, either between or among groups. We examine variation in axial regionalization in saurichthyid fishes, a clade of extinct non-teleostean actinopterygians with highly variable axial skeletal morphology but an otherwise conservative body plan, and compare these findings to other non-teleostean actinopterygians to assess conservation of a regionalized axial skeleton within bony fishes. We document up to eight distinct regions in the vertebral column of Triassic Saurichthys: (1) a postoccipital region, (2) an anterior and (3) a posterior abdominal region, (4) a transitional region spanning the abdominalcaudal boundary, (5) an anterior and (6) a posterior caudal region and (7) preural and (8) ural regions. Based on taphonomical and morphological evidence, the transitional region appears to function in axial stiffening in the area of the median fins, whereas the abdominal region is highly flexible. The degree to which these axial regions are osteologically differentiated is highly variable across Saurichthyidae, implying iterative evolution of differentiation and de-differentiation over relatively short geological timescales. Such variably expressed regionalization was also identified in the outgroup non-teleostean actinopterygians Birgeria and Australosomus. Despite variation in morphological disparity, the regions identified in saurichthyids correlate well with those documented in some teleosts and Paleozoic actinopterygians, suggesting potential deep patterning homology but independent evolution of specific regionalized axial morphologies in response to changing functional demands.
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