Baleen boom and bust

A year and a half ago
Marx and Fordyce 2015 entitled an academic paper on whale evolution, “Baleen boom and bust.” The authors report, “The phylogeny of 90 modern and dated fossil species suggests three major phases in baleen whale history: an early adaptive radiation (36–30 Ma), a shift towards bulk filter-feeding (30–23 Ma) and a climate-driven diversity loss around 3 Ma.”

As part of their introduction,
Marx and Fordyce report, “Past studies have fundamentally disagreed on the phylogenetic position, and even monophyly, of many extant and extinct taxa, with major implications for tree topology and molecular estimates of divergence times. Such problems probably reflect limited taxonomic sampling, which is known to compromise both phylogenetic accuracy  and macroevolutionary inferences. Another key problem is the presentation of morphological data as simple character-based scorings, which are usually unsupported by illustrations and hence difficult to comprehend or repeat.”

That’s what I keep saying,
although I’m new to mysticetes. To their credit Marx and Fordyce score 90 taxa for 37,000+ molecular and 272 morphological characters. The project is illustrated with 400 annotated specimens at morphobank.org: project 687. Their oldest taxon is Himalaycetus (Bajpai and Gingerich 1998) from the early Eocene (53.5mya), so this stacks up to be an excellent paper…

except for a few things…

First a little backstory on Himalayacetus
The specimen is the central portion of a left dentary with molar teeth. In vivo the dentqry would have been about 30 cm long, so in the range of Pakicetus. Baipai and Gingerich compared Himalayacetus to Sinonyx, which they considered a mesonychid, and a “representative middle Eocene archaeocete.” In the LRT, you might remember, Sinonyx nested with tenrecs, not mesonychids. Based on chemical analysis, Himalayacetus frequented fresh and salt water, but was found in marine strata.

And here’s where Marx and Fordyce made their mistake(s).
Rather than testing a wide gamut of mammals as potential whale ancestors, Bajpai and Gingerich settled on tradition as they wrote, “Following Van Valen (1966), archaeocetes are generally regarded as descendants of Mesonychia, with which they were long confused.”

As readers all know by now,
“generally regarded” doesn’t cut the mustard anymore, not when testing will tell you what is… and what is not. The LRT shows that mesonychids are distant relatives of mysticetes. Hippos and desmostylians, like Behemotops, are much closer. Desmostylians have been overlooked by whale workers. And Sinonyx is not a mesonychid. It nests with tenrecs, including some giant ones like Andrewsarchus, even when given the opportunity to nest with mesonychids and hippos, which likewise do not nest with artiodactyls in the LRT.

Bajpai and Gingerich report, “The time of divergence of extant Cetacea from extant Artiodactyla is unchanged and lies in the range of 62.5–66.4 Ma, at or near the beginning of the Cenozoic.” Then they report, “Some systematists using molecular genetic clocks suggest divergence of Cetacea from other orders of mammals in the Mesozoic as early as 100 Ma, but the quantified likelihood of such a hypothesis is vanishingly small from the point of view of known fossils and radiometric calibration of the geologic time scale.”

In other words,
they took data and verbally diminished its importance before scientific testing. They assumed that whales are monophyletic. It’s really not their fault. The hypothesis of a separation of odontocetes from mysticetes is only a few weeks old and they published about 70 weeks ago and were no doubt writing their manuscript months to years before that.

So to sum up, except for the invalidated relationship
between basal mysticetes and basal odontocetes, the interrelationships between most of the whale taxa in Marx and Fordyce are valid —

…with a second exception that
some toothed whales are, but should not be, in the lineage of mysticetes…

…with a third exception that
primitive gray whales (Eschritus robustus) nest as highly derived in Marx and Fordyce, while highly derived right whales (Eubalaena australis) nest closer to the origin of baleen whales. So the phylogenetic order of those mysticetes is reversed…

…with a fourth exception that
Janjucetus is considered a basal mysticete and it is not that closely related. It nests with Anthracobune in the LRT and, since that clade is more primitive than the Desmostylia, Janjucetus probably had legs.

That these workers 
accepted the “generally regarded” ancestors without testing is something that can be repaired. All they have to do is add the intervening taxa listed by the LRT to their analysis. When that happens, Janjucetus will nest with Anthracobune. Baleen whales will be derived from desmostylians. Gray whales will nest primitively. Odontocete whales will nest with a long line of extinct and extant tenrecs. And all the currently known fossil toothed whales with nest with odontocetes.

All of these arguments also apply to 
Geisler et al. 2011, who used Sus (pig), Bos (cattle) and Hippopotamus as outgroups with a basal split between mysticetes and odontocetes. Of course there is a HUGE morphological gap between hippos and whales that is conveniently overlooked everywhere but in the LRT.

