Platanistoid

Alexandra T. Boersma & Nicholas D. Pyenson, 2016, Arktocara yakataga, a new fossil odontocete (Mammalia, Cetacea) from the Oligocene of Alaska and the antiquity of Platanistoidea, Peerj 2321, pp. 1-41 : 25-28

Platanistoid systematics

The present day concept of Platanistoidea has its origins with Simpson (1945), although by the late 20th century, it became clear that genera such as Inia, Pontoporia, and Lipotes were more closely related to Delphinoidea than to Platanista (Muizon, 1984; Muizon, 1985; Muizon, 1987), especially with the advent of molecular datasets in the 21st century (see Geisler et al., 2011 for a comprehensive review). Muizon (1984) provided the first modern articulation of Platanistoidea to include the numerous fossil forms that appeared to be most closely related to Platanista than any other odontocete, living or extinct, including Platanistidae , Squalodelphinidae (DSqualodelphidae sensu Muizon, 1984, an alternative spelling that according to Rice (1998) was wrongly formed), and Squalodontidae . Later, Muizon (1987) described two synapomorphies for Platanistoidea: a loss or reduction of the coracoid process and supraspinatus fossa of the scapula; and the acromion process located on the anterior edge of the scapula. In a review of fossil and extant Delphinida, Muizon (1988a) added another extinct family, Dalpiazinidae , to the aggregate of extinct families in Platanistoidea, tentatively placing it as sister group to Squalodontidae within Platanistoidea.

Muizon (1994) modified this diagnosis of the Platanistoidea to include three more characteristics: a deep subcircular fossa located dorsal to the spiny process of the squamosal; a hook­like articular process or rim on the periotic; and the migration of the palatines dorsolaterally, surrounded by the maxilla and pterygoid which partly overlap them. The type and only specimen of Arktocara does not possess any of the elements required to evaluate these synapomorphies, though the palatine is located dorsolaterally (ventral of the sphenopalatine foramina) and is slightly overlapped by the pterygoid (though not the maxilla) (see ‘Description’, palatine).

Muizon (1994) maintained that Dalpiazinidae may be a sister group to Squalodontidae , but admitted that the available material referable to Dalpiazinidae was too fragmentary to evaluate any synapomorphies of Platanistoidea. As a result, Fordyce (1994) excluded Dalpiazinidae from his analysis of Platanistoidea. Dalpiazina ombonii (Muizon, 1988b) (IGUP 26405), which was originally given the genus name Champsodelphis by Longhi (1898), was later reviewed by and given its new genus by Muizon (1988b), and is the only described member of this group. Based on observations by one of us (NDP) of the type specimen, we follow Fordyce (1994) in excluding this taxon from consideration as a platanistoid until a more detailed study can resolve the confusing history of associated material that forms the basis for this taxon (and potential membership of other odontocetes).

In his description of Waipatia maerewhenua, Fordyce (1994) articulated the current concept of Platanistoidea (and largely the basis for the node­based definition used here), which narrowed Muizon’s (1987, 1991) definition to include only the families Squalodontidae , Squalodelphinidae , and Platanistidae , although Fordyce (1994) hinted at possibly platanistoid affinities of other taxa, such as Prosqualodon davidis. Fordyce (1994) also added two synapomorphies: the anterior process of the periotic roughly cylindrical in cross section; and the anterior process smoothly deflected ventrally. Fordyce (1994) ’s diagnosis of Platanistoidea also omitted any mention of synapomorphies related to the palatines, and noted that the previous two synapomorphies of the scapula were equivocal, as they are not seen in all platanistoids. The type specimen of Arktocara has no associated tympanoperiotics, but the periotics of both Allodelphis pratti and Zarhinocetus errabundus possess both periotic synapomorphies of the Platanistoidea ( Fig. 9 ).

