1. The European roe deer Capreolus capreolus is a typical faunal element of the Holocene. It was already present in Europe at least 600 000 years ago and it has been known from both glacial and interglacial phases since then. With nearly 3000 fossil and subfossil records, it is one of the most frequent mammals in the Late Quaternary.
Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation. Mysticetes are generally assumed to have symmetrical skulls and lack high-frequency hearing. Here we show that protocetid and basilosaurid archaeocete skulls are distinctly and directionally asymmetrical. Archaeocete asymmetry involves curvature and axial torsion of the cranium, but no telescoping. Cranial asymmetry evolved in Eocene archaeocetes as part of a complex of traits linked to directional hearing (such as pan-bone thinning of the lower jaws, mandibular fat pads, and isolation of the ear region), probably enabling them to hear the higher sonic frequencies of sound-producing fish on which they preyed. Ultrasonic echolocation evolved in Oligocene odontocetes, enabling them to find silent prey. Asymmetry and much of the sonic-frequency range of directional hearing were lost in Oligocene mysticetes during the shift to lowfrequency hearing and bulk-straining predation.Cetacea | land-to-sea transition M ost mammals have bilaterally symmetrical skulls. Symmetrical crania characterize the artiodactyls closely related to whales, and symmetrical crania characterize mysticetes within Cetacea (1) (Fig. 1A). Odontocetes are exceptional because most odontocete crania are asymmetrical, with dorsal cranial bones shifted posteriorly and to the left side (1-8). Living odontocetes have a hypertrophied melon, nasal sacs, and phonic lips used to produce high-frequency sound (> 20 kHz) (9-11). Mysticetes lack these specializations of the nasal apparatus, use low-frequency sound (11,12), and may use the larynx (13) to produce lowfrequency sound. Coupling of high-frequency echolocation with facial and cranial asymmetry in living odontocetes, and the absence of both in living artiodactyls and living mysticetes, make it reasonable to expect that asymmetry originated in odontocetes (5-7). However, it is unresolved how the cranial asymmetry of odontocetes evolved in the transition from archaeocetes to modern whales, and the history becomes even more complex when archaeocetes themselves are considered.Eocene archaeocete whales gave rise to all modern toothed and baleen whales during or near the Eocene-Oligocene transition (14-16). Archaeocetes were previously thought to have symmetrical skulls (3, 5, 7). Asymmetry observed in fossil crania has often been assumed to be an artifact of deformation following burial, and it has been ignored or even removed in published drawings [as was done initially for three of the skulls we studied (17-19)]. ResultsHere we document and quantify asymmetry in archaeocete crania. Further observations on exceptionally well-preserved archaeocete crania and dentaries suggest a link between cranial asymmetry and the ability to locate sound sources in water.We quantified midline suture deviation, δx, from a straight rostrocaudal axis [RC, after Ness...
Odontoceti and Mysticeti (toothed and baleen whales) originated from Eocene archaeocetes that had evolved from terrestrial artiodactyls. Cranial asymmetry is known in odontocetes that can hear ultrasound (>20,000 Hz) and has been linked to the split function of the nasal passage in breathing and vocalization. Recent results indicate that archaeocetes also had asymmetric crania. Their asymmetry has been linked to directional hearing in water, although hearing frequencies are still under debate. Mysticetes capable of low-frequency and infrasonic hearing (<20 Hz) are assumed to have symmetric crania. This study aims to resolve whether mysticete crania are indeed symmetric and whether mysticete cranial symmetry is plesiomorphic or secondary. Cranial shape was analyzed applying geometric morphometrics to three-dimensional (3D) cranial models of fossil and modern mysticetes, Eocene archaeocetes, modern artiodactyls, and modern odontocetes. Statistical tests include analysis of variance, principal components analysis, and discriminant function analysis. Results suggest that symmetric shape difference reflects general trends in cetacean evolution. Asymmetry includes significant fluctuating and directional asymmetry, the latter being very small. Mysticete crania are as symmetric as those of terrestrial artiodactyls and archaeocetes, without significant differences within Mysticeti. Odontocete crania are more asymmetric. These results indicate that (1) all mysticetes have symmetric crania, (2) archaeocete cranial asymmetry is not conspicuous in most of the skull but may yet be conspicuous in the rostrum, (3) directional cranial asymmetry is an odontocete specialization, and (4) directional cranial asymmetry is more likely related to echolocation than hearing.
Basilosauridae are cosmopolitan fully-aquatic archaeocete whales, represented by larger Basilosaurus isis and smaller Dorudon atrox in the middle-to-late Eocene Gehannam and Birket Qarun Formations of Egypt (ca. 38-36.5 Ma). Adult and juvenile Dorudon but only adult Basilosaurus are found in these shallow-marine deposits. Lethal bite marks on juvenile Dorudon skulls sparked the idea that adult Basilosaurus invaded calving grounds of D. atrox to prey on their young. However, there has been no direct evidence to support this idea. In this study, bite marks on specimens of juvenile D. atrox that have previously been described but not assigned to a particular tracemaker are reinvestigated, and additional bone modifications are analyzed. Applying computed tomography (CT), digital surface scanning, and three-dimensional (3D) reconstruction, the juvenile D. atrox specimens were digitally placed into the mouth of an adult B. isis. Bite marks match the dentition of B. isis. Imprints of tooth casts of B. isis in modeling clay furthermore resemble bite marks on these D. atrox specimens in shape and size. B. isis was likely a predator that included juvenile D. atrox in its diet. Prey was predominantly captured from a lateral position across the head and sometimes adjusted in the mouth prior to a more powerful bite. Scavenging of B. isis on D. atrox calves is also possible. The diet of Basilosaurus and dietary differences within the genus resemble those known in modern killer whales (Orcinus orca). B. isis is the only archaeocete known to date that possibly preyed on other cetaceans.
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