New paper on Plesiadapis suffers from taxon exclusion

Boyer and Gingerich 2019
bring us an excellent and comprehensive review of Plesiadapis (Figs. 1-3), a rodent relative (clade: Glires, Figs. 4, 5) traditionally and wrongly considered a basal primate with rodent-like teeth.

Figure 1. From Boyer and Gingerich 2019, Plesiadapis skeleton and in vivo.

Figure 1. From Boyer and Gingerich 2019, Plesiadapis skeleton and in vivo.

This primate-mimic
nests with another primate mimic, Daubentonia (Fig. 3), the extant aye-aye, a taxon barely mentioned and not analyzed by Boyer and Gingerich.

Plesiadapis

Figure 2. Plesiadapis, formerly considered a basal primate, is here considered a member of Glires close to Carpolestes and Daubentonia. See figure 3.

From the abstract
“Plesiadapis cookei is a large-bodied plesiadapiform euarchontan (and potential stem primate) known from many localities of middle Clarkforkian North American Land Mammal age, late Paleocene epoch, in the Clarks Fork Basin of northwestern Wyoming.”

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

Figure 3. Ignacius and Plesiadapis nest basal to Daubentonia in the LRT.

From the abstract
“On a broader scale, cladistic analysis of higher-level taxa… indicates that plesiadapids and carpolestids exhibit a greater number of identical character states than previously thought … Even so, analysis of combined data from dentition, cranium, and postcrania still robustly support a link between plesiadapids, saxonellids, and carpolestids (Plesiadapoidea) and does not contradict previous hypotheses suggesting a special relationship of plesiadapoids to euprimates (Euprimateformes).”

Figure 4. From Boyer and Gingerich 2019, cladograms nesting Plesiadapis.

Figure 4. From Boyer and Gingerich 2019, cladograms nesting Plesiadapis. Too few taxa. Where is Daubentonia? Where are the derived rodents and multitubercuates? Compare to figure 5.

Too few taxa,
alas is the one obvious issue with Boyer and Gingerich 2019 (Fig. 4).

Figure 1. Subset of the LRT focusing on Glires and subclades within.

Figure 5. Subset of the LRT focusing on Glires and subclades within.

Not much else to say.
The large reptile tree (LRT, 1583+ taxa; subset Fig. 5) is an online resource that can and should be employed. Current traditions and textbooks are out of date on this subject. At least consider the taxon list in your more focused studies so you don’t overlook any obvious taxa. Test them yourselves. Don’t make the same mistake.


References
Boyer DM and Gingerich PD 2019. Skeleton of Late Paleocene Plesiadapis cookei (Mammal, Euarchonta): life history, locomotion, and phylogenetic relationships. University of Michigan Papers on Paleontology 38:269pp.

wiki/Plesiadapis

A post-dentary reversal between rodents and multituberculates

Yesterday I promised a look at the new Jurassic gliding mammal, Arboroharamiya (Han et al. 2017), known from two crushed, but complete specimens (Figs. 1, 2). Originally this genus was considered a euharamiyid, close to the Jurassic squirrel-like Shenshou (Fig. 3) derived from trithelodont pre-mammals close to Haramiyavia.

Figure 1. The holotype specimen of Arboroharamiya HG-M017 in situ with DGS tracings added.

Figure 1. The holotype specimen of Arboroharamiya HG-M017 in situ with DGS tracings added. The skull in figure 5 comes from this specimen.

The two specimens are superficially distinct
due to the width of their extraordinary gliding membranes, reinforced with stiff fibers. I have not tested the paratype specimen in the LRT yet.

Figure 2. The paratype specimen of Arboroharamiya HG-M018, in situ. DGS color tracing added. The skull is in poor shape.

Figure 2. The paratype specimen of Arboroharamiya HG-M018, in situ. DGS color tracing added. The skull is in poor shape.

Contra Han et al. 2017
In the large reptile tree Arboroharamiya nests with Carpolestes, Ignacius, Plesiadapis, Daubentonia and Paulchaffatia, taxa excluded from Han et al. The extant rodents, Rattus and Mus, are also related and included in the Han et al. cladogram (Fig. 3).

