A new archaic homodont toothed cetacean (Mammalia, Cetacea, Odontoceti) from the early Miocene of Peru Olivier LAMBERT Institut royal des Sciences naturelles de Belgique, D.O. Terre et Histoire de la Vie, 29 rue Vautier, B-1000 Brussels (Belgium) [email protected] Christian DE MUIZON Département Histoire de la Terre, Muséum national d’Histoire naturelle, Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements (CR2P: CNRS, MNHN, UPMC-Paris 06; Sorbonne Universités), case postale 38, 57 rue Cuvier, F-75231 Paris cedex 05 (France) [email protected] Giovanni BIANUCCI Università di Pisa, Dipartimento di Scienze della Terra, via S. Maria 53, I-56126 Pisa (Italy) [email protected] Published on 27 March 2015 urn:lsid:zoobank.org:pub:B3DF8C10-7139-4CBA-9093-0795B6ACBBC7 Lambert O., De Muizon C. & Bianucci G. 2015. — A new archaic homodont toothed cetacean (Mammalia, Cetacea, Odontoceti) from the Early Miocene of Peru. Geodiversitas 37 (1): 79-108. http://dx.doi.org/10.5252/g2015n1a4 ABSTRACT Apart from a few exceptions, extant odontocetes (toothed cetaceans) exhibit a roughly homodont dentition. e transition from basilosaurid-like double-rooted cheek teeth with accessory denticles to single-rooted conical teeth occurred during the late Oligocene-early Miocene. At that time, several clades of now extinct, homodont and predominantly long-snouted odontocetes appeared in the fossil record. Among them, members of the genera Argyrocetus Lydekker, 1893 and Macrodelphinus Wilson, 1935, from the early Miocene of the Northeast Pacific and Argentina, were tentatively attributed to the family Eurhinodelphinidae. However, due to the frag- mentary state of the specimens, unambiguous apomorphies of the family could not be detected. Based on two well-preserved skulls with associated mandibular elements, discovered in early Miocene layers of the Chilcatay Formation (Pisco Basin, Peru), we report on a new genus and species of long-snouted homodont odontocete, Chilcacetus cavirhinus n. gen., n. sp. Characterized by, among others, the presence of alveoli on the anterior premaxillary portion of the rostrum, the lack of a lateral groove on the rostrum, anterodorsally elevated nasals, a possibly autapomorphic cavity between nasals and mesethmoid in the posterior wall of the bony nares, a high temporal fossa, and the absence of ankylosis along the mandibular symphysis, C. cavirhinus n. gen., n. sp. does not fit in any of the known odontocete families, but shares several morphological features with Argyrocetus spp. KEY WORDS and Macrodelphinus. Our phylogenetic analysis, based on 77 characters for 35 odontocete taxa, suggests the Cetacea, existence of an early Miocene Eastern Pacific long-snouted homodont odontocete clade (with an hypothetical Odontoceti, South Atlantic member, the poorly known Argyrocetus patagonicus Lydekker, 1893), distinct from the only homodont, Early Miocene, superficially similar eoplatanistids and eurhinodelphinids. Furthermore, our consensus tree indicates an early Peru, branching of this new clade compared to other homodont odontocete lineages. Unfortunately, the results of phylogeny, new genus, the cladistic analysis presented here are not well supported; a reappraisal of Argyrocetus and Macrodelphinus is new species. needed to more clearly define the new clade and bolster its phylogentic position. GEODIVERSITAS • 2015 • 37 (1) © Publications scientifiques du Muséum national d’Histoire naturelle, Paris. www.geodiversitas.com 79 Lambert O. et al. RÉSUMÉ Un nouveau cétacé à dents (Mammalia, Cetacea, Odontoceti) archaïque homodonte du Miocène inférieur du Pérou. À quelques exceptions près, les odontocètes (cétacés à dents) actuels possèdent une dentition approximativement homodonte. Le passage de dents de type basilosauridé à racine double et munies de denticules accessoires à des dents coniques à racine simple s’est produit au cours de l’Oligocène supérieur-Miocène inférieur. À cette époque, plusieurs clades d’odontocètes homo- dontes et majoritairement longirostres apparaissent dans le registre fossile. Parmi ceux-ci, les espèces des genres Argyrocetus Lydekker, 1893 et Macrodelphinus Wilson, 1935, du Miocène inférieur du Pacifique Nord-Est et d’Argentine, ont été attribuées à titre d’essai à la famille Eurhi- nodelphinidae. Cependant, à cause de l’état fragmentaire des spécimens, aucune apomorphie non-ambigüe de la famille n’a pu être détectée. Sur base de deux crânes bien préservés, associés à des éléments de mandibule, découverts dans les couches du Miocène inférieur de la Formation Chilcatay (Bassin Pisco, Pérou), nous rendons compte d’un nouveau genre et d’une nouvelle espèce d’odontocète longirostre et homodonte, Chilcacetus cavirhinus n. gen., n. sp. Caractérisé entre autres par la présence d’alvéoles