Patterns of ossification in the manus of the harbor porpoise (Phocoena phocoena): Hyperphalangy and delta‐shaped bones

Abstract

With the transition from terrestrial to aquatic habitats, cetacean forelimbs have undergone significant modifications in bone morphology and soft tissue distribution. Some, but not all, of these modifications are also demonstrated in other lineages of extant and extinct secondarily aquatic tetrapods. This study examines the ontogenetic pattern of ossification of the manus of the harbor porpoise (Phocoena phocoena), using plain film radiography. Two modifications examined are hyperphalangy (number of phalanges per digit increased beyond the typical mammalian number) and the morphology of delta-shaped bones. Hyperphalangy in Phocoena phocoena is apparent in digits 2 and 3. Phalangeal counts in all digits are variable (sometimes between the right and left flippers of the same individual) and are not necessarily correlated with age. Phalangeal ossification and epiphyseal fusion proceeds along the proximo-distal axis within each digit. In addition, digits 2 and 3 are at a more advanced stage of ossification than more abaxial digits. Delta-shaped bones appear to be a normal stage in the ossification of phalanges in all digits except the third, and may persist in the adult in certain digits. In humans, this morphology is a developmental anomaly usually associated with other malformations, such as polydactyly or syndactyly. Delta-shaped bones in the cetacean manus display a consistent orientation and the process by which they are formed may be similar to that in extinct marine reptiles.

The trouble with flippers: a report on the prevalence of digital anomalies in Cetacea

“…Metacarpal IV was delta‐shaped with a convex posterior aspect, and was apposed to the rounded margin of the accessory rounded bone. This morphology resembled the ‘kissing delta’ phalanx condition described in accounts of human polydactyly (Dawson, 2003; Elliot et al. , 2004).…”

Timing of epiphyseal development in the flipper skeleton of the harbour porpoise (Phocoena phocoena) as an indicator of paedomorphosis

“…Only three specimens had epiphyses on the first phalanx of the fifth digit. Unlike Dawson (2003) we found secondary centres of ossification on the fifth metacarpal, usually in the form of a ring-like shape around the periphery of the primary centre of ossification, covering the proximal, ulnar and distal sides of the bone. This is also seen in Black Sea harbour porpoises, where Golídin (2004) found the development to be similar to that of the carpal bones, where the secondary centres of ossification cover the entire periphery.…”

“…The development of flipper epiphyses has been thoroughly studied in the striped dolphin ( Stenella coeruleoalba ) by Calzada and Aguilar (1996) and DiGiancamillo et al (1998), providing a delphinid benchmark for diagnosing phocoenid paedomorphosis. Dawson (2003) investigated morphology and ossification patterns in a small sample of harbour porpoises for which age estimates were not available.…”

Growth plate formation and development in alligator and mouse metapodials: evolutionary and functional implications

“…4) (Thomson, 1869). While a few exceptions are known, these occur in taxa with highly derived skeletal morphology (e.g., Cetacea) in which metacarpal and phalangeal growth is greatly restricted (Ogden et al, '81;Dawson, 2003;Galatius et al, 2006). Other possible exceptions have also been noted [i.e., Fig.…”