Which brings up the Yamatocetus illustration problem.
Yamatocetus (Fig. 1, lower left) is a cetiothere mysticete, but Marx et al. 2016 illustrated their Yamatocetus with tiny teeth that don’t appear to be present in photos of the specimen. You can see those photos here: morphobank.org: project 687. Moreover, teeth or not, Yamatocetus nests in the LRT after several toothless taxa. Flat straight jaws in mysticetes is a derived trait, which I can see could be confusing if you are in the monophyletic whales camp. Moreover, considering the extreme flatness of the Yamatocetus rostrum with rather sharp edges, if even rudimentary teeth were present they should, like all marginal teeth, erupt from the jaw margins. If oriented toward the dentary there is little to no room for tooth roots. In short, I think the illustrated teeth are dubious. Send me datum to the contrary if you have it.

Also note
the rather substantial morphological leap from the unnamed NMV P2525677 suction feeding odotocete (Fig. 1, upper right) to passively filtering mysticete Yamatocetus, (Fig. 1, lower left). Odontocetes are active hunters, as were their tenrec ancestors. Mysticetes are passive grazers, as were their desmostylian ancestors.

The origin of baleen
still appears to belong to derived desmostylians (Fig. 3), with concave ventral rostral margins, as noted earlier here and preserved in basal mysticetes.

Figure 2. from Marx et al. 2016. presumes the monophyly of whales and the origin of mysticetes from odontocetes with small teeth. Both are not supported by the LRT. Upper left: Dorudon. Upper right: NMV P2525677. Lower left: Yamatocetus Lower right: Eschrichtius.

Figure 2. from Marx et al. 2016. presumes the monophyly of whales and the origin of mysticetes from odontocetes with small teeth. Both are not supported by the LRT. Upper left: Dorudon. Upper right: NMV P2525677. Lower left: Yamatocetus Lower right: Eschrichtius. When desmostylians are added, Eschrichtius becomes the basalmost mysticete and Yamatocetus is derived.  Were teeth present in Yamatocetus? I have yet to see evidence for teeth, which should be absent in such a derived taxon. I have not seen photos of the Yamatocetus mandible. Does it exist?

Before leaving this topic
the convergence of mysticetes with odontocetes is truly so remarkable that it has gone unnoticed for all this time by both professionals and amateurs alike. To those who dismiss the ability of the LRT to lump and separate such closely convergent taxa, this has been a test of that ability.

Figure 1. Paleoparadoxia. Note the gap left by the diastema between the anterior and posterior teeth. With the phylogenetic placement of desmostylians at the base of the Mysticeti, can this gap be where baleen (light blue) originated? Good question.. The loss of teeth and the elaboration of the baleen in transitional taxa would take us to the toothless, baleen endowed whales.

Figure 3. Paleoparadoxia. Note the gap left by the diastema between the anterior and posterior teeth. With the phylogenetic placement of desmostylians at the base of the Mysticeti, can this gap be where baleen (light blue) originated? Good question.. The loss of teeth and the elaboration of the baleen in transitional taxa would take us to the toothless, baleen endowed whales. Note the similarity of Eschrichtius (Fig. 1, lower right).

References
Demere TA, McGowen MR, Berta A & Gatesy J. 2008. Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales, Systematic Biology, 57 (1) 15-37. DOI: 10.1080/10635150701884632\
Geisler JH, McGowen MR, Yang G and Gatesy J 2011. A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea. Evolutionary Biology 11:112.
Gingerich PD 2005. Aquatic Adaptation and Swimming Mode Inferred from Skeletal Proportions in the Miocene Desmostylian Desmostylus. Journal of Mammalian Evolution, Vol. 12, Nos. 1/2, June 2005.
Marx FG and Fordyce RE 2015. Baleen boom and bust: a synthesis of mysticete phylogeny, diversity and disparity, Royal Society open Science 2:14034.
Marx FG, Hocking DP, Park T, Ziegler T, Evans AR and Fitzgerald EMG 2016. Suction feeding preceded filtering in baleen whale evolution. Memoirs of Museum Victoria 75:71-82.
Okazaki Y 2012. A new mysticete form the upper Oligocene Ashiya Group, Kyushu, Japan and its significance to mysticete evolution. Bulletin of the Kitakyushu Museum of Natural History and Human History Series A (Natural History) 10:129-152.
Shikama T 1966. Postcranial skeletons of Japanese Desmostylia. Palaeontol. Soc. Japan Spec. Pap. 12: 1–202.

 

 

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