Teeth in the shrew/rodent/rabbit/multituberculate clade

The problem:
Always ready for a review, I noticed in the rat/rabbit clade of the large reptile tree (LRT, 1272 taxa) canine teeth (and sometimes nearby others) were lost creating a diastema in seven subclades (Fig. 1). The biggest worry was the apparent reappearance of a full arcade of teeth in highly derived taxa, like Paulchaffotia and Carpolestes, after a several clades without a full arcade (including rodents and the aye-aye). Generally, that’s not supposed to happen. So I reviewed all the data and made a helpful image (Fig. 2).

Figure 1. Subset of the LRT focusing on the clade of rodents, shrews, rabbits and multituberculates. White taxa have a small or large tooth gap between the incisors and premolars.

Figure 1. Subset of the LRT focusing on the clade of rodents, shrews, rabbits and multituberculates. White taxa have a small or large tooth gap between the incisors and premolars.

The solution:
After trying and failing to force all taxa with a diastema together, the LRT recovered a cladogram in which canine teeth disappeared creating a diastema seven times by convergence in the rabbit/rodent clade (Fig. 1). Apparently unknown taxa with small canines linked the last taxa with canines (hedgehogs) with the first taxa with canines beyond rodents (multituberculates).

You might remember
that marsupials and large placental ungulates also produced taxa with a similar diastema. So it is a common convergent trait.

When charts don’t help, sometimes pictures  do.
Here (Fig. 2) are several taxa from the the subset cladogram above (Fig. 1) so you can see for yourself how evolution works in tiny steps that slowly add up to large changes. Particularly interesting here is the central place of hedgehogs (with a full arcade of teeth) basal to higher clades with a full arcade of teeth alongside yet another clade or two with lost canines (diastema).

Figure 2. A selection of taxa from figure 1 more or less to scale and in phylogenetic order (pink arrows). Hope this helps with the concept of a gradual accumulation of traits. The hedgehogs Erinaceus and Echinops are transitional to the higher taxa with teeth and without.

Figure 2. A selection of taxa from figure 1 more or less to scale and in phylogenetic order (pink arrows). Hope this helps with the concept of a gradual accumulation of traits. The hedgehogs Erinaceus and Echinops are transitional to the higher taxa with teeth and without.

Note:
The rodent-like ‘primates’ Ignacius, Plesiadapis and Daubentonia (Figs. 1, 2) are more closely related to rodents in the LRT (contra Gunnell et al. 2018.) That’s heresy, still waiting to be confirmed or refuted by testing by other workers. Note how similar Ignacius is to the hedgehog, Erinaceus (Fig. 3).

Figure 3. The hedgehog, Erinanceus, compared to Ignacius from the Paleocene.

Figure 3. The hedgehog, Erinanceus, compared to Ignacius from the Paleocene. Note the reduction to loss of the canine in the latter.

References
Gunnell GF et al. (9 co-authors) 2018. Fossil lemurs from Egypt and Kenya suggest an African origin for Madagacar’s  aye-aye. Nature Communications 9(3193).

 

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