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).

 

When pre-primates split from pre-rodents

A quick look at two closely related taxa today.
Hapalodectes (IVPP V5235, Fig. 1; Paleocene. China) nests at the base of the clade of primates (lemurs through humans, Fig. 3). Notice the narrow, transverse premaxilla and large canine.

Figure 1. The IVPP V5235 specimen of Haplodectes. Note the large canine and small, transverse premaxilla, traits shared with higher primates.

Figure 1. The IVPP V5235 specimen of Hapalodectes. Note the large canine and small, transverse premaxilla, traits shared with higher primates.

On the other hand
Ptilocercus
(Fig. 2; extant, Thailand, the pen-tailed tree shrew) nests at the base of the clade of rodents and rabbits and kin. Notice the large premaxilla (yellow) and small canine (orange). This represents the genesis of gnawing teeth in this clade.

Figure 2. The skull of Ptilocercus, nesting at the base of the rodent/rabbit clade. Note the large premaxilla (yellow) and small canine (orange).

Figure 2. The skull of Ptilocercus, nesting at the base of the rodent/rabbit clade. Note the large premaxilla (yellow) and small canine (orange).

These mouse-sized arboreal taxa
are late survivors of an earlier Middle Jurassic radiation. Both have a complete circumorbital ring, atypical for most mammals.

Figure 3. Subset of the LRT focused on Primates and basal Glires.

Figure 3. Subset of the LRT focused on Primates and basal Glires.

 

A tiny Late Jurassic pre-rabbit: Henkelotherium

This earlier tracing and nesting
saw improvement with a higher resolution image.

Figure 1. Henkelotherium, a traditional pantothere, nests as a Late Jurassic pre-rabbit in the LRT.

Figure 1. Henkelotherium, a traditional pantothere, nests as a Late Jurassic pre-rabbit in the LRT. Image about 3.5x life size.

Henkelotherium guimarotae (Krebs 1991; Late Jurassic 150 mya; Figs. 1-3) was traditionally considered a pantothere or eupantothere. Here Henkelotherium nests with rabbits as a Late Jurassic member of the clade Glires. Like its sisters, the manus was small and the pes had long digits with sharp claws. The lumbar region was long and flexible. A tiny taxon, the image below (Fig. 2) is about twice natural size.

Figure 2. Henkelotherium reconstructed from DGS tracings in figure 1. Note the tiny manus and large pes, traits that continue into extant rabbits.

Figure 2. Henkelotherium reconstructed from DGS tracings in figure 1. Note the tiny manus and large pes, traits that continue into extant rabbits. Image about twice life size. What looks like an eye here is just a tracing of bone cracks, probably in the frontal.

Wikipedia reports
“Eupantotheres are derived compared to symmetrodonts in having wider upper than lower teeth (although they still lack the protocone of tribosphenic forms.” The large reptile tree does not recover this clade. Rather the clade Lagomorpha should be extended to include the taxa found here (Fig. 3).

Figure 3. The Lagomorpha clade with the addition of Henkelotherium.

Figure 3. The Lagomorpha clade with the addition of Henkelotherium.

References
Krebs B 1991. Skelett von Henkelotherium guimarotae gen. et sp. nov. (Eupantotheria, Mammalia) aus dem Oberen Jura von Portugal. Berl Geowiss Abh A.: 133:1–110.

wiki/Henkelotherium

Rats! – (or where Mickey Mouse diverged from Walt Disney)

Updated January 3, 2019, seven years and about 1000+ taxa later.

We don’t talk about mammals much,
but as reptiles they (we) do qualify as subjects to be covered by ReptileEvolution.com.

A new online study by Wu et al. (2012) finds evidence for a post-Cretaceous origin for rodents. Rodents (everything from porcupines and guinea pigs to squirrels, mice and multituberculates) are related to rabbits (lagomorphs).

The Wu et al 2012 study on rodents and their post-Cretaceous appearance.

The Wu et al 2012 study on rodents and their post-Cretaceous appearance.

How are they all related? 
Near (but not at) the base of the primates is an interesting set of taxa known as tree shrews, like Tupaia (Raffles 1821, Fig. 1). Essentially they are basal rodents/rabbits/multituberculates.

Tupaia, the large tree shrew,

Figure 1. Tupaia, the large tree shrew, a living taxon close to the base of rabbits and rodents with origins in the Paleocene, just following the Cretaceous. Click to learn more.

The most common tree shrew, Tupaia was found to be basal to the equally arboreal and highly derived Plesiadapis (Fig. 3) and terrestrial rabbits, and rodents, like the porcupine. It’s worthwhile to see the porcupine skull and how close it resembles that of Plesiadapis.

Plesiadapis

Figure 3. Plesiadapis, formerly considered a basal primate, is here considered a basal arboreal lagomorph (rabbit ancestor).

The other arboreal tree shrew, 
Ptilocercus, is basal to Tupaia, derived from basal primates and carnivores.

Ptilocercus, pen-tailed tree shrew

Figure 2. Ptilocercus, pen-tailed tree shrew, a living relative to the ancestor of bats and colugos.

Based on the nesting of multituberculates
all these tree shrews, rodents and rabbits had their origin in the Jurassic, not the Paleocene (contra Wu et al. 2012.

References
Wu S, Wu W, Zhang Z, Ye J, Ni X, Sun J, Edwards SV, Meng J and Organ CL 2012. Molecular and Paleontological Evidence for a Post-Cretaceous Origin of Rodents. PLoS ONE 7(10): e46445. doi:10.1371/journal.pone.0046445
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0046445