New Paratethyan dwarf baleen whales mark the origin of cetotheres

Abstract

Background: Family Cetotheriidae sensu stricto and several closely related taxa comprise the Cetotherioidea and represent a lineage of Neogene baleen whales that includes the smallest edentulous baleen whales in Earth history. Most of known cetotheriids came from the Late Miocene to Quaternary, and the earliest records from the latest Middle Miocene. The Paratethys region shows a great diversity of Middle to Late Miocene cetotheriids. That includes nominative taxon of the family, Cetotherium rathkii, and this suggests that the earliest cetotheriids may have lived in that region.

Materials and methods: Here, Ciuciulea davidi, a new genus and species from the Middle Miocene of southeastern Europe, is described as the chronologically earliest and earliest diverging member of Cetotheriidae. Also, a new specimen of Otradnocetus, a basal Cetotherioidea sensu Gol'din & Steeman, 2015 is identified from the Late Miocene deposits of Caucasus and compared with Otradnocetus virodovi from the Middle Miocene of the same region.

Results and discussion: Ciuciulea davidi is a dwarf whale displaying primitive traits: posterior ends of facial bones forming a single transverse line, a narrow occipital shield, and a relatively long interparietal region. Meanwhile, it shares some cetotheriid apomorphies: posteriorly telescoped wedge-shaped facial bones and an ovoid tympanic bulla with shallow lateral and medial furrows, a short anterior lobe and a short sigmoid process. Phylogenetic analysis suggests that Parietobalaena and Otradnocetus are branches diverging before the clade Cetotheriidae + Neobalaenidae. This is confirmed by the stepwise evolution of the anatomy of the squamosal, mandible, and ear bones across these groups. The re-described juvenile specimen of Otradnocetus differs from O. virodovi in the more primitive anatomy of the mandible and the autapomorphic anatomy of the humerus. Records of the earliest cetotheriids and related taxa in the Paratethys support the idea that this could be the region where Cetotheriidae evolved before their worldwide dispersal and radiation.

Keywords: Baleen whales; Biogeography; Cetotheriidae; Ciuciulea; Miocene; Otradnocetus; Paratethys; Phylogeny; Taxonomy.

Figure 1. Geographical position of localities of Ciuciulea davidi (Moldova) and Otradnocetus spp. (Russia).

(A) Europe; (B) the type locality of Ciuciulea davidi in Moldova.

Figure 2. The skull of Ciuciulea davidi, ZIRM V 28/1 (holotype), from the Middle Miocene of Moldova.

(A) Dorsal view; (B) ventral view. The scale bars equal 10 cm.

Figure 3. The skull of Ciuciulea davidi, ZIRM V 28/1, from the Middle Miocene of Moldova (holotype) (cont.).

(A) Lateral view; (B) posterior view. The scale bars equal 10 cm.

Figure 4. The left periotic bone of Ciuciulea davidi, ZIRM V 28/1 (holotype).

(A) Ventral view; (B) lateral view. Abbreviations: ant, anterior; ap, anterior process; dors, dorsal; fo, fenestra ovalis; lat, lateral; maf, mallear fossa; mpg, median promontorial groove; pc, pars cochlearis; pcc, posterior cochlear crest; sta, stapedial fossa; tofc, tympanic opening of the facial canal. The identical scale bars equal one cm.

Figure 5. The left tympanic bulla of Ciuciulea davidi, ZIRM V 28/1 (holotype).

(A) Ventral view; (B) dorsal view; (C) lateral view; (D) medial view, (E) posterior view. Abbreviations: adc, anterodorsal crest; als, anterolateral shelf; ant, anterior; bap, bulla anterior pedicle; bpp, bulla posterior pedicle; cp, conical process; dors, dorsal; dpp, dorsal posterior prominence; Eu, Eustachian outlet; ir, involucral ridge; iv, involucrum; lat, lateral; lf, lateral furrow; med, medial; mf, medial furrow; mr, main ridge; sf, groove posterior to the sigmoid process (“sigmoid fissure”); sp, sigmoid process; vent, ventral. The scale bars equal one cm.

Figure 6. Postcranial skeleton elements of Ciuciulea davidi, ZIRM V 28/1 (holotype).

(A) Cervical and thoracic vertebrae, left lateral view; (B) lumbar vertebrae, left dorsolateral view; (C) right humerus, lateral view; (D) right humerus, medial view; (E) right humerus, posterior view; (F) right humerus, anterior view. The scale bars equal five cm.

Figure 7. The skull and mandibles of Otradnocetus virodovi, GNM CO-1–3.

General view, with a sketch drawing from the original description (Mchedlidze, 1984: 21). The scale bar equals 20 cm.

Figure 8. The skull of O. virodovi, GNM CO-1.

(A) Dorsal view; (B) ventral view. The scale bar equals 20 cm.

Figure 9. The skull of O. virodovi, GNM CO-1 (cont.).

(A) Right lateral view; (B) posterior view. The scale bar equals 20 cm.

Figure 10. The left ramus of mandible of O. virodovi, GNM CO-3.

(A) Posterior part, lateral view; (B) posterior view; (C) medial view. The scale bars equal five cm. Figure 10C courtesy of Maia Bukhsianidze.

Figure 11. Forelimb bones of Otradnocetus.

(A) O. virodovi, GNM CO-5–12, left forelimb in lateral view; (B) Otradnocetus sp., VSEGEI 2401, left forelimb: humerus in posterolateral view; other bones in lateral view; (C) Otradnocetus sp., VSEGEI 2401, mounted right forelimb with re-constructed radius and ulna: humerus in posteroventrolateral view; manus bones, lateral view. The scale bars equal five cm.

Figure 12. The skeleton of Otradnocetus sp., VSEGEI 2401, from the Middle Miocene of the northwestern Caucasus (Russia), right lateral view.

The scale bar equals one m. Courtesy of Grigory Prokopov.

Figure 13. Specific features of skeleton of Otradnocetus sp., VSEGEI 2401.

(A) Left squamosal, dorsal view; (B) left squamosal, ventral view; (C) left tympanic bulla, ventral view; (D) left tympanic bulla, medial view; (E) posterior portion of the right mandible ramus, lateral view; (F) caudal vertebrae with chevron bones, right lateral view; (G) a chevron bone, ventral view. The scale bars equal five cm.

Figure 14. Consensus phylogenetic tree of Neogene and recent Mysticeti, with newly described taxa.

Eight trees, best score = 513 steps, CI = 0.32, RI = 0.66. Branch support (GC) values are indicated below the branches.