Morphological modi®cation of the cetacean vertebral column for axial locomotion was one of many changes that occurred during and after the invasion of aquatic habitats by the ancestors of living whales. These modi®cations resulted in a column that can be divided into neck, chest, torso, and¯uke structural and functional units. Variations in these units allow subdivision of living whales into three morphological groups that re¯ect differences in regional¯exibility of the column and of locomotor style. Both odontocete and mysticete cetaceans exhibit trends toward reduction of relative centrum length in the neck and torso, restricting the¯exibility of the column with increasing adaptation to aquatic lifestyles. Mysticetes and physeterid odontocetes retain the archaeocete pattern of constant vertebral shape along the torso, which acts as an undulatory unit. The location of secondary sites of reduced centrum length in non-physeterid odontocetes subdivides them into subgroups, one with reduced posterior, and the other with reduced anterior, torso¯exibility. The relationships between particular osteological patterns and swimming styles in living taxa are used to support predictions of swimming style in fossil species after adoption of a¯uke in the late Eocene.

Vertebral anatomy in delphinid cetaceans exhibits marked heterogeneity. Description and functional interpretation of this variability is facilitated by the recognition of structural units along the column whose boundaries transgress those of the classical mammalian series. Vertebral anatomy of the killer whale (Orcinus orca) and the Atlantic whitesided dolphin (Lagenorhynchus acutus) lie near the ends of an anatomical continuum. Primitive columns resemble those of living delphinapterid delphinoids in exhibiting minimal intervertebral variation, low counts and spoolshaped vertebrae. Derived columns are more regionalized, displaying traits that limit mobility in the anterior torso, enhance flexibility at the point of neural spine syncliny and increase dorsoventral displacement of prefluke vertebrae. Reconstruction of the historical sequence of anatomical innovations identifies syncliny as an early and critical step in delphinid column evolution. Trait distribution supports evolutionary isolation of Pseudorca and Orcinus from remaining delphinids, inclusion of Feresa and Peponocephala among delphinine delphinids, and subdivision of delphinines on the basis of centrum dimensions, neural spine inclination and count. Details of vertebral anatomy can also be used to place fragmentary postcranial material, particularly that of fossils, in functional and evolutionary context.

Mammalian cervical count has been fixed at seven for more than 200 million years. The rare exceptions to this evolutionary constraint have intrigued anatomists since the time of Cuvier, but the developmental processes that generate them are unknown. Here we evaluate competing hypotheses for the evolutionary origin of cervical variants in Bradypus and Choloepus, tree sloths that have broken the seven cervical vertebrae barrier independently and in opposite directions. Transitional and mediolaterally disjunct anatomy characterizes the cervicothoracic vertebral boundary in each genus, although polarities are reversed. The thoracolumbar, lumbosacral, and sacrocaudal boundaries are also disrupted, and are more extreme in individuals with more extreme cervical counts. Hypotheses of homologous, homeotic, meristic, or associational transformations of traditional vertebral column anatomy are not supported by these data. We identify global homeotic repatterning of abaxial relative to primaxial mesodermal derivatives as the origin of the anomalous cervical counts of tree sloths. This interpretation emphasizes the strong resistance of the "rule of seven" to evolutionary change, as morphological stasis has been maintained primaxially coincident with the generation of a functionally longer (Bradypus) or shorter (Choloepus) neck.

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Why is mammalian cervical count fixed across the historically long and ecologically broad mammalian radiation? Multiple lines of evidence, considered together, suggest a link between fixed cervical count and the muscularization of the diaphragm, a key innovation in mammalian history. We test this hypothesis by documenting the anteroposterior (AP) movement of the diaphragm, a lateral plate derivative, relative to that of the somitic thoracolumbar transition in unusually patterned mammals, by comparing the temporal occurrence of an osteological proxy for the diaphragm and fixed cervical counts in the fossil record, and by quantifying morphological differentiation within the mammalian cervical series. We then integrate these anatomical observations with details of diaphragm function and development to propose a sequence of innovations in mammalian evolution that could have led to fixed cervical count. We argue that the novel commitment of migratory muscle precursor cells (MMPs) of mid-cervical somites to a fate in the abaxial diaphragm defined these somites as a new and uniquely mammalian modular subunit. We further argue that the coordination of primaxial abaxial patterning constrained subsequent AP migration of the forelimb, thereby secondarily fixing cervical count.