Neuromuscular anatomy and evolution of the cetacean forelimb.
*Biological Evolution; Adaptation; Animals; Body Size; Cetacea/*anatomy & histology/physiology; Forelimb/*anatomy & histology/physiology; Humpback Whale/anatomy & histology; Movement; Muscle; Neuroanatomy; Physiological; Skeletal/*innervation/physiology; Sperm Whale/anatomy & histology
The forelimb of cetaceans (whales, dolphins, and porpoises) has been radically modified during the limb-to-flipper transition. Extant cetaceans have a soft tissue flipper encasing the manus and acting as a hydrofoil to generate lift. The neuromuscular anatomy that controls flipper movement, however, is poorly understood. This study documents flipper neuromuscular anatomy and tests the hypothesis that antebrachial muscle robustness is related to body size. Data were gathered during dissections of 22 flippers, representing 15 species (7 odontocetes, 15 mysticetes). Results were compared with published descriptions of both artiodactyls and secondarily aquatic vertebrates. Results indicate muscle robustness is best predicted by taxonomic distribution and is not a function of body size. All cetaceans have atrophied triceps muscles, an immobile cubital joint, and lack most connective tissue structures and manus muscles. Forelimbs retain only three muscle groups: triceps (only the scapular head is functional as the humeral heads are vestigal), and antebrachial extensors and flexors. Well-developed flexor and extensor muscles were found in mysticetes and basal odontocetes (i.e., physeterids, kogiids, and ziphiids), whereas later diverging odontocetes (i.e., monodontids, phocoenids, and delphinids) lack or reduce these muscles. Balaenopterid mysticetes (e.g., fin and minke whales) may actively change flipper curvature, while basal odontocetes (e.g., sperm and beaked whales) probably stiffen the flipper through isometric contraction. Later diverging odontocetes lack musculature supporting digital movements and are unable to manipulate flipper curvature. Cetacean forelimbs are unique in that they have lost agility and several soft tissue structures, but retain sensory innervations.
Cooper Lisa Noelle; Dawson Susan D; Reidenberg Joy S; Berta Annalisa
Anatomical record (Hoboken, N.J. : 2007)
2007
2007-09
Article information provided for research and reference use only. All rights are retained by the journal listed under publisher and/or the creator(s).
<a href="http://doi.org/10.1002/ar.20571" target="_blank" rel="noreferrer noopener">10.1002/ar.20571</a>
Evolution of hyperphalangy and digit reduction in the cetacean manus.
*Adaptation; *Biological Evolution; Animals; Cetacea/*anatomy & histology/physiology; Forelimb/*anatomy & histology/physiology; Morphogenesis; Newborn; Phylogeny; Physiological
Cetaceans (whales, dolphins, and porpoises) have a soft tissue flipper that encases most of the forelimb, and elongated digits with an increased number of phalanges (hyperphalangy). In addition, some cetaceans exhibit a reduction in digit number. Although toothed cetaceans (odontocetes) are pentadactylous, most baleen whales (mysticetes) are tetradactylous and also lack a metacarpal. This study conducts a survey of cetacean metacarpal and phalangeal morphologies, traces the evolution of hyperphalangy in a phylogenetic context, optimizes characters onto previously published cetacean phylogenies, and tests various digit loss hypotheses. Dissections were performed on 16 cetacean flippers representing 10 species (8 mysticetes, 2 odontocetes). Phalangeal count data were derived from forelimb radiographs (36 odontocetes, 5 mysticetes), osteological specimens of articulated forelimbs (8 mysticetes), and were supplemented with published counts. Modal phalangeal counts were coded as ordered and unpolarized characters and optimized onto two known cetacean phylogenies. Results indicate that digital ray I is reduced in many cetaceans (except Globicephala) and all elements of digital ray I were lost in tetradactylous mysticetes. Fossil evidence indicates this ray may have been lost approximately 14 Ma. Most odontocetes also reduce the number of phalangeal elements in digit V, while mysticetes typically retain the plesiomorphic condition of three phalanges. Results from modal phalangeal counts show the greatest degree of hyperphalangy in digits II and III in odontocetes and digits III and IV in mysticetes. Fossil evidence indicates cetacean hyperphalangy evolved by at least 7-8 Ma. Digit loss and digit positioning may underlie disparate flipper shapes, with narrow, elongate flippers facilitating fast swimming and broad flippers aiding slow turns. Hyperphalangy may help distribute leading edge forces, and multiple interphalangeal joints may smooth leading edge flipper contour.
Cooper Lisa Noelle; Berta Annalisa; Dawson Susan D; Reidenberg Joy S
Anatomical record (Hoboken, N.J. : 2007)
2007
2007-06
Article information provided for research and reference use only. All rights are retained by the journal listed under publisher and/or the creator(s).
<a href="http://doi.org/10.1002/ar.20532" target="_blank" rel="noreferrer noopener">10.1002/ar.20532</a>