Mysticeti

Mysticeti

Baleen whales

Michel C. Milinkovitch and Olivier Lambert

close boxContaining group: Cetacea

Introduction

All living mysticetes are large filter-feeders although the exact means by which baleens are used vary among species (gulp-feeding for balaenopterids, skim-feeding for balaenids, and bottom ploughing for eschrichtiids).

Characteristics

The morphological modifications linked to filter-feeding are:

• Loss of functional teeth (vestigial teeth are lost before birth),

• Development of large body size and head,

• Shortening of the neck,

• Expanded maxillae (on which the epithelially-derived baleen plates develop),

• Mandibular rami only attached to each other by a ligament (at the symphysis).

Discussion of Phylogenetic Relationships

The relationships among extinct and extant groups of mysticetes have been less extensively studied than the relationships among odontocetes. Four families are currently recognized: Balaenidae, Balaenopteridae, the Neobalaenidae (including a single extant species, the pygmy right whale, Caperea marginata), and the Eschrichtiidae (including a single extant species, the gray whale, Eschrichtius robustus).

Some molecular analyses positioned the skim-feeding balaenids (right whales) as the basal lineage of extant mysticetes (e.g., Arnason and Gulberg, 1994, 1996; Gatesy 1998). Uncertainty remains regarding the phylogenetic position of neobalaenids: Caperea might be the sister group to balaenopterids (rorquals) and Eschrichtius (gray whale) (Arnason and Gulberg 1994), or to balaenids (Gatesy 1998). This latter hypothesis is also supported by the morphological studies of McLeod et al. (1993) and Bisconti (2000).

Discussion of some problematic extinct groups

Several fossil groups of mysticetes (e.g., Aetiocetidae and Cetotheriidae) are still taxonomically problematic; it is very likely that some of them form paraphyletic groups. Systematic revisions, additional anatomical data and cladistic analyses will be required before reaching reasonable hypotheses regarding the phylogeny of these groups.

• The Oligocene family Aetiocetidae (review in Barnes et al. 1994) includes primitive toothed mysticetes (some of them having been erroneously placed within archaeocetes), showing a mix of archaeocete-like and mysticete-like skull features. This family persisted into Late Oligocene, at a time where more derived baleen-bearing whales had already evolved. Specimens have been found on both sides of the North Pacific.

• The fossil specimens from other archaic toothed mysticete families (Llanocetidae, Mammalodontidae, and Kekenodontidae) have not been studied extensively enough to be meaningfully included into phylogenetic analyses. Members of these families, may have used their teeth for bulk feeding rather than for

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