Prenatal cranial ossification of the humpback whale (Megaptera novaeangliae)

Abstract

Being descendants of small terrestrial ungulate mammals, whales underwent enormous transformations during their evolutionary history, that is, extensive changes in anatomy, physiology, and behavior were evolved during secondary adaptations to life in water. However, still only little is known about whale ontogenetic development, which help to identify the timing and sequence of critical evolutionary events, such as modification of the cetacean ear. This is particularly true for baleen whales (Mysticeti), the group including the humpback whale Megaptera novaeangliae. We use high-resolution X-ray computed tomography to reinvestigate humpback whale fetuses from the Kükenthal collection at the Museum für Naturkunde, Berlin, thus, extending historic descriptions of their skeletogenesis and providing for the first time sequences of cranial ossification for this species. Principally, the ossification sequence of prenatal Megaptera follows a typical mammalian pattern with the anterior dermal bones being the first ossifying elements in the skull, starting with the dentary. In contrast to other mammals, the ectotympanic bone ossifies at an early stage. Alveolar structure can be observed in both the maxillae and dentaries in these early prenatal specimens but evidence for teeth is lacking. Although the possibility of obtaining new embryological material is unlikely due to conservation issues, our study shows that reexamination of existing specimens employing new technologies still holds promise for filling gaps in our knowledge of whale evolution and ontogeny.

A Study of the Head during Prenatal and Perinatal Development of Two Fetuses and One Newborn Striped Dolphin (Stenella coeruleoalba, Meyen 1833) Using Dissections, Sectional Anatomy, CT, and MRI: Anatomical and Functional Implications in Cetaceans and Terrestrial Mammals

“…Cranial Cavity Three fontanelles were observed in our odontocetes fetus studied: occipital, frontal, and mastoid [1] (the last one less clear) and are confused in Stenella attenuata [55]. Studies in mysticetes could not add more information for comparison with dolphins [63].…”

Investigation of failure mechanisms in silicon based half cells during the first cycle by micro X-ray tomography and radiography

“…X-ray microtomography and radiography was conducted with a laboratory X-ray CT system [43]. To be fully compatible with X-ray imaging and at the same time to completely represent working LIBs, we have designed and manufactured two dedicated proof-of-concept electrochemical cells: a cell for tomography (tomo-cell) and a cell for radiography (radio-cell).…”

Prenatal Development of the Humpback Whale: Growth Rate, Tooth Loss and Skull Shape Changes in an Evolutionary Framework

“…This method of staging is preferred because it is the most detailed classification available for cetacean embryos and early fetuses, and also makes it possible to compare the developmental sequence of the humpback whale with that published for another cetacean (pantropical spotted dolphin; Moran et al, ). Using a staging system based on external features rather than determining the stage of the gestation using measurements such as total length of the specimens avoids errors introduced through specimen preservation (e.g., shrinkage due to alcohol preservation; Hampe et al, ). Therefore, while total length, especially of larger fetuses, can be used to help establish the growth rate of the species, it should not be used alone to estimate gestational stage.…”

“…Although little information is available on the ontogeny of baleen whales, descriptive anatomy of mysticete fetuses revealed that mineralized tooth germs develop in both upper and lower jaws in utero , but teeth never erupt, and neonates bear baleen plates (e.g., Ridewood, ; Van Van Dissel‐Scherft and Vervoort, ; Karlsen, ; Tomilin, ; Fudge et al, ). More recent work using computed tomography (CT) and other quantitative methods increased our understanding of the ossification sequence of mysticetes and other general skull growth patterns (Armfield et al, ; Roston et al, ; Hampe et al, ), but have yet to provide much information on the development of teeth or baleen. New data on gene expression during teeth and baleen development have also been collected using histology and protein‐staining techniques (Ishikawa and Amasaki, ; Ishikawa et al, ; Thewissen et al, ).…”