Many modifications to the mammalian bauplan associated with the obligate aquatic lives of cetaceans—fusiform bodies, flukes, flippers, and blowholes—are evident at a glance. But among the most strikingly unusual and divergent features of modern cetacean anatomy are the arrangements of their cranial bones: (1) bones that are situated at opposite ends of the skull in other mammals are positioned close together, their proximity resulting from (2) these bones extensively overlapping the bones that ordinarily would separate them. The term “telescoping” is commonly used to describe the odd anatomy of modern cetacean skulls, yet its usage and the particular skull features to which it refers vary widely. Placing the term in historical and biological context, this review offers an explicit definition of telescoping that includes the two criteria enumerated above. Defining telescoping in this way draws attention to many specific biological questions that are raised by the unusual anatomy of cetacean skulls; highlights the central role of sutures as the locus for changes in the sizes, shapes, mechanical properties, and connectivity of cranial bones; and emphasizes the importance of sutures in skull development and evolution. The unusual arrangements of cranial bones and sutures referred to as telescoping are not easily explained by what is known about cranial development in more conventional mammals. Discovering the evolutionary‐developmental processes that produce the extensive overlap characteristic of cetacean telescoping will give insights into both cetacean evolution and the “rules” that more generally govern mammalian skull function, development, and evolution. Anat Rec, 302:1055–1073, 2019. © 2019 Wiley Periodicals, Inc.
The external anatomy of a 130-mm blue whale fetus (Balaenoptera musculus) is described, and its internal anatomy is reconstructed noninvasively from microCT scans. The specimen lies developmentally at the junction of the embryonic and fetal periods. Similarly to the embryos of many odontocetes, it lacks a caudal fluke and dorsal fin, but it also exhibits an elongated rostrum, resorbed umbilical hernia, partially exposed cornea, and spatial separation of the anus and genitalia seen in early odontocete fetuses. Dermal ossification of the cranial bones has begun, but the endochondral skeleton is completely cartilaginous. The shape and position of the maxilla suggest that the earliest stages of anterior skull telescoping have begun, but there is no indication of occipital overlap posteriorly. The nasopharynx, larynx, and heart already display the distinctive morphology characteristic of Balaenoptera. This study develops a model of body length changes during blue whale development by integrating the large International Whaling Statistics (IWS) database, historical observations of blue whale migration and reproduction, and descriptions of fetal growth trends in other mammals. The model predicts an age of 65 days postconception for the specimen. The early developmental milestones of Balaenoptera mirror those of the odontocete Stenella to a remarkable extent, but the first appearance of the caudal fluke and dorsal fin are delayed relative to other morphological transitions. The accelerated prenatal growth characteristic of Balaenoptera occurs during fetal, not embryonic, development. Anat Rec, 296:709-722, 2013. V C 2013 Wiley Periodicals, Inc.Key words: blue whale; fetus; mysticete; development; microCT The development of a blue whale (Balaenoptera musculus) from a zygote to a 7.5-m, 2,000-3,000 kg neonate (Reidenberg and Laitman, 2008) is remarkable for both its rapid increase in size and for the reorganization of multiple mammalian anatomical systems for life in the water. Study of the transitions of size and morphology during blue whale development is limited by access to fetal specimens. Many thousands of fetuses were recovered during the decades of active commercial whaling, but very few were preserved for scientific examination. Embryos and small fetuses are particularly rare, with the result that the early ontogeny of the species is largely unknown.
Tests of phenotypic convergence can provide evidence of adaptive evolution, and the popularity of such studies has grown in recent years due to the development of novel, quantitative methods for identifying and/or measuring convergence. Two commonly used methods include (i) distance-based methods that measure morphological distances between lineages in phylomorphospace and (ii) fitting evolutionary models to morphological datasets to test whether lineages have evolved toward adaptive peaks. Here, we demonstrate that both types of convergence measures are influenced by the position of putatively convergent taxa in morphospace such that morphological outliers are statistically more likely to exhibit convergence by chance. A more substantial issue is that some methods will often misidentify divergent lineages as being convergent. These issues likely influence the results of many studies, especially those that focus on morphological outliers. To help address these problems, we developed a new distance-based method for measuring convergence that incorporates distances between lineages through time and minimizes the possibility of divergent taxa being misidentified as convergent. We advocate the use of this method when the phylogenetic tips of putatively convergent lineages are of the same or similar geologic ages (e.g., extant taxa), meaning that convergence among the lineages is expected to be synchronous. We conclude by emphasizing that all available convergence measures are imperfect, and researchers should recognize the limitations of these methods and use multiple lines of evidence when inferring and measuring convergence.
Reorientation of the nasal passage away from the anteroposterior axis has evolved rarely in mammals. Unlike other mammals, cetaceans (e.g., whales, dolphins, and porpoises) have evolved a "blowhole": posteriorly repositioned nares that open dorsad.Accompanying the evolution of the blowhole, the nasal passage has rotated dorsally.Neonatal cetaceans possess a blowhole, but early in development, cetacean embryos exhibit head morphologies that resemble those of other mammals. Previous workers have proposed two developmental models for how the nasal passage reorients during prenatal ontogeny. In one model, which focused on external changes in the
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