Figure 1. From Han et al. 2017, a cladogram that nests Arboroharamiya close to Xianshou and Shenshou. Colors added to showing the shuffling of various clades in the LRT. Cyan = Eutheria. Red = Metatheria. Yellow = Prototheria. Gray = Trithelodontia. White are untested or basal cynodonts.

Figure 3. From Han et al. 2017, a cladogram that nests Arboroharamiya close to Xianshou and Shenshou. Colors added to showing the shuffling of various clades in the LRT. Cyan = Eutheria. Red = Metatheria. Yellow = Prototheria. Gray = Trithelodontia. White are untested or basal cynodonts. Silhouettes are gliders. The Allotheria is not recovered by the LRT.

Arboroharamiya provides an unprecedented look
at the post-dentary in taxa transitional between rodents + plesiadapiformes and multituberculates (Fig. 5). Earlier here, here and here multituberculates were shown to have pre-mammal post-dentary/ear bones, yet nested with placental and rodent taxa. This is a reversal or atavism, a neotonous development due to the backward shifting of the squamosal (another reversal) favoring the development of larger jaw muscles to power that uniquely shaped cutting tool, the lower last premolar. It has never been so clear as in Arboroharamiya, though.

Figure 4. Subset of the LRT nesting Arboroharamiya with Carpolestes within Rodentia

Figure 4. Subset of the LRT nesting Arboroharamiya with Carpolestes within Rodentia

Han et al. reported, “The lower jaws are in an occlusal position and the auditory bones are fully separated from the dentary.” In the new interpretation (Fig. 5) the neotonous articular is back in contact with the neotonous quadrate (both auditory bones in derived mammals) as the squamosal shifts posteriorly to its more primitive and neotonous position toward the back of the skull. Essentially the back of the skull in Arboroharamiya and multituberculates are embryonic relative to rodents.

Reversals
can be confusing because they are a form of convergence arising from neotony. The LRT separates convergent taxa by nesting them correctly with a wide suite of traits and testing them with a wide gamut of taxa.

Figure 3. Images from Han et al. Color and white labels added. Here the malleus, incus and stapes have reverted to their pre-mammal states and configurations. Note the quadrate is in contact with the articular, as in pre-mammals as the dentary and squamosal become a sliding joint, carried by larger jaw muscles. Also note the various ectotympanic bones (yellow) also present, typical of Theria.

Figure 3. Images from Han et al. Color and white labels added. Here the malleus, incus and stapes have reverted to their pre-mammal states and configurations. Note the quadrate is in contact with the articular, as in pre-mammals as the dentary and squamosal become a sliding joint, carried by larger jaw muscles. Also note the various ectotympanic bones (yellow) also present, typical of Theria.

When a few traits say: pre-mammal
and a suite of traits say: rodent descendant, go with the standard for phylogenetic analysis. Only maximum parsimony reveals reversals when they appear. If you relied on just the post-dentary traits here you’d be ‘Pulling a Larry Martin‘ and nesting Arboroharamiya with pre-mammals.

I didn’t think I’d have to
keep referring to the dear departed professor from Kansas, Dr. Larry Martin, but he did like to play that game. I’m encouraging others not to, whether they know they are doing so or not.

References
Han G, Mao F-Y, Bi-SD, Wang Y-Q and Meng J 2017. A Jurassic gliding euharamiyidan mammal with an ear of five auditory bones. Nature 551:451–457.

 

Evolution of multituberculates illustrated

Updated the next day, January 5, 2019 with new interpretations of the post-dentary bones in figure 3, detailed here.

With the addition of four taxa
to the large reptile tree (LRT, 1370 taxa), a review of the Bremer scores helped cement relationships in the Primates + Glires clade (Figs. 1, 2). Yesterday we looked at plesiadapiform taxa (within Glires, Fig. 2) leading to the aye-aye, Daubentonia. Today we’ll look at a sister clade within Glires, one that produced the clade Multituberculata.

