| Summary: | Baleen whales (mysticetes) form an essential part of modern ocean ecosystems, and show some of the most extreme adaptations evolved by any mammal. The evolutionary history of these animals has attracted much attention in recent years, but many disagreements and question marks remain. This study takes a fresh look at the phylogenetic relationships and diversity of mysticetes, with a special emphasis on the relationships of the enigmatic living pygmy right whale, Caperea marginata, and the evolution of morphological diversity (disparity) over the past 34 Ma. Detailed morphological comparisons making use of extensive collections housed at New Zealand institutions reveal the pygmy right whale to be the last survivor of the once diverse, and formerly thought extinct, family Cetotheriidae. A comprehensive phylogenetic analysis of 69 mysticetes species and 232 characters supports this result. The same analysis furthermore confirms the monophyly of the major extinct and extant mysticete families (Aetiocetidae, Balaenidae, Balaenopteridae, Cetotheriidae, Eschrichtiidae, Mammalodontidae), in addition to an expanded Eomysticetidae and a clade of toothed mysticetes comprising aetiocetids, mammalodontids, and an undescribed specimen from the early Oligocene of New Zealand. Unlike for most previous analyses, the present data matrix was assembled using the online morphology database MorphoBank, where every scoring was illustrated with around 3,800 annotated, high-quality photographs. Mysticete disparity is found to have increased quickly during the early history of the clade. Following a distinct peak during the Late Oligocene, disparity rapidly declines towards the Middle Miocene, before eventually starting to recover again during the Late Miocene and Pliocene. Disparity and taxonomic diversity are entirely decoupled. The disparity spike coincides with the co-occurrence of several small, relatively short-lived and very disparate taxa, hinting at the existence of an adaptive radiation leading to several distinct morphotypes early in mysticete evolutionary history. Using the phylogeny to reconstruct missing character scores in the data matrix has virtually no effect on the results, despite the matrix being less than 60% complete. By contrast, the inclusion of reconstructed hypothetical ancestors (internal branches) notably changes the morphospace distribution of the observed taxa. In a plot of the first two axis of a Principal Coordinate Analysis, roughly half of the ancestral taxa fall outside the range defined by the observed taxa, indicating that reconstructed values at internal nodes can combine to reveal novel information about unsampled ancestral morphologies.
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