Deinogalerix: not a giant extinct hedgehog, but close!

Rather,
Deinogalerix (Fig. 1, 2) is a giant moonrat, (Fig. 3) according to its nesting in the large reptile tree (LRT, 1399 taxa)

Figure 1. Skull of Deinogalerix with bones colored in DGS overlay.

Figure 1. Skull of Deinogalerix with bones colored in DGS overlay. Note the separation of the prefrontal and lacrimal along with the large size of the premolars relative to the small molars.

Deinogalerix koenigswaldi  (Freudenthal 1972; Villiera et al. 2013; Late Miocene 10-5mya; skull length 20cm, snout-vent length 60cm) is the extinct giant moon rat (not hedgehog), restricted to a Mediterranean island, now part of a peninsula. Giant premolars and tiny molars make the dentition unusual. Seven species have been identified.

Figure 2. Deinogalerix skeleton.

Figure 2. Deinogalerix skeleton. Snout to vent length = 60cm.

Echinosorex gymnura (Blainville 1838; length to vent up to 40cm, tail up to 30cm, Fig. 3) is the extant moonrat, or gymnure, an omnivore that looks like an opossum or rat. Here it nests with Pholidocercus, a Messel pit armadillo-mimic we looked at earlier here. Distinct from most Glires, the canines are large.

Figure 3. Echinosorex, the extant moonrat, looks like an opossum, but nests with Deinogalerix in the large reptile tree.

Figure 3. Echinosorex, the extant moonrat, looks like an opossum, but nests with Deinogalerix in the large reptile tree.

References
Freudenthal M 1972. Deinogalerix koenigswaldi nov. gen., nov. spec., a giant insectivore from the Neogene of Italy. Scripta Geologica. 14: 1–19.
Villiera B, Van Den Hoek Ostendeb L, De Vosb J and Paviaa M 2013. New discoveries on the giant hedgehog Deinogalerix from the Miocene of Gargano (Apulia, Italy). Geobios. 46 (1–2): 63–75.

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Taxon exclusion mars Mesozoic mammal study

King and Beck 2019
bring us a new phylogenetic analysis restricted to Mesozoic mammals. This represents a massive case of taxon exclusion of basal mammals as demonstrated earlier here, because so many basal mammals are still alive! Think of all the tree shrews, arboreal didelphids, and nearly every little creeping taxon in Glires that nest basal to known Mesozoic mammals. You cannot restrict the taxon list to just those extremely rare Mesozoic mammals.

On the plus side,
King and Beck confirm earlier tree topologies recovered by the large reptile tree (LRT, 1394 taxa) that nest haramiyidans apart from euharamiyidans when they reported, “Tip-dating applied to Mesozoic mammals firmly rejects a monophyletic Allotheria, and strongly supports diphyly of haramiyidans, with the late Triassic Haramiyavia and Thomasia forming a clade with tritylodontids, which is distant from the middle Jurassic euharamiyidans.”

Taxon exclusion
made the authors nest Vintana with euharamiyidans rather than the wombats recovered by the LRT. Other than Vintana, no wombats were included in the King and Beck study—because they are alive. Furthermore, no rodents and aye-ayes were included—because they are alive. So, sans living taxa, King and Beck’s cladogram had no idea where to nest euharamiyidans in the mammal family tree. The wide gamut taxon list of the LRT solves most problems that arise from restricted studies like the King and Beck study.

Figure 4. Mesozoic euthrerians (placentals, in black). Later taxa in light gray. All taxa more primitive than Mesozoic taxa were likely also present in the Jurassic and Cretaceous. None appear after Onychodectes. Madagascar separated from Africa 165-135 mya, deep into the Cretaceous with a population of tenrecs attached. No rafting was necessary. 

Figure 1. Mesozoic euthrerians (placentals, in black). Later taxa in light gray. All taxa more primitive than Mesozoic taxa were likely also present in the Jurassic and Cretaceous. None appear after Onychodectes. Look at all the taxa King and Beck could have added to their analysis!  …and we’re not even looking at the Metatheria and Prototheria here.

More on Mesozoic mammals
here.

References
King B and Beck R 2019. Bayesian Tip-dated Phylogenetics: Topological Effects,
2 Stratigraphic Fit and the Early Evolution of Mammals. PeerJ
doi: http://dx.doi.org/10.1101/533885.