The traditional, but invalid outgroup taxon,
Haramiyavia, is a pre-mammal trithelodontid not related to the rodent-and plesiadapiform- related members of the Multituberculata in the LRT. More on that hypothesis below.

In Figure 1
look for the gradual accumulation of traits in derived taxa. Carpolestes (Late Paleocene) is a late survivor from a Jurassic radiation. Paulchoffatia is Latest Jurassic. Megaconus is Middle Jurassic. Vilevolodon, Xianshou and Rugosodon are Late Jurassic. Kryptobaatar is Late Cretaceous. Ptilodus is Paleocene. So this radiation had its genesis in the Early Jurassic and some clades, like Carpolestes, had late survivors.

Figure 1. LRT taxa in the lineage of multituberculates arising from Carpolestes and Paulchoffatia.

Figure 1. LRT taxa in the lineage of multituberculates arising from Carpolestes and Paulchoffatia. Carpolestes is a sister to Ignacius. The new taxon, Arboroharamiya, nests with Xianshou in the Han et al. cladogram.

It’s worth noting
that the one key trait that highlights many multituberculates, the oddly enlarged last premolar of the dentary, is also a trait found in the basal taxon, Carpolestes, but not in Paulchoffatia, (Fig. 1). Paulchoffatia has the odd mandible (dentary) without a distinct retroarticular process common to multituberculates, convergent with Daubentonia. That there is also no distinct glenoid process (jaw joint) in clade members made these jaw bones even harder to understand. Then I realized the jaw joints were mobile, slung in place by muscles, as in rodents and primates, rather than a cylindrical dentary/squamosal joint, as in Carnivorans.

There is one more elephant in the room
that needs to be discussed. Earlier we looked at the splints of bone at the back of the jaws in multituberculates identified as posterior jaw bones (Fig. 3), a traditional pre-mammal trait. Multis move the squamosal to the back of the skull and reduce the ear bone coverings (ectotympanics) that nearly all other placentals use to cover the middle ear bones. This reversal to the pre-mammal condition is key to the traditional hypothesis shared by all mammal experts that multis are pre-mammals. Embryo primitive therians have posterior jaw bones, but these turn into tiny middle ear bones during ontogeny. In multis their retention in adults is yet another example of neotony.

Why lose/reverse those excellent placental middle ear bones?
‘Why’ questions get into the realm of speculation. With that proviso, here we go.

Figure 2. Jaw muscles of the Late Cretaceous multituberculate, Catopsbaatar.

Figure 2. Jaw muscles of the Late Cretaceous multituberculate, Catopsbaatar.

The over-development of the lower last premolar
indicates some sort of preference or adaptation for food requiring such a tooth. The coincident and neotonous migration of the squamosals to the back of the skull (the pre-mammal Sinoconodon condition) enlarged the temporal chewing muscles (Fig. 2). The neotonous lack of development of tiny middle ear bones was tied in to that posterior migration. Evidently Jurassic and Cretaceous arboreal multis did not need the hearing capabilities provided by the tiny middle ear bones of most therians, but they needed larger jaw muscles. Evidently they were safe in the trees because there were few to no arboreal predators of mammals back then. Multis and rodents had the trees to themselves. Evidently that changed in the Tertiary, when multis became extinct, perhaps because birds of prey (hawks and owls) became widespread and only rodents could hear them coming. That’s a lot of guesswork. Confirmation or refutation should follow.

Figure 3. Images from Han et al. Color and white labels added. Here the malleus, incus and stapes have reverted to their pre-mammal states and configurations. Note the quadrate is in contact with the articular, as in pre-mammals as the dentary and squamosal become a sliding joint, carried by larger jaw muscles. Also note the various ectotympanic bones (yellow) also present, typical of Theria.

Figure 3. Images from Han et al. Color and white labels added. Here the malleus, incus and stapes have reverted to their pre-mammal states and configurations. Note the quadrate is in contact with the articular, as in pre-mammals as the dentary and squamosal become a sliding joint, carried by larger jaw muscles. Also note the various ectotympanic bones (yellow) also present, typical of Theria.

