The Emergence of Whales, Chp. 5

Synopsis of Chapter 5: Phylogenetic and Morphometric Reassessment of the Dental Evidence for a Mesonychian and Cetacean Clade

Maureen O’Leary, Department of Cell Biology, NY University School of Medicine (when the paper was written, she was at the Dept. of Anatomical Sciences, Health Sciences Center, SUNY-Stony Brook)

This is a tough chapter to summarize. It’s about teeth. It’s virtually all about teeth. There are detailed pictures and diagrams of teeth. There are numerous terms that refer to features on teeth (paracones, metaconids, trigon basin, cusps, reentrant grooves, etc.)

Reading her first paragraph provides some insight about the uncertainty regarding exactly what group the cetaceans are descended from. The question is artiodactyl vs. mesonychian, but they are both ungulate groups! In her first sentence, O’Leary calls the mesonychians “an aberrant group of carnivorous mammals nested within the ungulates”.

She reviews what’s been happening, notably the molecular AND morphological evidence for the Artiodactyla connection. But, a body of morphological work – mostly dental and cranial – supports the mesonychian connection, and there’s not much fossil evidence for the Artiodactyl association (this was written before Thewissen discovered the fragmentary astragali, which lean toward but don’t force the Artiodactyl ancestry.) Figure 1 shows seven (!!) different schematic diagrams of possible cetacean origin routes. Even though that sound daunting, the differences in some cases amount to where a particular representative goes. Notably, Dissacus and Andrewsarchus get shuffled around.

She proceeds to analyze (and criticize) some previous work, including critical comments aimed at earlier papers by Thewissen! (Yes, scientists all band together to present a common front, don’t they?)

The main new thing that O’Leary is doing here will be looking at the pakicetids, protocetids, and ambulocetids. She’s also going to be doing this: “Eigenshape analysis describes the occlusal outline shapes of the P^4’s and M^1’s of early cetaceans to determine whether or not archaic cetaceans most closely resembly mesonychians or other mammals such as Chriacus, Arctocyon, and Diacodexis.” The P^4 is going to be very important, becomes it assumes a very distinctive morphology early in cetacean evolution.

She then describes systematic paleontology of a new mesonychid from Pakistan (unnamed as yet).

Next: description of methods for phylogenetic analysis. Used specimens from AMNH and casts from six other different collections. She assembled a character taxon matrix with pakicetids, Ambulocetus, mesonychidae (8) and Hapalodectidae (1), Triisodontinae (4), Artiodactyla (1), with artocyonids as outgroup members. She lists 23 character states, then describes the eigenshape analysis method.

RESULTS

Of the next 10 pages, four are completely or more than 50% devoted to detailed drawings of archaocete or mesonychian teeth. I can see where the paraconid is located. O’Leary proceeds to describe the dental apomorphies for Mesonychia, Cetacea, Acreodi, and Cete. Unless you’re a paleodentist, it won’t do any good to describe them. But some things are basic, like “Cetaceans differ from mesonychians in the position of the paraconid on the lower molars.” That’s that type of thing O’Leary is examining. (For those of you who don’t know, an apomorpy is a new genetic characteristic to a clade. Also known as a derived trait: it means that a character/trait is inferred to be a modified version of a more primitive condition of that character, hence the apomorphy arose in the evolution of the clade.)

What is Acreodi? It’s a name used by Prothero et al. for Cetacea+Mesonychia. Cete = Cetacea+Mesonychidae+ Hapalodectidae+Triisodontinae.

A key result:

“…mesonychians are autapomorphic in the position of the paraconid. This derived feature is present and retained throughout the entire mesonychian clade and apparently did not undergo any reversals. Thus, although mesonychians (Mesonychidae and Hapalodectidae) may be the sister taxon to the cetaceans, they are too derived dentally to have been ancestral to cetaceans.” (!!)

O’Leary devotes a few paragraphs to toothwear patterns. Basically, the teeth of archaeocetes don’t show the same toothwear patterns as mesonychids. Relatively unworn teeth show some minor similarities; heavily worn molars show markedly different toothwear patterns. Here’s an example of what can be learned from this analysis (end of the toothwear section):

“The mesonychian chewing mechanism involved more crushing and possibly a component of mediolateral movement especially in late stages of wear. The chewing mechanism of archaic cetaceans involved heavy shearing and almost exclusively orthal motions, even in advanced stages of wear.”

To the DISCUSSION:

I’m going to summarize the main points:

1. Mesonychia is monophyletic (Hapalodectidae+Mesonychidae, excluding Andrewsarchus)

2. Nalacetus (remember what I said in Chapter 1) is the most primitive known cetacean. (!!)

3. Cetacea + Mesonychia = clade Acreodi, sister taxon Microclaenodon. Andrewsarchus and Diacodexis are somewhat removed from Acreodi.

4. Discussion of how more specimens, particularly Pakicetus and Nalacetus clarified the position of mesonychians, i.e. sister taxon, not ancestral. Here’s a prediction (for those that think evolution doesn’t make predictions):

“This phylogenetic analysis predicts that the common ancestor of cetaceans and mesonychians had a relatively primitive molar trigonid with a lingually positioned paraconid, similar to the triisodontine condition.” (Now they’ll have to go look for it.)

6. Toothwear patterns show divergence of chewing styles.

7. The new mesonychian tooth from Pakistan potentially offers further insight into “the internal structure of clade Cete”.

So, has anything been resolved here? Yes, apparently: mesonychians are not ancestral to Cetacea. Much further on, we’ll found out if there’s a synthesis afoot.

But not before considering the immense Chapter 6:

“Relationships of Cetacea to Terrestrial Ungulates and the Evolution of Cranial Vasculature in Cete”

which has some really interesting anatomical information about whales that I never knew before.