Pholidocercus: a long tailed armadillo-mimic hedgehog

Reversals in this taxon make it interesting.
Pholidocercus hassiacus (Fig. 1; von Koenigswald & Storch 1983; HLMD Me 7577; Middle Eocene) is a member of the rabbit/rodent/multituberculate clade Glires, but without the large anterior incisors that are found in most other members. This is a reversal hearkening back to basal placentals.

Figure 1. Only one of the several Messel Pit Pholidocercus specimens. This one has a truncated tail and a halo of soft tissue (pre-spines).

Figure 1. Only one of the several Messel Pit Pholidocercus specimens. This one has a truncated tail and a halo of soft tissue (pre-spines).

Three upper molars are present,
as in primates, and basal members of Glires, like Ptilocercus, the tree shrew. Other hedgehogs have only two upper molars.

Four upper premolars are present,
one more than in basal placentals and other hedgehogs.

Other hedgehogs have a stub for a tail.
Yet another reversal, Pholidocercus has a long, tail. It is bony and armored,  analogous to that of an armadillo (genus: Dasypus). Sister hedgehogs have just a stub for a tail. The curling of all hedgehogs for defense also recalls the spinal flexion of armadillos for defense. This is a trait basal therian mothers originally used to help guide their newborns from birth canal to teat.

Figure 2. Pholidocercus skull with DGS colors added. Distinct from most members of the Glires, the canine becomes more robust in the hedgehog clade. Note the posterior jaw joint, the opposite of mouse-like rodents.

Figure 2. Pholidocercus skull with DGS colors added. Distinct from most members of the Glires, the canine becomes more robust in the hedgehog clade. Note the posterior jaw joint, the opposite of mouse-like rodents. The short jugal is typical of this clade. No elongate dentary incisors here, yet another reversal to a basal placental condition.

Those sacral neural spines
(Fig. 1) are taller than in sister taxa. Armadillos also have tall sacral spines.

The clade Lipotyphyla, according to Wikipedia
“is a formerly used order of mammals, including the members of the order Eulipotyphla as well as two other families of the former order Insectivora, Chrysochloridae and Tenrecidae. However, molecular studies found the golden moles and tenrecs to be unrelated to the others.” 

The clade Eulipotyphyla, according to Wikipedia
“comprises the hedgehogs and gymnures (family Erinaceidae, formerly also the order Erinaceomorpha), solenodons (family Solenodontidae), the desmansmoles, and shrew-like moles (family Talpidae) and true shrews (family Soricidae).”

The clade Erinaceidae, according to Wikipedia
“Erinaceidae contains the well-known hedgehogs (subfamily Erinaceinae) of Eurasia and Africa and the gymnures or moonrats (subfamily Galericinae) of South-east Asia.”

The LRT largely confirms this clade,
but moles (genus: Talpa) nest separately in the clade Carnivora with the mongoose, Herpestes.

When you come across a taxon like Pholidocercus
first you eyeball it and declare it a… a… well, there are so many reversals here that it is best to avoid pulling a Larry Martin and just add it to a wide gamut phylogenetic analysis to let a large suite of traits decide for themselves based on maximum parsimony. Luckily the LRT had enough taxa to nest Pholidocercus with confidence with the hedgehogs, despite the several distinguishing traits and reversals.

Just added to the LRT:
Echinosorex, the extant moonrat. It also has a long tail and nests with Pholidocercus.

References
von Koenigswald W and Storch Gh 1983. Pholidocercus hassiacus, ein Amphilermuride aus dem Eozan der “Grube Messel” bei Darmstadt (Mammalia: Lipotyphla). Senchenberg Lethaia 64:447–459.

wiki/Hedgehogs
wiki/Erinaceus
wiki/Echinops
wiki/Pholidocercus

The Miacis-Mustela split within Carnivora

Nesting as the proximal outgroup to all placentals in the LRT
is the slinky, omnivorous, often inverted marsupial didelphid, Caluromys. So that’s the morphology we start with. In the large reptile tree (LRT, 1376 taxa; subset in Fig. 2) Carnivora is the first placental clade to split off. Earlier we looked at the similarity in skulls between Caluromys and didelphid-like basalmost Carnivora, Volitantia, Primates, Glires and Maelestes (at the base of the tenrec-odontocete clade).

Always seeking ‘a gradual accumulation of derived traits’,
basal Carnivora, like civets (e.g. Nandinia) in the LRT are likewise slinky, omnivorous and often inverted. That starts to fade away with the raccoons, Procyon and Ailurus and later evolves to hypercarnivory in the extant mongoose (Herpestes). Based on the appearance of the mongoose sister, Cryptoprocta in Madagascar (135 mya) along with the Paleocene appearance of derived Carnivora, like Miacis and Palaeosinopa, basal Carnivora had their genesis early in the Mesozoic.