A recent paper by Han et al. 2017
on the Late Jurassic pre-mulltituberculate euharamiyidan, Arboroharamiya (Fig. 3), documents precisely the status of the middle ear/posteror jaw bones along with the phylogenetic reduction of the ectotympanic that frames the ear drum and forms a thin shell around the middle ear bones in more primitive members of the clade Glires (Fig. 4, evidently there is more variation in this, and I will take a look at that in the future). Han et al. report for Arboroharamiya, “The lower jaws are in an occlusal position and the auditory bones are fully separated from the dentary.” That is the mammal condition.

The Han et al cladograms
include a rabbit and a rodent, but suffer from massive taxon exclusion. As a result they mix up prototherians, metatherians and eutherians as if shuffling a deck of cards, as compared to the LRT. My first impression is that they use too many taxa known only form dental traits when they should have deleted those until a robust tree topology was created and established with a large suite of traits from more complete taxa, as in the LRT.  I will add Arboroharamiya to the LRT shortly.

Figure 2b. Subset of the LRT focusing on Primates + Glires.

Figure 4. Subset of the LRT focusing on Primates + Glires.

Unfortunately,
and I hate to report this, mammal experts have been guilty of depending on a short or long list of traits (which can and often do converge and reverse) to identify taxa and clades. As readers know, paleontologists should only depend on a phenomic phylogenetic analysis that tests a large suite of bone characters and a wide gamut of taxa. Analysis proves time and again to be the only way to confidently identify taxa and lump’n’split clades. Cladograms, when done correctly, weed out convergence. Otherwise, reversals, like the neotonous reappearance of post-dentary bones and the reotonous disappearance of ectotympanics, can be troublesome to deal with, causing massive confusion. A phylogenetic analysis quickly and confidently identifies reversals because all possible candidates are tested at one time. 

Unfortunately,
d
iscovering this little insight is yet another reason why other workers have dismissed the LRT, have attempted to discredit the LRT, and is causing confusion in yet another upcoming class of future paleontologists. Paleo students have to choose between relying on a short list of traits or performing a phenomic phylogenetic analysis. Only the latter actually works (see below) and avoids mixing in convergent traits.

If you don’t remember
‘amphibian-like reptiles,’ those are taxa, like Gephyrostegus, Eldeceeon and Silvanerpeton, that nest at the base of all reptiles in the LRT, but have no traditional reptile traits. Everyone else considers them anamniotes. In the LRT, based solely on their last common ancestor status/nesting, these taxa are known to have evolved the amniotic membrane, the one trait, by definition, that unites all reptiles (including birds and mammals) and labels the above basal taxa, ‘amphibian-lke reptiles.’

References
Han G, Mao F-Y, Bi-SD, Wang Y-Q and Meng J 2017. A Jurassic gliding euharamiyidan mammal with an ear of five auditory bones. Nature 551:451–457.
Urban et al. (6 co-authors) 2017. A new developmental mechanism for the separation of the mammalian middle ear ossicles from the jaw. Proceedings of the Royal Society B: Biological Sciences https://doi.org/10.1098/rspb.2016.2416

The aye-aye and squirrel revisited in µCT scans

Morris, Cobbb and Cox 2018
used µCT scans to compare the traditional ‘unusual primate’ from Madagascar, Daubentonia (the aye-aye, Fig. 1) to the grey squirrel, Sciurus. In the large reptile tree (LRT) these two both nest within the larger clade Glires and the smaller clade, Rodentia, not close to Primates. Daubentonia could have nested with any one of several included primates, but did not do so, although other putative primates with rodent-like teeth, Ignacius and Plesiadapis, also nests within Rodentia in the Daubentonia clade.

Figure 1. From Morris, Cobb and Cox 2018, µCT images of Daubentonia and Sciurus.

Figure 1. From Morris, Cobb and Cox 2018, µCT images of Daubentonia and Sciurus.

