Paedomorphic Ossification in Porpoises with an Emphasis on the Vaquita (<I>Phocoena sinus</I>)

Abstract

Heterochrony, the change in timing of developmental processes, is thought to be a key process shaping the numerous limb morphologies of tetrapods. Through a delayed offset in digit development, all cetaceans (i.e., whales, dolphins, and porpoises) have evolved supernumary phalanges (hyperphalangy). Moreover, some toothed cetaceans further alter digital morphologies by delayed endochondral and perichondral ossification of individual elements. In the harbor porpoise (Phocoena phocoena), these paedomorphic patterns have created poorly ossified phalangeal elements. However, no studies have addressed this morphology in other porpoise taxa. This study documents the timing of carpal and digital epiphyseal ossification in the poorly studied vaquita (Phocoena sinus) based on radiographs (n = 18) of known-age specimens. Patterns of vaquita manus ossification were compared between other porpoise and delphinid taxa. Adult vaquitas are paedomorphic in carpal, metacarpal, and digital development as they maintain a juvenile ossification pattern relative to that of other porpoise species of equivalent ages. Vaquitas also ossify fewer carpal elements as compared to other porpoise and some delphinid cetaceans, and ossification arrests relative to that of the harbor porpoise. Vaquitas also display sexual dimorphism as females reach a greater body size and display more ossified elements in the manus relative to their paedomorphic male cohorts.

A new Oligo-Miocene dolphin from New Zealand: Otekaikea huata expands diversity of the early Platanistoide

“…The skull lacks obvious roughened or eroded bone that might indicate a gerontic age. Otekaikea huata also has an ossified carpus, which supports an adult age class based on a flipper radiograph study on Phocoena sinus by Mellor et al (2009). Skull topography.…”

Interspecific variation of ontogeny and skull shape among porpoises (Phocoenidae)

“…Quantitative ontogenetic studies of P. phocoena have also revealed incomplete fusion of vertebral and flipper skeleton epiphyses, as well as pedomorphic phalangeal morphology (Galatius and Kinze,2003; Galatius et al,2006). Similar and more profound pedomorphosis in the flipper skeleton is seen in P. sinus with retention of rounded rather than angular carpal elements (Mellor et al,2009). Since porpoises attain sexual maturity at much younger ages than most cetaceans, phocoenid pedomorphosis is suggested as the result of progenetic evolution (Ichishima and Kimura,2005; Galatius et al,2006; Galatius,2010), a truncation of somatic development at an immature stage caused by accelerated sexual maturity relative to a nonpedomorphic ancestor (Gould,1977; Alberch et al,1979).…”

“…From the few cranial suture data on these species that are presented here, they seem to show a degree of fusion similar to the two similarly sized species treated in depth here. The flipper skeleton of adult P. sinus , the smallest phocoenid, has been found to be profoundly pedomorphic (Mellor et al,2009).…”

Ossification Pattern of Estuarine Dolphin (Sotalia guianensis) Forelimbs, from the Coast of the State of Espírito Santo, Brazil

“…Mellor et al [27] studied older, physically mature Phocoena sinus individuals, whose humeral, ulnar and metacarpal ossification was complete. Ossification was unfinished only in the distal radial epiphysis of 11-year-old females.…”

“…Nevertheless, we could not compare the progression of the ossification of S . guianensis with that of Phocoena , since Mellor et al [27] only studied mature animals.…”