dans la partie prémaxillaire du rostre, l’absence d’un sillon latéral sur le rostre, des nasaux élevés antérodorsalement, une cavité entre nasaux et mésethmoïde sur la paroi latérale des narines osseuses (peut-être autapomorphique), une fosse temporale élevée, et l’absence d’ankylose de la symphyse mandibulaire, C. cavirhinus n. gen., n. sp. ne rentre dans aucune des familles connues d’odontocètes mais partage plusieurs traits morphologiques avec Argyrocetus spp. et Macrodelphinus. Notre analyse phylogénétique, appliquant 77 caractères à 35 taxons d’odontocètes, suggère l’existence d’un clade de dauphins longirostres homodontes dans MOTS CLÉS le Pacifique Est (avec un membre hypothétique dans l’Atlantique Sud, le mal connu Argyroce- Cetacea, tus patagonicus Lydekker, 1893), bien distinct des superficiellement similaires éoplatanistidés et Odontoceti, homodonte, eurhinodelphinidés. De plus, l’arbre de consensus indique le branchement précoce du nouveau Miocène inférieur, clade par rapport aux autres lignées d’odontocètes homodontes. Malheureusement, les résultats Pérou, de l’analyse cladistique sont relativement faiblement soutenus; une réévaluation des genres Argy- phylogénie, genre nouveau, rocetus et Macrodelphinus est nécessaire pour mieux définir ce nouveau clade et le positionner de espèce nouvelle. façon plus robuste dans un cadre phylogénétique. INTRODUCTION ticularly elongated snout bearing numerous, proportion- ally small teeth. Hypotheses concerning the phylogenetic Apart from minor morphological differences between an- relationships of these long-snouted dolphins with extant terior and posterior teeth in a few species (e.g., the river families have been proposed, but as yet no consensus has dolphins Inia (d’Orbigny, 1834) and Platanista (Wagler, emerged, in part because some taxa were not included in 1830)), extant toothed cetaceans (suborder Odontoceti) large cladistic analyses (e.g., Muizon 1991; Fordyce 1994; are generally considered homodont, with much less change Geisler & Sanders 2003; Lambert 2005a; Barnes 2006; along the tooth row than in basilosaurid archaeocetes, the Geisler et al. 2011; Aguirre-Fernández & Fordyce 2014; latter being characterized by double-rooted cheek teeth Lambert et al. 2014). Furthermore, among the early Mio- with transversely flattened crowns bearing accessory den- cene long-snouted forms, the familial affinities of several ticles. Several families of heterodont odontocetes (Agoro- other taxa are still debated. Indeed, due to the fragmen- phiidae, Simocetidae, Squalodontidae, Waipatiidae, and tary state of the type series, relationships of members of Xenorophidae), dating from the Oligocene and early to the genera Argyrocetus Lydekker, 1893 (three species from middle Miocene, are currently known; these are consid- the Northeast Pacific and Argentina) and Macrodelphinus ered morphologically and phylogenetically intermediary Wilson, 1935 (one species from the Northeast Pacific) are between basilosaurid archaeocetes and extant homodont not convincingly resolved (Kellogg 1932; Wilson 1935; odontocete families (Muizon 1991; Fordyce 1994, 2002; Barnes 1976; Lambert 2005b). Uhen 2008; Geisler et al. 2011). During the late Oligo- Based on two skulls from the early Miocene of the Pisco cene-early Miocene, several groups of now extinct roughly Basin (Peru), here we describe a new genus and species of homodont odontocetes arose, including Allodelphinidae, homodont long-snouted odontocete, sharing morphological Eoplatanistidae, Eurinodelphinidae, the probably non- similarities with Macrodelphinus and species of the genus monophyletic Kentriodontidae, the platanistid subfamily Argyrocetus. Better preserved than published specimens of Pomatodelphininae, and Squalodelphinidae (Fordyce & these two genera, the new taxon helps resolve their relation- Muizon 2001). Except for squalodelphinids and most of ships and contributes to filling in the gap(s) between early, the kentriodontids, these clades are characterized by a par- heterodont, odontocetes, and extant families. 80 GEODIVERSITAS • 2015 • 37 (1) Archaic homodont toothed cetacean from Peru MATERIAL AND METHODS SYSTEMATIC PALAEONTOLOGY ABBREVIATIONS Order CETACEA Brisson, 1762 BDNLTM Bünde Doberg und Tabak Museum, Bünde, Germany; Suborder ODONTOCETI Flower, 1867 IRSNB Institut Royal des Sciences Naturelles de Belgique, Family incertae sedis Brussels, Belgium; LACM Natural History Museum of Los Angeles County, Los Angeles, USA; Genus Chilcacetus n. gen. MGP Museo di Geologia e Paleontologia, Università di Padova, Italy; TYPE AND ONLY SPECIES. — Chilcacetus cavirhinus n. sp. MLP Museo de La Plata, La Plata, Argentina; MNHL Muséum d’Histoire naturelle de Lyon, France; ETYMOLOGY. — From Chilcatay, the geological formation from