A new fossil dolphinDilophodelphis fordyceiprovides insight into the evolution of supraorbital crests in Platanistoidea (Mammalia, Cetacea)

Abstract

Many odontocete groups have developed enlarged facial crests, although these crests differ in topography, composition and function. The most elaborate crests occur in the South Asian river dolphin (Platanista gangetica), in which they rise dorsally as delicate, pneumatized wings anterior of the facial bones. Their position wrapping around the melon suggests their involvement in sound propagation for echolocation. To better understand the origin of crests in this lineage, we examined facial crests among fossil and living Platanistoidea, including a new taxon, Dilophodelphis fordycei, nov. gen. and sp., described herein, from the Early Miocene Astoria Formation of Oregon, USA. We measured the physical extent and thickness of platanistoid crests, categorized their relative position and used computed tomography scans to examine their internal morphology and relative bone density. Integrating these traits in a phylogenetic context, we determined that the onset of crest elaboration or enlargement and the evolution of crest pneumatization among the platanistoids were separate events, with crest enlargement beginning in the Oligocene. However, we find no evidence for pneumatization until possibly the Early Miocene, although certainly by the Middle Miocene. Such an evolutionary context, including data from the fossil record, should inform modelling efforts that seek to understand the diversity of sound generation morphology in Odontoceti.

A new large squalodelphinid (Cetacea, Odontoceti) from Peru sheds light on the Early Miocene platanistoid disparity and ecology

“…The strict consensus tree obtained here shows the same relationships within the homodont platanistoids as the tree of Godfrey et al [ 15 ], summarized in the basalmost position of Allodelphinidae and the sister group relationship between Platanistidae and Squalodelphinidae, both families resulting as monophyletic groups. This analysis also confirms the position of Dilophodelphis within the platanistids, as already proposed by Boersma et al [ 44 ] using a matrix modified from Godfrey et al [ 15 ].…”

“…On the medial portion of the ventral surface of the orbit roof, a fossa is partially filled by sediment ( figure 4 a , b ). A similar fossa, but slightly larger and deeper, has been observed in Huaridelphis and Notocetus , and interpreted as corresponding to an extension of the pterygoid sinus in the orbit region [ 13 , 44 ].…”

“…Our consensus tree supports the referral of Macrosqualodelphis to the family Squalodelphinidae, of which it is the earliest diverging lineage. The relationships between other squalodelphinids remain unresolved, as in previous analyses [ 13 , 15 , 44 ]. The 50% majority-rule consensus tree provides a more satisfactory result, with the specimen MUSM 603 branching before the two other South American genera Huaridelphis and Notocetus (unresolved relationships), and a clade including all the squalodelphinids from the North Atlantic realm.…”

Extensive Diversity and Disparity of the Early Miocene Platanistoids (Cetacea, Odontoceti) in the Southeastern Pacific (Chilcatay Formation, Peru)

“…However, the Adams consensus tree shows a more satisfactory result, with Furcacetus in a derived position among squalodelphinids, forming a clade together with Huaridelphis and Medocinia + Squalodelphis. Interestingly enough, in our new analysis Dilophodelphis is placed within the Squalodelphinidae, instead of Platanistidae as proposed by Boersma et al [101] and Bianucci et al [26]. Also recovered by Kimura [133], this different familial attribution could be due here to some changes of character states when re-examining the holotype of this genus and to the reformulation of the character 9 dealing with the dorsal crest in the antorbital-supraorbital region.…”

A Total-Group Phylogenetic Metatree for Cetacea and the Importance of Fossil Data in Diversification Analyses

“…The second piece of information noted was whether the matrix was based on a previous study. For example, the matrix of Godfrey et al (2017) was based on an earlier study by Lambert et al (2014) and was itself later used by Boersma et al (2017). This information was added to each matrix's XML file for use in determining relative weighting or study redundancy when assembling the final MRP matrix.…”