The Morris, Cobb and Cox study labeled the obvious similariities
of the aye-aye and squirrel “Convergence—the independent evolution of similar phenotypes in distantly related clades”.

Unfortunately,
the Morris, Cobb and Cox cladogram, compiled from three earlier genomic studies, did not include any fossil taxa. In their study, Daubentonia nests with other taxa from the island of Madagascar, adding evidence to the curious notion that placental mammals fall into genomic clades determined by land masses, like Afrotheria.

According to Sterling and McCreless 2007 (citations deleted), “Owen’s definitive study of aye-aye anatomy (Owen, 1866) finally quelled the debate over the species’ taxonomic position, focusing attention away from the animal’s rodentlike anterior teeth and towards its primatelike characteristics, such as a postorbital bar, stereoscopic vision, and an opposable hallux. Although its placement within the primates is still being debated, Daubentonia is considered a member of the family Indridae; as a sister taxon to the other Malagasy primates; and as the most basal branch of the strepsirrhines.”

Owen 1866 had no idea
of the fossil taxa that now surround Daubentonia in the LRT, attracting it away from primates and toward fossil rodents, none of which have a postorbital bar, but all of which have an opposable hallux and rodent-like dentition. In the LRT a postorbital bar is retained in taxa basal to Glires (Ptilocercus and Tupaia), but lost in all derived taxa, except Daubentonia. A postorbital process appears in the Late Cretaceous multituberculate, Catopsbaatar. A pseudo-postorbital bar created by the anteriorly displaced squamosal appears in the clade that includes Pectinator, Chinchilla and Allactaga.

Figure 2. Lemur catta skull in 3 views.

Figure 2. Lemur catta skull in 3 views. Compare this skull to that of Daubentonia in figure 1.

A little backstory
Gmelin 1788 and Cuvier 1797 assigned Daubentonia to the Rodentia, under the genus and species: Sciurus madagascariensis. Geoffroy 1795 coined the present genus name, in honor of his professor L-J-M Daubenton. Shaw 1800 called Daubentonia a “long-fingered lemur” perhaps because it lived in the land of lemurs, Madagascar. Wikipedia provides no references for phylogenetic studies that include Daubentonia and fossil taxa nesting as sisters and near-sisters in the LRT. So… taxon exclusion once again appears to have unnecessarily created an enigma.

Oxnard 1981 reported,
“The differences that have been found are large enough that it can be confidently asserted that in its postcranial skeleton, Daubentonia is more different from the primates as a whole than is any other primate genus. paralleling the enormous differences of Daubentonia from other primates in its dentition, skull and cheiridia, that we may prefer to keep open minds about its taxonomic placement.”

No other primates
have the ever-growing rodent-like incisors that Daubentonia has. But all rodents do.

There is one family of rodents native to Madagascar,
the Nesomyinae. Genera include:

  1. Brachytarsomys
  2. Brachyuromys
  3. Eliurus
  4. Gymnuromys
  5. Hypogeomys
  6. Macrotarsomys
  7. Monticolomys
  8. Nesomys
  9. Voalavo

None of these taxa
are currently included in the LRT. None appear to be more closely related to Daubentonia that to Mus and Rattus.

Sciurus was recently added to the LRT
and, not surprisingly, it nested with Ratufa, the giant squirrel, within the clade Rodentia.

Lemur catta was also recently added to the LRT
just to be fair, because Morris, Cobb and Cox 2018 nested Daubentonia with Lemur catta. Not surprisingly Lemur nested at the base of the Notharctus clade, derived from the IVPP V 5235 specimen of Hapalodectes.

The worldwide dispersion of post-Cretaceous basal primates
actually signals a radiation preceding the splitting of Madagascar and South America from Africa in the Early Cretaceous (rather than lemurs rafting to Madagascar and New World monkey to South America), as discussed earlier here. Madagascar provided a refuge for lemurs and lemur-like rodents, like Daubentonia.

