Saturday, December 22, 2012

Evolution of the pygmy right whale Caperea and the resurrection of the Cetotheriidae from extinction


I have a special place in my heart for small baleen whales. Large baleen whales have skulls (modern and fossil) that are just a pain in the ass to examine, move, photograph, measure, or excavate. When I first got started with marine mammal paleontology as an undergraduate, baleen whale earbones were great to work with as they still provide reasonable phylogenetic information, but are easy to identify in the field, easier to collect than complete skulls, and when a significant collection of them is established, inferring the diversity of a baleen whale assemblage is elementary. The discovery of a well preserved skull of Herpetocetus bramblei in the Purisima Formation in June 2007 (and to a lesser extent, a mandible of the same species from the preceding summer) catapulted me into the study of fossil cetotheriid mysticetes. Cetotheres were formerly considered a wastebasket group, a neglected heap of misfit toothless mysticetes that didn’t belong in modern families. These were found to belong to two phylogenetic groups: one monophyletic clade termed the Cetotheriidae sensu stricto (Cetotherium, Piscobalaena, Herpetocetus), and another group forming a paraphyletic group of early balaenopteroids, sometimes jokingly referred to as “Kelloggitheres” (Parietobalaena, Diorocetus, Pelocetus). Given my interest in true cetotheres, I was pleased to see the publication of a new article in Proceedings of the Royal Society by my adviser Ewan Fordyce and fellow labmate Felix Marx on a phylogenetic reinterpretation of the modern pygmy right whale, Caperea marginata, as an extant cetothere.



The skeleton of the pygmy right whale, Caperea marginata. From Bisconti (2012).


The modern pygmy right whale inhabits the southern ocean, and is rarely observed alive; as of 2008, only 25 sightings of Caperea have been made at sea. We know more about Caperea from dead individuals than live ones – much like working with fossils. Caperea is approximately 5-6 meters in length as an adult, dark gray, and bears a bowed rostrum with long baleen, like balaenid right whales (true right whales). Caperea feeds on copepods and euphausiids, probably employing a similar skim feeding behavior as in right whales. Unlike balaenids, the pygmy right has a falcate dorsal fin. Caperea has on rare occasions been observed in massive gregarious accumulations. The skeleton of Caperea is one of the most distinct among baleen whales: an extreme degree of cranial telescoping, a relatively short rostrum, nearly absent zygomatic processes of the squamosal, mandibles that are transversely flattened with a sinusoidal outline, bizarre earbones, posterior ribs that are expanded and flattened, and transverse processes of the lumbar vertebrae that are expanded into canoe-paddle shapes and overlap one another.


Phylogenetic results from Fordyce and Marx (2012), showing the revised phylogenetic position of Caperea marginata as a cetotheriid.




Comparison of the skulls of Caperea marginata (above) and Herpetocetus transatlanticus (below) in dorsal (left) and ventral (right) views. From Fordyce and Marx (2012).

Two previous phylogenetic hypotheses for the pygmy right whale exist. As the common name implies, morphologists have historically considered it to be related to the Balaenidae – forming a clade called the Balaenoidea. Most previous phylogenetic analyses have supported a monophyletic Balaenoidea (see summary in Churchill et al., 2012). However, molecular analyses have generally failed to yield such a relationship, and instead have found Caperea to be slightly more closely related to balaenopteroid mysticetes rather than balaenids. A recent morphological analysis by Felix Marx (2011) resulted in this intermediate position, unique among morphological analyses. In his phylogeny, Caperea showed up as a sister to the balaenopteroids, with cetotheriids, “kelloggitheres”, and balaenids further down the tree. In the new phylogeny by Fordyce and Marx (2012), Caperea was bumped down one node on the tree into the Cetotheriidae – and as a sister taxon to the Herpetocetinae in particular (Nannocetus + Herpetocetus). This resulted from the reinterpretation of several anatomical features, based on new observations of the skull morphology of Caperea. For example, cetotheriids and Caperea share a plug-like extension of the petrosal (earbone) that is exposed on the lateral surface of the skull. Like Herpetocetus, a large flange of the petrosal sticks out laterally and articulates with the squamosal bone. Certain features are obvious only in juveniles, and are obliterated during ontogeny by changes in skull shape. For example, juvenile Caperea show a rectangular extension of the maxilla that extends toward the top of the skull; this is a feature that is absent in all modern and fossil balaenids, but present in cetotheriids and balaenopteroids. Many other characters relating to the basicranium were interpreted as similar in cetotheriids, which in the analysis pulled Caperea into the Cetotheriidae.





Comparison of the earbones of Caperea marginata (left) and Herpetocetus transatlanticus (right); petrosal in articulation with squamosal above, and tympanic below. From Fordyce and Marx (2012).

So what’s so important about the reclassification of Caperea as a cetotheriid? For one, it allies a modern baleen whale with a group formerly considered to be totally extinct. The last member of the family – Herpetocetus – lasted until about 2 million years ago in the eastern North Pacific. I’m not so certain that Caperea is necessarily so closely related to the herpetocetines in particular – the fossil neobalaenid Miocaperea is contemporaneous with the oldest herpetocetine, Nannocetus eremus, and is already extremely derived; indeed, the case for it requiring a genus separate from Caperea is thin. This indicates that the hallmark pygmy right whale morphology had already evolved by the time derived herpetocetines evolved – suggesting if anything that these two groups (pygmy right whales, and herpetocetines) evolved in parallel. Neobalaenines paralleled the morphology of herpetocetines, separated from the herpetocetines in the southern hemisphere. It is unclear whether or not herpetocetines utilized a similar feeding ecology – but it is fascinating to imagine the northern hemisphere analog of Caperea going extinct at the end of the Pliocene, and the only straggler of a formerly more diverse group of dwarf baleen whales surviving in the southern ocean.

Further reading-





 References-

Churchill M, Berta A, Demere TA. 2011 The systematics of right whales (Mysticeti: Balaenidae). Mar. Mamm. Sci. 28, 497–521.

Fordyce RE, Marx FG. 2012 The pygmy right whale Caperea marginata: the last of the cetotheres. Proc R Soc B 280:20122645.

Marx FG. 2011 The more the merrier? A large cladistic analysis of mysticetes, and comments on the transition from teeth to baleen. J. Mamm. Evol. 18, 77–100.

1 comment:

OKEAN Shept said...

In 2012 the fossilized occipital bone of a skull of a Miocene whale of tsetoteriya (Cetotherium), or the sorts Brandtocetus (and on age it precisely corresponds to a holotype of a type of B. chongulek - bottoms of the late Sarmatian) is found
So far it is kept and нечищена. The closed-up channel of a spinal opening is visible. Weight is more than 10 kg.
Find Kerch place, garden sites on the seashore with breaks to 10 m, and clay layers, plaster crystals.
It is found on a surface about the road.
http://vk.com/album204569914_195885301
At whom doesn't suffice for a full skeleton, write okean@ukr.net