Gray 1821 defined Vivveridae
as consisting of the genera ViverraGenettaHerpestes, and Suricata. All tested taxa are basal members of the Carnivora in the LRT (subset in Fig. 2), so this clade is paraphyletic. Bowdich 1821 defined the clade Carnivora as it is used today.

Most derived Carnivora forsake their veggies
as they become highly specialized for predation. Two clades diverge from a common mongoose-like ancestor: one from a sister to Late Paleocene Miacis (Fig. 1), the other from a sister to Mustela, the European mink. Both are small, long-torsoed and short-legged still resembling the placental outgroup taxon, Caluromys.

Figure 1. Mustela and Miacis (the mink/weasel) compared to scale.

Figure 1. Mustela and Miacis (the mink/weasel) compared to scale.

In the LRT
Miacis is basal to the clade of sea lions, dogs, cats, hyaenas and kin (Fig. 2).

Miacis parvivorus (Cope 1872; Heinrich et al. 2008; 30cm in length; Late Paleocene-Late Eocene) was originally considered a pre-carnivore, but here nests as a derived member of the Carnivora, arising from a Mesozoic sister to Herpestes, the mongoose. It was a sister to Mustela and Hyopsodus in the LRT. Miacis had a full arcade of 11 teeth (x4), but the canines and carnassial were smaller. Miacis had retractable claws, like a cat, and was likely arboreal.

In the LRT
extant Mustela is basal to the clade of wolverines, bears, seals and kin (Fig. 2).

Mustela lutreola (Linneaus 1761; extant European mink; up to 43cm in length) is a fast and agile animal related to weasels and polecats. Mustela lives in a burrow. It swims and dives skilfully. It is able to run along stream beds and stay underwater for one to two minutes. Mustela is basal to PuijilaUrsus and other bears, Phoca and other seals.

Importantly 
note the relatively close affinity of dogs (Canis) and cats (Panthera), in the LRT. That becomes a factor in a genomic study below.

Figure 3. Subset of the LRT focusing on Carnivora, the basalmost eutherian clade. Talpa is the European mole. Shrews and shrew-moles nest within the clade Glires.

Figure 2. Subset of the LRT focusing on Carnivora, the basalmost eutherian clade. The two derived clades arising from Mustela and Miacis are shown here.

 

 

How does the clade Carnivora look to traditional paleontologists?
Flynn et al.2005 (Fig. 3) attempted to “assess the impact of increased sampling on resolving enigmatic relationships within the placental clade, Carnivora, by using genomic testing” (so no fossils there). In Flynn et al. the extant Carnivora have their first dichotomy splitting cats from dogs (which are also closely related in the LRT, Fig. 2). No outgroup appears in the Flynn et al. cladogram, which mixes primitive and derived taxa, relative to the LRT. Note, seals + sea lions are monophyletic when fossils are not included. Minks are highly derived here, the opposite of the topology in the LRT. So some relationships are simply inverted, which sometimes happens when the outgroup is not correctly defined.

Figure 4. Carnivora according to Flynn et al. 2005 based on genomic testing.

Figure 3. Carnivora according to Flynn et al. 2005 based on genomic testing. Cryptoprocota is a ‘Malagasy carnivore.”

Once again,
genomic testing does not replicate phenomic testing in deep time. That’s why the LRT is here. So you can test traits vs. genes, always seeking ‘a gradual accumulation of traits’ that echoes or models evolutionary events, without relying on the hope and faith that must come from any analysis that omits fossil taxa. The LRT also provides a list of outgroup taxa back to Devonian tetrapods.

Based on a trait list,
or a photo (Fig. 1), it is easy to see that Miacis and Mustela are closely related. However, in phylogenetic analysis each of these sisters nest at the base of a different derived clade of Carnivora. Cats and dogs remain closely related in the LRT, but both are highly derived relative to the outgroup and basal taxa. The LRT reveals cats are convergent with basal Carnivora, like the cat-like civets.