References
Cuvier G 1797. Tableau e´lementaire de l’histoire naturelle des animaux. Paris, France: Baudouin.
Gmelin JF 1788. Caroli a Linné systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima tertia, aucta, reformata. – pp. [1-12], 1-500. Lipsiae. (Beer).
Groves CP 2005. Order Primates. pp. 111–184 In Wilson, D. E.; Reeder, D. M. Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press.
Morris PJR, Cobb SNF and Cox PG 2018. Convergent evolution in the Euarchontoglires. Biology Letters 14: 20180366. http://dx.doi.org/10.1098/rsbl.2018.0366
Owen R 1866. On the aye-aye (Chiromys, Cuvier: Chiromys madagascariensis, Desm.; Sciurus madagascariensis, Gmelin, Sonnerat; Lemur psilodactylus, Schreber, Shaw). Transactions of the Zoological Society of London 5:33–101.
Oxnard CE 1981. The uniqueness of Daubentonia. American Journal of Physical Anthropology 54(1):1–21.
Sterling EJ and McCreless 2007. Adaptations in the Aye-Aye: A review. Chapter 8 (pp. 159–184) in Lemurs, Ecology and Adaptation. Gould L and Sauther ML Eds. Part of the 47 volume Developments in Primatology: Progress and Prospect book series. Springer Nature Switzerland.

PR on Morris, Cobb and Cox 2018
wiki/Aye-aye

SVP 2018: More complete material of Mixodectes

I know nothing about Mixodectes as I write this.
But based on the abstract description, I will put it into a phylogenetic perspective.

Sargis et al. 2018 report: 
“Mixodectids are eutherian mammals from the Paleocene of North America that have been considered close relatives of the extinct plagiomenids, microsyopid plesiadapiforms,and/or dermopterans, making them relevant to better understanding euarchontan relationships. We analyzed a new dentally associated skeleton of Mixodectes pungent (NMMNH P-54501). It is the most complete skeleton of a mixodectid known, preserving a partial skull with all teeth erupted and previously unknown elements of the axial skeleton, forelimbs, and hind limbs, all with epiphyses fused.”

The authors believe plesiadapiforms are basal to primates,
which is invalid based on the results in the large reptile tree (LRT, 1315 taxa). Plesiadapiforms are more closely related to carpolestids (including Daubentonia, the extant aye-aye) and multituberculates.

The NMMNH P-54501 mixodectid has

  1. Humeral traits indicating a mobile shoulder and elbow.
  2. The humerus has a large medial epicondyle and the proximal phalanges have
    pronounced flexor sheath ridges, both indicating powerful flexion of the digits.
  3. Pelvis traits as in arboreal euarchontans.
  4. The femur suggests a habitually flexed knee.
  5. The astragalus and calcaneum indicates mobility in the ankle joints and is often present in arboreal taxa capable of pedal inversion.
  6. The authors do not discuss the teeth…which are important: are they rodent-like (with large incisors as in Glires)?… or carnivore like (with canine fangs as in other primates)?

Sargis et al. conclude: “In summary, the postcranial morphology of Mixodectes is very similar to that of arboreal euarchontans, including plesiadapiforms, supporting inferences based on less complete material that mixodectids were both arboreal and members of Euarchonta.”

Euarchonta (Waddell et al. 1999) = Scandentia (tree shrews), Dermoptera (colugos), Plesiadapiformes (Plesiadapis) and Primates (lemurs to humans). Together these taxa are not monophyletic in the LRT (subset Fig. 1).

Figure 3. Subset of the LRT focusing on Glires, rodents and multituberculates.

Figure 1. Subset of the LRT focusing on Glires, rodents and multituberculates. Primates are the sister clade to the clade shown above.

References
Sargis EJ et al. (4 co-authors) 2018. Functional morphology of a remarkably complete skeleton of Mixodectes pnugens: evidence for arboreality in an enigmatic eutherian from the Early Paleocene. SVP abstracts.

SVP 2018: Rodent, plesiadapiform and multituberculate teeth similarities

You heard it here first.
Birlenbach and Fox 2018 found similarity in the rodent, multituberculate (Fig. 1) and plesiadapiform teeth. In the large reptile tree (LRT, 1038 taxa, subset Fig. 2) that is so because they are closely related to one another.