More recent revisions of the Platanistoidea have supported the exclusion of Squalodontidae , restructuring Platanistoidea to some combination of the families

Platanistidae , Allodelphinidae , Squalodelphinidae and Waipatiidae . Lambert, Bianucci & Urbina (2014) ’s description of the squalodelphinid Huaridelphis pointed to the inclusion of Platanistidae , Allodelphinidae and Squalodelphinidae in a monophyletic Platanistoidea based on a number of descriptive synapomorphies: deeply grooved rostral suture between the premaxilla and maxilla; elevation of the antorbital region higher than dorsal margin of rostrum base in lateral view; widening of cranium; presence of a deep fossa in orbit roof; vertex distinctly shifted to the left compared with the sagittal plane of the skull; reduction of the ventral exposure of palatine; hamular fossa of the pterygoid sinus extended anteriorly on the palatal surface of rostrum; presence of an articular rim on the periotic; elongation of anterior spine on the tympanic bulla and associated anterolateral convexity; loss of double rooted posterior teeth; and tooth count greater than 25. Of these synapomorphies, Arktocara lacks two: the antorbital region is not higher than the rostrum base, and the vertex is not shifted to the left. Both Squalodon and Waipatia were excluded from Platanistoidea in the results, though a broader sample size may change the relationship between the heterodont and homodont platanistoids.

In contrast to Lambert, Bianucci & Urbina (2014), Tanaka & Fordyce’ s (2015a) equally weighted strict consensus recovered a monophyletic Platanistoidea that included both Waipatia maerewhenua and Waipatia hectori ( Tanaka & Fordyce, 2015b), both Otekaikea spp., Platanistidae, Squalodelphis fabianii, and Notocetus vanbenedeni (i.e., a paraphyletic Squalodelphinidae ). However, in their implied weighting strict consensus, Squalodon was added to Platanistoidea. Allodelphinidae was not included in their analysis. Tanaka & Fordyce (2015a) diagnosed Platanistoidea sensu stricto (i.e., with Squalodon excluded) based on 6 synapomorphies: presence of the posterior dorsal infraorbital foramina of the maxilla (character 59); C­shaped or weakly curved parabullary sulcus (character 169); presence of the articular rim on the periotic (character 186); presence of the anterior spine of the tympanic bulla (character 195); presence of the anterolateral convexity of the tympanic bulla with anterolateral notch (character 196); and presence of the ventral groove (median furrow) of bulla anteriorly (character 212). Tanaka & Fordyce (2015a) also mentioned that character 59 was seen in other odontocete lineages besides the Platanistoidea, and it is the only character that is preserved in Arktocara.

In a broad review of Allodelphinidae, Kimura & Barnes (2016) described three new allodelphinids from the Miocene of western North America and revised the

definition of Platanistoidea to include Waipatiidae , Squalodelphinidae , Allodelphinidae , Squalodontidae , and Platanistidae . Kimura & Barnes (2016), however, did not provide a computer­assisted phylogenetic analysis to support their claim about the familial level relationships among platanistoids, pointing instead to a matrix and an analysis in Barnes (2006) that included only two outgroups in a taxon list that exclusively contained presumed platanistoids. More crucially, Kimura & Barnes (2016) did not perform a phylogenetic analysis nor code the character states for the three novel allodelphinid taxa that they described (i.e., Goedertius oregonensis, Ninjadelphis ujiharai, and Zarhinocetus donnamatsonae Kimura & Barnes, 2016).

Our phylogenetic analysis herein addresses some of the shortfalls of previous studies by including type genera belonging to all potential platanistoid families that have been presented in recent phylogenetic analyses (i.e., Lambert, Bianucci & Urbina, 2014; Tanaka & Fordyce, 2015a; Kimura & Barnes, 2016). We resolved a monophyletic Platanistoidea that included Platanistidae , Waipatiidae (Waipatia maerewhenua C Waipatia hectori C Otekaikea marplesi C Otekaikea huata), Allodelphinidae and a

