Cetothere Skeletons From the Miocene Choptank Formation of Maryland and Virginia 1. THE SKELETON OF A MIOCENE CHOPTANK CETOTHERE Discoveiy of additional mysticete skulls in Oligocene liiarine formations would furnish a much more authori-tative basis for sjjeculative opinions relative to their early geological history. The apically attenuated tri-angular supraoccipital shield of Cetotheriopsis linti-anus (Brandt, 1873, pi. 19), which was recovered near Linz, Austria, from the white quartz sands of the Oligocene Chattian stage, represents an advanced phase in the remodeling of the mysticete skull. The pro-nounced forward thrust of the ix>st«rior cranial bones of this skull clearly supfwrts the conclusion that the re-modeling of the cranial architecture of these Tertiaiy whalebone whales had progi^ssed beyond the initial oven'iding of the intertemix>ral region by the supraoc-cipital prior to the Miocene. Furthermore, three distinct types of posterior cranial remodeling represented by Morenocetus parvtis (Ca-brera, 1926, fig. 1 ) , '■'■Plesiocetus''' dyticus ( Cabrera, 1926, fig. 3), and Aglaocetus m-oreni (Kellogg, 1934), and for which no prior geological antecedents are as yet known, have been excavated in the lower Miocene Patagonian marine fomiation. The skull of Aetiocetus cotylalveus (Emlong, 1966), which was discovered in the upj^er Oligocene marine Yaquina formation of northwestern Oregon, and which has the horizontally divided jxisterior end of the max-illary projecting backward above and below the su-praorbital process of the frontal, a typical mysticete interlocking structural relationship, cannot be consid-ered to be in the direct antecedent lineage of the mysti-cetes, but does suggest what may have been basic ancestral skull architecture. This Oligocene cetacean seems to have had the usual diphyodont mammalian dentition, the crowns of the cheek teeth having serrated edges. Vestigial cheek teeth are present in the jaws of fetal Recent mysticetes. On all archaeocete skulls thus far described, including Protoeetus, Prozeuglodon, Hafiilosauru.H. Donidon, and Zygorhiza, molar teeth are located on the hinder end of the maxillary, which projects backward beneath the supraorbital process of the frontal, and not anterior to this process as in Patrhcetus, M icrozeuglodon, and Aetiocetus^ and prob-ably also in Agorophius and Archueodelphis. The direction, elongation, and curvature of the upper and lower transverse processes of the sixth cervical vertebra indicate that the cervical extension of the thoracic retia mirabilia had been previously developed and acquired by this Oligocene Aetiocetus. On the AetiocetuH skull the transversely narrowed posterior ends of the palatine bones, separated medi-ally by the vertical plate of the vomer, are prolonged backward beyond the level of the anterior end of the pterygoid fossa, much farther backward than on the skull of any known Eocene archiiecK^ete. Tliis backward prolongation of the palatines increased the length of the internal choanae. Tlie narial fossa in front of the elongated nasal bones is situated at the middle of the length of the skull. The conformation of the bones en-closing the internal choanae may not, however, possess any particular phylogenetic significance. Tlie backward shift of the external nares and the associated narial fossa in the mesorostral trough does not ap{>ear to have any causal lelationship with the prolongation of the internal choanae on all skulls of Recent cetacea:is. For instance, the palatines on the skull of the finback, Bala-enoptera physalus, are not extended backward beyond the level of the anterior end of the pterygoid fossa although the relatively small nasal bones behind the narial fossa are situated almost entirely ix>sterior to the level of the anterior end of the orbit. Conversely on the skull of the bowhead, Balaena mysticetus, the pala-tines extend backward to the occipital condyles and the narial fossa is located far forward on the rostrum, approximately behind the middle of the length of the
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