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Among mammals, modern cetaceans (whales, dolphins, and porpoises) are unusual in the absence of hind limbs. However, cetacean embryos do initiate hind-limb bud development. In dolphins, the bud arrests and degenerates around the fifth gestational week. Initial limb outgrowth in amniotes is maintained by two signaling centers, the apical ectodermal ridge (AER) and the zone of polarizing activity (ZPA). Our data indicate that the cetacean hind-limb bud forms an AER and that this structure expresses Fgf8 initially, but that neither the AER nor Fgf8 expression is maintained. Moreover, Sonic hedgehog (Shh), which mediates the signaling activity of the ZPA, is absent from the dolphin hind-limb bud. We find that failure to establish a ZPA is associated with the absence of Hand2, an upstream regulator of Shh. Interpreting our results in the context of both the cetacean fossil record and the known functions of Shh suggests that reduction of Shh expression may have occurred Ϸ41 million years ago and led to the loss of distal limb elements. The total loss of Shh expression may account for the further loss of hind-limb elements that occurred near the origin of the modern suborders of cetaceans Ϸ34 million years ago. Integration of paleontological and developmental data suggests that hind-limb size was reduced by gradually operating microevolutionary changes. Long after locomotor function was totally lost, modulation of developmental control genes eliminated most of the hind-limb skeleton. Hence, macroevolutionary changes in gene expression did not drive the initial reduction in hind-limb size.cetacea ͉ delphinidae ͉ evo-devo ͉ limb development ͉ whale evolution

Numerous authors have attributed the linear scars on male beaked whales to intraspecific fighting. The species Mesoplodon carlhubbsi was examined from a structural viewpoint to describe functionally how such scarring may result. In this species, linear scars occur in two patterns: single scars and parallel scars. Parallel scars occur when both of the large teeth possessed by the aggressor male come in contact simultaneously with the inflicted whale. In mature males, the teeth project from the lower jaw outside the mouth up along both sides of the rostrum to a height greater than the top of the rostrum. This indicates that the teeth are used with the mouth closed by bringing the dorsal aspect of the rostrum in contact with the soon to be inflicted whale. The densely ossified mesorostral canal evident in mature males probably functions in reinforcing the rostrum as the males engage in fighting. Two evolutionary trends in Mesoplodon dentition are hypothesized: (i) the change in position of the teeth in the mandibles from an apical (primitive) position back to a more posterior position; (ii) once situated farther back on the mandibles, to raise the apex of the teeth above the level of the rostrum. Comments on other Mesoplodon species are noted for comparisons.

INTROOUCTION-The nasal morphology of extant cetaceans has evolved significantly from the typical architecture of terrestrial mammals. The need to occlude the nasal passages from water as these animals dived provided strong selection for such changes in morphology. The position of the external nares has shifted posteriorly to a more dorsal position on the head to facilitate respiration with a minimum of head movement as the animal surfaces. The telescoping of the cranial bones as described by Miller (1923) resuIted primarily by the repositioning of the na res (Raven and Gregory, 1933). In addition to these modifications of a purely respiratory nature, odontocetes have further evolved a complex series of nasal diverticula superficial to the cranium that are involved with sound production. Based on our dissections of numerous cetaceans (Mead, 1975;Heyning, 1989), we attempt to provide an overview of the salient points in the evolution of the nasal morphology and speculate about so me of the selection pressures that might have created the present diversity of nasal morphologies.In order to establish the evolutionary history of nasal morphology, we use here a previously published cIadistic analysis of the relationships among the fa mi lies of extant odontocetes (Fig. I;Heyning, 1989). Based on this analysis, Platanista gangetica is placed in the family Platanistidae, and the remaining extant genera of "river" dolphins, Inia, Lipotes, and Pontoporia, are placed within the family Iniidae. We also work on the strongly founded assumption that the two extant cetacean suborders, the Odontoceti and Mysticeti, are monophyletic (Van

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