Another flawed aye-aye origin paper: Gunnell et al. 2018

Earlier we looked at µCT scans of the aye-aye (Figs. 1, 4), Daubentonia made by Morris, Cobb and Cox 2018 and comparisons to Lemur catta (Fig. 2), a taxon often considered a sister to Daubentonia.

Figure 1. Daubentonia was considered a primate for over 150 years. Here it nests with Plesiadapis, rodents and rabbits.

Figure 1. Daubentonia was considered a primate for over 150 years. Here it nests with Plesiadapis, rodents and multituberculates + carpolestids.

 

Figure 2. Lemur catta in vivo and skeleton.

Figure 2a. Lemur catta in vivo and skeleton.

Figure 2. Lemur catta skull in 3 views.

Figure 2b. Lemur catta skull in 3 views. Compare this skull to Daubentonia in figure 4. Note the large canines missing in Daubentonia, replaced by giant incisors and no canines.

Gunnell et al. 2018
reidentified the fossil jaw bone of Propotto leaky (Simpson 1967, 20mya; Fig. 3). “In a study published August 21 in the journal Nature Communications, researchers have re-examined Propotto’s fossilized remains and suggest that the strange creature wasn’t a bat, but an ancient relative of the aye-aye, the bucktoothed nocturnal primate that represents one of the earliest branches of the lemur family tree.” 

Figure 1. Propotto and Plesiopithecus nest with Daubentonia in Gunnell et al. 2018, which does not test many rodents, despite the rodent-like teeth shown here.

Figure 3. Propotto and Plesiopithecus nest with Daubentonia in Gunnell et al. 2018, which does not test many rodents, despite the rodent-like teeth shown here.

Unfortunately
when I ran the Gunnell et al. matrix the clade of rodent-toothed taxa (Daubentonia, Propotto and Pleisopithecus) nested with the primate-toothed Lemur catta. All primates in the large reptile tree (LRT, 1372 taxa) have large canines and two small incisors (except humans and kin where the canines are not fangs). Rodents have the opposite, small to absent canines together with single giant incisors. Rodent-toothed Carpolestes and Plesiadapis (Fig. 6) were tested by Morris, Cobb and Cox 2018, but nested far from the Daubentonia clade. That is strange. No other rodents were tested to eliminate the possibility that rodent-toothed taxa might actually be closer to rodents than primates or that Carpolestes and Pleisadapis might be rodents themselves. In the LRT they are primate-like rodents, not rodent-like primates.

Strangely, but traditionally,
the outgroup taxa for primates in the Morris, Cobb and Cox 2018 study were Tupaia and Ptilocercus, two taxa that nest not with primates, but with Glires (shrews + rodents + multituberculates and kin) in the LRT, which includes more taxa.

A toothless diastema
occurs between the one to two premolars and the giant dentary incisors of Daubentonia, Plesiadapis, Ignacius and most rodents. I don’t see that morphology in figure 3 where three premolars fill the space between the molars and incisors of Propotto and Plesiopithecus. Such a mandible morphology is found in more basal members of Glires, like the hedgehog (Echinops), Apatemys and some shrews, like Scutisorex. None of these taxa were tested by the Gunnell team in their study of Propotto and Plesiopithecus.

The Gunnell et al. cladogram may have suffered from
too many dental traits and too few Glires taxa. It did not deliver the expected ‘gradual accumulation of traits’ that mark every good cladogram (because that’s how evolution works). Rather, like too many cladograms we’ve looked at over the years, sister taxa just don’t look like each other and the enigma taxon looks too much like something else in the cladogram.

Quote mining from the Duke U PR online article:
Propotto: “In 1967, paleontologist George Gaylord Simpson inspected the fragments and classified the specimen as a previously unknown member of the loris family, nocturnal primates with enormous eyes. But a colleague named Alan Walker took a look and thought otherwise, eventually convincing Simpson that the bones belonged to a bat.