References
Bowdich TE 1821. An analysis of the natural classifications of Mammalia, for the use of students and travelers. 115 pp.
Cope ED 1872. Third account of new vertebrata from the Bridger Eocene of Wyoming Territory. Proceedings of the American Philosophical Society 12(86): 469-472.
Flynn JJ, Finarelli JA, Zehr S, Hsu J, Nedbal MA 2005. Molecular phylogeny of the Carnivora (Mammalia): Assessing the impact of increased sampling on resolving enigmatic relationships. Systematic Biology. 54 (2): 317–37.
Heinrich RE, Strait SG and Houde P 2008. Earliest Eocene Miacidae (Mammalia: Carnivora) from northwestern Wyoming. Journal of Paleontology. 82 (1): 154–162.
Linneaus C von 1761. Fauna Suecica sistens Animalia Sueciae Regni: Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes. Distributa per Classes, Ordines, Genera, Species, cum Differentiis Specierum, Synonymis Auctorum, Nominibus Incolarum, Locis Natalium, Descriptionibus insectorum. Editio altera, auctior. Stockholmiae: L. Salvii, 48 + 578 pp.,

wiki/Mustela
wiki/Miacis
wiki/Hyopsodus
wiki/Carnivora

Enigmatic oreodont, Merycoidodon, joins the LRT

Something of an enigma.
Wikipedia reports, “Merycoidodon is an extinct genus of terrestrial herbivore.” That’s rather vague for a common sheep-sized fossil from the USA.

Figure 1. Merycoidodon reconstruction traced by an unknown artist from an AMNH mount photo.

Figure 1. Merycoidodon reconstruction traced by an unknown artist from an AMNH mount photo.

In Late Eocene
to Late Oligocene (38–16mya) deposits, Merycoidodon (Leidy 1848) lived in large herds, principally in South Dakota. but also found from Alberta to Florida, typically preferring well-watered areas. Leidy considered it a member of the ruminantoid Pachydermata‘.

Figure 2. Merycoidodon skull. Colors added.

Figure 2. Merycoidodon skull. Colors added.

Merycoidodontoidea
Wikipedia reports, “Merycoidodontoidea, sometimes called “oreodonts,” or “ruminating hogs”, is an extinct superfamily of prehistoric cud-chewing artiodactyls with short faces and fang-like canine teeth. As their name implies, some of the better known forms were generally hog-like, and the group has traditionally been placed within the Suina (pigs, peccaries and their ancestors), though some recent work suggests they may have been more closely related to camels.” Evidently the phylogenetic nesting of Merycoidodon is not clear to the Wikipedia writers. That may be due to its generalize appearance.

Spaulding et al. 2009
nested Merycoidodon ancestral to Camelus + Lama, derived from Hyracotherium and Cainotherium, among tested taxa. The Spaulding et al. cladogram separated hippos from mesonychids, nesting hippos with Diacodexis (largely incomplete) and Indohyus, an omitted tenrec in the LRT.

Figure 3. the Merycoidodon cladogram includes hippos, whales and a number of extinct taxa.

Figure 3. the Merycoidodon cladogram includes hippos, whales and a number of extinct taxa.

In the large reptile tree
(LRT, 1376 taxa) Merycoidodon nests firmly as the proximal outgroup at the base of the Mesonyx to mysticete (baleen whale) clade (subset Fig. 3). Merycoidodon also nests between the Phenacodus clade and the Homalodotherium clade + artiodactyl clades.

It is worth noting again
that hippos do not nest with artiodactyls in the LRT, breaking a traditional paradigm.

Figure 1. Mesonyx, the first known mesonychid was a sister to Hippopotamus in the large reptile tree. So maybe it was a plant eater.

Figure 5 Mesonyx nests between oredonts, like Merycoidodon, and hippos, like Hippopotamus.

I’ve been curious about oreodonts for decades.
What were they? Happy to finally test it and nest it where it belongs, basal to hippos, and transitional to modern hoofed ruminants. The generalized appearance of Merycoidodon is appropriate to its basal and transitional nesting. Based on its nesting basal to Ocepeia (middle Paleocene), the genesis of Merycoidodon must extend to the early Paleocene, if not before.

Figure 3. Hippopotamus. This stout, wide-faced, fanged mammal does not nest with deer.

Figure 6. Hippopotamus. This stout, wide-faced, fanged mammal does not nest with deer,but with Mesonyx.

References
Leidy 1848. On a new fossil genus and species of ruminantoid Pachydermata: Merycoidodon culbertsonii. Proceedings of the Academy of Natural Sciences of Philedelphia Vol IV, 47-51.

Merycoidodontidae (Thorpe 1923)
Mesonychidae (Cope 1880)

wiki/Merycoidodontoidea
wiki/Merycoidodon

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 5. 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