SVP abstracts 7: Coombs follows the traditional whale origin myth

Coombs 2020 studied whale skulls
using a traditional, but recently invalidated phylogeny. She did not understand the diphyly of the former clade ‘Cetacea’.

From the Coombs abstract:
“The extant clades of whales, Odontoceti (toothed whales) and Mysticeti (baleen whales), diverged ~39 Ma.”

According to the large reptile tree (LRT, 1749+ taxa) that divergence occurred way back when whale ancestors were still tree shrews. A tiny taxon, Anagale (Fig. 1; Late Cretaceous, 75-71mya) is near their last common ancestor.

Figure 1. We are very fortunate to have several of these basal placental taxa still living with us, as chronologically long-lived taxa. Starting with the extant Didelphis at the base of the Theria, phylogenetic miniaturization gave us the smaller Monodelphis domestics and the even smaller M. sores and M. kunsi, which gave rise to the larger Nandinia at the base of the Carnivora, Tupaia, at the base of the expanded Glires, Ptilocercus at the base of the expanded Archonta, and Maelestes at the base of the tenrecs + whales and the Condylarthra, aka the rest of the mammals.

Figure 1. We are very fortunate to have several of these basal placental taxa still living with us, as chronologically long-lived taxa. Starting with the extant Didelphis at the base of the Theria, phylogenetic miniaturization gave us the smaller Monodelphis domestics and the even smaller M. sores and M. kunsi, which gave rise to the larger Nandinia at the base of the Carnivora, Tupaia, at the base of the expanded Glires, Ptilocercus at the base of the expanded Archonta, and the Condylarthra, aka the rest of the mammals.

Continuing from the Coombs abstract:
“Odontocetes evolved high-frequency echolocation and cranial asymmetry, while mysticetes evolved larger masses and filter feeding.”

Actually odontocete ancestors, represented by extant tenrecs, developed echolocation and cranial asymmetry, by the Paleocene 65mya.

Mysticete ancestors did not develop filter feeding until the Oligocene, 34-23mya at the earliest. Mystacodon (Fig. 2; 36mya) was considered the earliest baleen whale, but this toothy whale nests with the odontocete clade.

FIgure 1. This toothy whale with a tiny pelvis is Mystcodon, originally promoted as the earliest known mysticete (baleen whale).

FIgure 2. This toothy whale with a tiny pelvis is Mystcodon, originally promoted as the earliest known mysticete (baleen whale).

Continuing from the Coombs abstract:
“Despite an excellent fossil record and unique morphology, there has been little quantitative study of shape evolution spanning cetacean diversity.”

Before making that statement, Coombs should add taxa to start with a valid phylogeny, lacking at present. Ancestors to both whale clades (Fig. 3) have been traditionally overlooked due to taxon exclusion.

“To quantify morphological disparity and evolutionary rate in cranial shape and to identify ecological correlates of shape variation across Cetacea, I gathered 3D scans of specimens representing 84 living (72 odontocetes, 12 mysticetes) and 72 Eocene to Pliocene fossil (45 odontocetes, 17 mysticetes, 10 archaeocetes) cetaceans. I then digitized 123 landmarks and 64 curves on these scans and conducted high-dimensional geometric morphometric and macroevolutionary analyses within a phylogenetic framework.”

The Coombs phylogenetic framework is fatally flawed due to taxon exclusion. Adding pertinent taxa will solve this problem.

Figure 4. Subset of the LRT focusing on the odontocetes and their ancestors.

Figure 3. Subset of the LRT focusing on the odontocetes and their ancestors.

Continuing from the Coombs abstract:
“The largest component of cranial variation (PC1 = 39.9%) reflects a posterior shift in the nares and separates odontocete and mysticete modes of cranial telescoping. Rostrum length is the major component of variation on PC2 (20.7%) with dolicocephalic [having a long skull] (e.g., Pontoporia blainvillei) and brachycephalic [having a short skull] (e.g., Kogia sima) crania representing the extremes.”

Figure 3. The oreodont-mesonychid-hippo-desmoystlian-mysticete clade subset of the LRT

Figure 4. The oreodont-mesonychid-hippo-desmoystlian-mysticete clade subset of the LRT

Continuing from the Coombs abstract:
“Cranial asymmetry in archaeocetes is high in the rostrum, squamosal, jugal, and orbit, possibly reflecting preservational deformation. In odontocetes, it is highest in the naso-facial region. Mysticetes show levels of asymmetry similar to terrestrial artiodactyls.”

In other words: essentially no asymmetry. Why? Because mysticetes and odontocetes had different ancestors. Artiodactyls had nothing to do with whales ever since the LRT pulled hippos out of the artiodactyls and into the mesonychids (Fig. 4).

Figure 1. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Figure 5. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Continuing from the Coombs abstract:
“Significant rate shifts in asymmetry are observed in the stem odontocetes Xenorophidae (∼30 Ma), Physeteroidea (∼27 Ma), Squalodelphinidae (~27 Ma), and Monodontidae (~7 Ma). Rapid evolution of both cranial shape and asymmetry in cetaceans occurred in the Middle-Late Oligocene and peaks in the Middle Late Miocene, largely due to subclade-specific diversification of rostrum and facial morphology.”

Coombs’ results, no matter how carefully measured, are incomplete because they are not recovered within a valid phylogenetic context. Add pertinent taxa to resolve this issue.


References
Coombs E 2020. Cranial morphology in whales: A study spanning the evolutionary history and diversity of the Cetacean skull. SVP abstracts 2020.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.