polyphyletic Squalodelphinidae . We note that, for Phocageneus venustus, we followed Tanaka & Fordyce (2015a) ’s coding, which is primarily based on USNM 21039 ( Kellogg, 1957). Lambert, Bianucci & Urbina (2014) provide a valuable discussion of material that has been referred to this taxon. Our analysis departs most sharply from Tanaka & Fordyce (2015a) with the addition of the four allodelphinid genera. Though our recovery of a monophyletic Waipatiidae consisting of all described species of Waipatia and Otekaikea is consistent with Tanaka & Fordyce (2015b), the authors chose to limit their definition of Waipatiidae to both species of Waipatia, differing from the results in Tanaka & Fordyce (2014) where the authors definied Waipatiidae as including Waipatia maerewhenua and Otekaikea marplesi. Our results are consistent with Tanaka & Fordyce (2015a) ’s findings with the resolution of a polyphyletic Squalodelphinidae , with Squalodelphis fabianii as a basal member of Platanistoidea and an unnamed clade of Notocetus vanbenedeni C Phocageneus venustus as the sister group to Platanistidae . A more detailed coding of Squalodelphinidae in future work, especially one that includes Huaridelphis raimondii, will provide more insight into the relationships among this group.

We diagnose a node­based Platanistoidea by the following synapomorphies: moderately elevated coronoid process (character 33*); premaxillae>65% of width of rostrum at antorbital notches (character 51*); alisphenoidal­squamosal suture coursing along groove for mandibular branch of trigeminal nerve in ventral view (character 147[1]); lateral groove or depression with profile of periotic becoming slightly to markedly sigmoidal in dorsal view (character 166[1]); anteroposterior ridge on dorsal side anterior process and body of periotic (character 167[1]); parabullary sulcus on the periotic weakly to strongly curved and c­shaped (character 169[1,2]); and ventral surface of the posterior process of the periotic not flat along a straight path perpendicular to its long axis (character 191[1,2]) Of these synapomorphies, the two marked by an asterisk () are equivocal across the group, demonstrating character state reversals or independent origins (characters 33, 51). Two characters are ambiguous and show independent origins (characters 169 and character 191), but we argue remain useful for characterizing this group.

Only one of the six synapomorphies presented by Tanaka & Fordyce (2015a) is consistent with ours (character 169). The other 5 characters are all equivocal across the Platanistoidea, but some are still useful for diagnosing members of certain sub­clades. For example, the presence of the articular rim or on the periotic (character 186) is seen in all platanistoids except Allodelphis pratti, where there is no distinguishable rim lateral to the posterior process and separated by a sulcus ( Fig. 9 ). In Zarhinocetus errabundus, this trait is present as an extremely reduced rim. Kimura & Barnes (2016) make no mention of an articular rim or process on the periotic of Ninjadelphis ujiharai, and there is no evidence of it from the published photos of the type specimen. The presence of the anterior spine of the tympanic bulla (character 195), the anterolateral convexity of the tympanic bulla with anterolateral notch (character 196), and the ventral groove (median furrow) of bulla anteriorly (character 212) are all ambiguous characters, represented by two states each across Platanistoidea. All of the latter traits are present in Allodelphis pratti and Zarhinocetus errabundus, with perhaps the exception of the ventral groove of the anterior surface of the bulla in Allodelphis pratti, which could not be determined from the photos of the referred specimen (UCMP 83791) provided by Kimura & Barnes (2016), nor was not mentioned in their description of this taxon.

Figures

Figure 9 Right periotic of the holotype of Allodelphis pratti (YPM 13408). Right periotic of Allodelphis pratti in dorsal (A, B), and lateral (C, D) views. (A, C), Illustrated periotic with low opacity mask and interpretive line art. The two periotic synapomorphies for the Platanistoidea are labelled: the parabullary sulcus, and the dorsal crest. (B, D), photography by James Di Loreto, Smithsonian Institution. Courtesy of the Division of Vertebrate Paleontology; YPM 13408, Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA; peabody. yale. edu.