For nearly half a century the creature’s identity appeared to have been settled, until 2016, when another paleontologist, the late Gregg Gunnell of Duke University, began taking a fresh look at the fossil. To Gunnell’s eye, the creature’s hind teeth were more reminiscent of a primate than a bat. He also noted the stump of a broken front tooth, just visible in cross section, which would have jutted out from its mouth like a dagger — a trait only known in aye-ayes, the only living primates with rodent-like teeth.

“Gregg wrote to us and said, ‘Tell me I’m crazy,’” Seiffert said.

The researchers found that Propotto shared a number of features with a similarly buck-toothed primate that lived 34 million years ago in Egypt called Plesiopithecus, and that both were ancient relatives of the aye-aye.

In the new study, Seiffert, Gunnell and colleagues propose that the ancestors of aye-ayes split from the rest of the lemur family tree roughly 40 million years ago, while still on the African continent, and the resulting two lineages didn’t make their separate ways to Madagascar until later.

The findings suggest they arrived around the same time as other mammals, such as rodents, Malagasy mongooses and hedgehog- and shrew-like animals called tenrecs. Frogs, snakes and lizards may have made the trip around the same time.”

In the LRT, all these taxa were already on Madagascar in the Mesozoic and did not have to raft over after the split from Africa. 

“Lemurs can’t swim, so some scientists hypothesize that the small-bodied creatures crossed the 250-mile-wide channel that lies between Africa and Madagascar after being swept out to sea in a storm, by holding on to tree limbs or floating mats of vegetation before finally washing ashore.

Figure 2. Skeleton of Daubentonia (aye-aye). Like other plesiadapids, it convergences with the lemuroid primates.

Figure 4. Skeleton of Daubentonia (aye-aye). Like other plesiadapids, it convergences with the lemuroid primates. Consider it a primate-like rodent, not a rodent-like primate. Compare this skull to figure 5.

“But if the arrival were more recent, they might have had a shorter distance to travel, thanks to lower sea levels when the Antarctic ice sheet was much larger. “It’s possible that lemurs weren’t in Madagascar at all until maybe the Miocene,” as recently as 23 million years ago, Boyer said. Some of the lowest sea levels were also during this time,” Heritage said.

Figure 4. Perodicticus potto, the extant potto, has a typical lemur dentition, lacking giant incisors.

Figure 5. Perodicticus potto, the extant potto, has a typical lemur dentition, lacking giant incisors. Compare this skull to figure 4. Note the large canines missing in Daubentonia. 

What about the extant potto, Perodicticus potto?
Perodicticus potto (Bosman 1704, Fig. 5) does not have large rodent-like lower incisors. Rather it has a skull somewhat midway between the lemurs and tarsioids (Fig. 3) with large canines.

Figure 1. Ignacius and Plesiadapis nest basal to Daubentonia in the LRT.

Figure 6.  Ignacius and Plesiadapis nest basal to Daubentonia in the LRT.

This brings up the unfortunate habit
of naming taxa that are not related to the taxa they are purportedly related to, like Propotto and Plesiopithecus (Fig. 3).

And yet another example of ‘Pulling a Larry Martin’:

Figure 7. How Gunnell et al. 'Pulled a Larry Martin'. They cherry-picked taxa. They focused on just a few traits in the mandible. They hope that four tiny incisors might evolve into two giant incisors.

Figure 7. How Gunnell et al. ‘Pulled a Larry Martin’. They cherry-picked taxa. They focused on just a few traits in the mandible. They hope that four tiny incisors might evolve into two giant incisors.

For those who don’t read captions.
How Gunnell et al. ‘Pulled a Larry Martin‘. (Fig. 7).

  1. They cherry-picked taxa, (= taxon exclusion, where is Lemur catta in figure 7?).
  2. They focused on just a few traits in the mandible.
  3. They hoped that four tiny incisors might evolve into two giant incisors.
  4. They did not recognize the convergence that the LRT recovered.

References
Gunnell GF et al. (9 co-authors) 2018. Fossil lemurs from Egypt and Kenya suggest an African origin for Madagascar’s aye-aye. Nature Communications. PDF
Simpson GG 1967. The tertiary lorisiform primates of Africa. Bull. Mus. Comp. Zool. 136, 39–62.

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