Friday, July 10, 2020

[PaleoMammalogy • 2020] Ankylorhiza tiedemani • Convergent Evolution of Swimming Adaptations in Modern Whales Revealed by A Large Macrophagous Dolphin from the Oligocene of South Carolina

Ankylorhiza tiedemani 

in Boessenecker, Churchill, Buchholtz, et al., 2020. 

• A large macroraptorial dolphin is reported from the Oligocene of South Carolina
• Well-preserved skeleton reveals stepwise evolution of locomotion in toothed whales
• Convergence in locomotor features is identified between baleen and toothed whales

Modern whales and dolphins are superbly adapted for marine life, with tail flukes being a key innovation shared by all extant species. Some dolphins can exceed speeds of 50 km/h, a feat accomplished by thrusting the flukes while adjusting attack angle with their flippers. These movements are driven by robust axial musculature anchored to a relatively rigid torso consisting of numerous short vertebrae, and controlled by hydrofoil-like flippers. Eocene skeletons of whales illustrate the transition from semiaquatic to aquatic locomotion, including development of a fusiform body and reduction of hindlimbs, but the rarity of Oligocene whale skeletons has hampered efforts to understand the evolution of fluke-powered, but forelimb-controlled, locomotion. We report a nearly complete skeleton of the extinct large dolphin Ankylorhiza tiedemani comb. n. from the Oligocene of South Carolina, previously known only from a partial rostrum. Its forelimb is intermediate in morphology between stem cetaceans and extant taxa, whereas its axial skeleton displays incipient rigidity at the base of the tail with a flexible lumbar region. The position of Ankylorhiza near the base of the odontocete radiation implies that several postcranial specializations of extant cetaceans, including a shortened humerus, narrow peduncle, and loss of radial tuberosity, evolved convergently in odontocetes and mysticetes. Craniodental morphology, tooth wear, torso vertebral morphology, and body size all suggest that Ankylorhiza was a macrophagous predator that could swim relatively fast, indicating that it was one of the few extinct cetaceans to occupy a niche similar to that of killer whales.

Keywords: Cetacea, Neocetimarine mammal, locomotion, Odontoceti, Cenozoic, apex predator, macrophagous, swimming, hydrofoil

Figure 1: Skull and Skeleton of Ankylorhiza tiedemani, CCNHM 103.
(A–M) Skull in lateral view (A); vertex in dorsal view (B); sternum in dorsal view (C); selected vertebrae in anterior view (D); skeletal reconstruction with preserved elements in white and missing elements in red (E); selected teeth in labial view (F); proximal phalanx III in lateral (top) and distal (bottom) views (G); proximal phalanx IV in proximal (top) and lateral (bottom) views (H); metacarpals in lateral view (I); ulna, pisiform, and pyramidal in lateral view (J); and humerus in medial (K), lateral (L), and anterior (M) view.
Abbreviations: c, capitulum; dpc, deltopectoral crest; fr, frontal; gt, greater tuberosity; if, infraspinatus fossa; ip, interparietal; lt, lesser tuberosity; mx, maxilla; na, nasal; nc, nuchal crest; of, olecranon facet; op, olecranon process; os, occipital shield; p, parietal; pi, pisiform; pmx, premaxilla; pop, postorbital process; py, pyramidal; rf, radial facet; tf, temporal fossa; tn, trochlear notch; uf, ulnar facet; zp, zygomatic process. 

 Systematic Paleontology
Order Cetacea; 
Suborder Odontoceti; 

Ankylorhiza gen. nov.

Etymology. Ankylorhiza. Genus is Greek for “fused roots,” referring to the advanced degree of postcanine tooth root fusion for stem Odontoceti.

Ankylorhiza tiedemani, comb. nov.

Holotype. AMNH 10445, partial rostrum, Ashley Formation, South Carolina [14].
Referred Specimens. CCNHM 103, partial skeleton (Figure 1); Chandler Bridge Formation, late Oligocene (24.7–23.5 Ma), South Carolina; CCNHM 220, partial skull and associated vertebrae; Ashley Formation, early Oligocene (29.0–26.57 Ma), South Carolina (Methods S1).

Diagnosis. Ankylorhiza tiedemani is a large (425 mm bizygomatic width; body length est. 4.8 m) stem odontocete with limited polydonty (10 postcanine teeth) and moderate heterodonty. Ankylorhiza differs from all other odontocetes in the extreme development of nuchal crests and differs from all other stem odontocetes in its larger size and combination of derived (mostly single-rooted teeth; simplified tooth crowns) and plesiomorphic characters (e.g., parietal exposed at vertex, symmetrical skull, prominent intertemporal constriction, and longer humerus with long deltopectoral crest; Methods S1: Supplemental Diagnosis of Ankylorhiza tiedemani). Ankylorhiza differs from all crown Odontoceti in possessing some double-rooted teeth, prominent intertemporal constriction, and widely open mesorostral gutter along the entire length of the rostrum.

Figure 4: Relationships of Ankylorhiza and Convergent Postcranial Evolution in Neoceti Illustrated on a Simplified Cladogram

 Robert W. Boessenecker, Morgan Churchill, Emily A. Buchholtz, Brian L. Beatty and Jonathan H. Geisler. 2020. Convergent Evolution of Swimming Adaptations in Modern Whales Revealed by A Large Macrophagous Dolphin from the Oligocene of South Carolina. Current Biology. In Press. DOI: 10.1016/j.cub.2020.06.012  

15-foot-long skeleton of extinct dolphin suggests parallel evolution among whales