Figure 1. Animation of the mandible of the multituberculate Kryptobaatar showing the sliding of the jaw joint producing separate biting and grinding actions, just like rodents, their closest relatives in the LRT.

Figure 1. Animation of the mandible of the multituberculate Kryptobaatar showing the sliding of the jaw joint producing separate biting and grinding actions, just like rodents, their closest relatives in the LRT.

Unfortunately
Birlenback and Fox are working from older, smaller and less complete charts that tells them these three taxa are not closely related.

Figure 3. Subset of the LRT focusing on Glires, rodents and multituberculates.

Figure 2. Subset of the LRT focusing on Glires, rodents and multituberculates.

References
Birlenbach DM and Fox DL 2018. Morphological similarity in the dentition of rodents, multituberculates and plesiadapiformes during the Late Paleocene in North America.

Purgatorius: What is it?

Wikipedia reports: 
“For many years, there has been a large debate as to whether Purgatorius is a primitive member of the Primates or a basal member of the Plesiadapiforms.” Here (Fig. 1) taxa from the Plesiadapiformes have giant procumbent (rat-like) incisors followed by a long diastema, followed by flat molars…completely UNLIKE Purgatorius. So what were they thinking?

Halliday et al. 2015
nested Purgatorius outside crown group placentals with Protunugulatum (Fig 1). That seems reasonable, though it is twice the size. However, the large reptile tree (LRT, 1044 taxa) was not able to replicate most of the Halliday team’s cladogram, which nested hyraxes with elephant shrews…and horses… and that clade with pre-odontocetes and an early artiodactyl. It just gets worse after that. Protunugulatum was originally allied with condylarths, large plant-eating mammals. Halliday et al. nested it outside the placentals. Wible et al. 2007 nested it with whales + artiodactyls (a clade not validated by the LRT).

Purgatorius is another one of those fossils
known from an incompleted mandible with teeth and little else. Based on a lack of other bones, this is the sort of fossil the LRT cannot successfully resolve and it does not make it onto the list. So we go to plan #2: visual comparisons.

Figure 1. Purgatorius compared to other basal and often Paleocene mammals.

Figure 1. Purgatorius compared to other basal and often Paleocene mammals. Given these choices, Purgatorius looks more like Palaechthon, the basal dermopteran, than any other taxa in the LRT. Taxa in yellow nest together in the LRT with primates. Taxa in pink nest with rats and rabbits. Maelestes is a basal tenrec.

Rat-sized Purgatorius unio
(Valen and Sloan 1965; Latest Cretaceous/Earliest Paleocene) gained some early notoriety as the earliest known primate. Ankle bones found in association with Purgatorius, but not articulation, show signs of being flexible like those of primates (Kaplan 2012).

I can describe Purgatorius in the simplest of terms
based on comparisons to related basal mammal taxa (Fig. 1) and without describing any molar cusps (except one).

  1. small in overall size (skull < 2cm in length)
  2. robust mandible with convex dorsal and ventral rims and straight in occlusal view
  3. incisors likely procumbent, but not large
  4. canine tiny
  5. three robust premolars and three robust molars with one very tall cusp
  6. Premolar #3 taller than other teeth

Based on a visual comparison
of candidate taxa (Fig. 1), Purgatorius looks more like Protungulatum and even more like Palaechthon. The latter nests with flying lemurs like Cynocephalus. So we’re close to the base of primates, but closer to their cousins, and far from plesiadapiformes.

Best I can do for now…

References
Halliday TJD, Upchurch P and Goswami A 2015. Resolving the relationships of Paleocene placental mammals Biological Reviews. | doi = 10.1111/brv.12242
Kaplan M 2012. Primates were always tree-dwellers. Nature. doi:10.1038/nature.2012.11423
Van Valen L and Sloan R 1965. The earliest primates. Science. 150(3697): 743–745.
Wible JR, Rougier GW, Novacek MJ and Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary.” Nature volume 447: 1003-1006

wiki/Purgatorius