Discussion of some problematic extinct groups

Several fossil groups of odontocetes (e.g., Agorophiidae, Squalodontidae, Eurhinodelphinidae, and Kentriodontidae) are still taxonomically problematic; it is very likely that some of them form paraphyletic groups.

• The poorly known Late Oligocene Agorophiidae probably form a grade in which all the primitive heterodont odontocetes retaining intertemporal dorsal exposition of the parietals (a primitive character which places them as a morphological link between archaeocetes and Squalodontidae; Rothausen, 1968) have been grouped (e.g., Agorophius, Archaeodelphis and Xenorophus; reviewed in Fordyce, 1981). A new family Simocetidae was recently proposed for the agorophid-like dolphin Simocetus, from the Oligocene of the Eastern North Pacific, indicating a more important morphological diversity among these archaic odontocetes (Fordyce, 2002).
• The family Squalodontidae has long been recognized as very basal among odontocetes. This Late Oligocene to Late Miocene taxon is characterized by primitive heterodont teeth. However, some derived characters of the scapula suggest phylogenetic affinities with Platanistidae and Early Miocene Squalodelphinidae (Muizon, 1987; Fordyce, 1994). The family Dalpiazinidae, including the small and nearly homodont early Miocene Dalpiazina ombonii, has been loosely attached to squalodontids (Muizon, 1991). The monogeneric heterodont family Waipatiidae, known for now from the Late Oligocene of New Zealand is either related to the clade (Platanistidae, Squalodelphinidae) (Fordyce, 1994), to the clade (Eurhinodelphinidae, Ziphiidae) (Lambert, 2005), or basal to crown-Cetacea (Geisler & Sanders, 2003).
• Eurhinodelphinidae is a family of small- to moderate-size Late Oligocene to Miocene long-snouted odontocetes, including, among others, the genera Eurhinodelphis, Schizodelphis, Ziphiodelphis and Xiphiacetus. The only clear synapomorphy for the family is the edentulous premaxillae that extent far beyond the maxillae, getting the rostrum longer than the mandible. This specialized morphology of the feeding apparatus, combined with retention of a long neck, might suggest that these dolphins occupied a coastal or even estuarine habitat (Lambert, 2005a). Nevertheless, several species are known from both sides of the North Atlantic, a feature that contrasts with the seemingly endemic distribution of other species. Interestingly, members of the family where found in fresh water deposits from the Late Oligocene of southern Australia (Fordyce, 1983). Among other hypotheses, Eurhinodelphinidae have been phylogenetically located as the sister-group to Delphinida (Muizon, 1990; Fordyce, 1994). However, several similarities with beaked whales (Ziphiidae) (with respect to the morphology of the face, palate, and ear bones) suggest closer relationships with the latter (Lambert, 2005b). Muizon (1988) positioned the Italian Miocene long-beaked dolphin genus Eoplatanista (family Eoplatanistidae) as sister-group of the eurhinodelphinids and relationships within the family are investigated in Lambert (2005a).
• Kentriodontidae is a Late Oligocene to Late Miocene paraphyletic family whose ecological diversity has previously been underestimated (e.g., recently described large members of the family from the east coast of North America, Dawson, 1996a, b, long-snouted species from Portugal, Lambert et al. 2005), and from which extant delphinoids probably evolved. These archaic dolphins with numerous teeth were fish-eaters, like extant true dolphins. Similarities of the basicranium and sinus fossae with extant taxa indicate that kentriodontids already used echolocation for navigation and finding preys (Ichishima et al. 1994). The diagnosis of the family lacks well-defined synapomorphies, while the definition of the three known subfamilies Kentriodontinae (e.g., Delphinodon, the relatively cosmopolitan Kentriodon, Macrokentriodon, Rudicetus, Tagicetus), Pithanodelphininae (Atocetus, Pithanodelphis), and Lophocetinae (e.g., Hadrodelphis, Lophocetus) is somewhat better supported (discussion in Muizon, 1988; Ichishima et al. 1994; Dawson, 1996a,b; Bianucci, 2001; Lambert et al. 2005).

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