Two Cretaceous mammals join the LRT: Asioryctes and Repenomamus

As in traditional reptiles
I’m seeking basal relationships of basal taxa in mammals. For those you have to go to the Middle and Early Cretaceous, not the Paleocene.

Asioryctes (mid-Cretaceous, 85 mya, Kielen-Jaworowska 1975, 1984, Fig. 1) was a basal eutherian mammal, not far from Eomaia (Ji et al. 2002), the basalmost eutherian. In the large reptile tree (LRT, subset Fig. 5) Asioryctes nested at the base of Tupaia + Glires (including Multituberculates). It retained most of the original arcade of teeth, but with smaller canines. The overbite is a trait of this clade.

Fig. 1. Asiorryctes skull on the tip of someone's finger. Overlays include a traditional tracing that does not have the same proportions, a stretched out version of the same and a DGS tracing from which data was scored.

Fig. 1. Asiorryctes skull on the tip of someone’s finger. Overlays include a traditional tracing that does not have the same proportions, a stretched out version of the same and a DGS tracing from which data was scored. Note the reduction in the canines (yellow), which are lost in this herbivorous clade.

Figure 2. Asioryctes in life. Post crania is imagined.

Figure 2. Asioryctes in life. Post crania is imagined. Image out of “From the Beginning – the Story of Human Evolution”.

The second Cretaceous mammal
is the predator, Repenomamus (Li et al. 2000, Hu et al. 2005; Early Cretaceous, Figs. 3, 4). One specimen of R. robustus was the size of the opossum, Didelphis, and included digested baby dinosaur bones, R. giganticus was half again as large, one of the largest known mammals of the dinosaur era. Despite its predatory nature, note the reduction in the canine teeth, a clade trait that is reversed in most primates.

Figure 3. Repenomamus in situ with bones colorized digitally (DGS) and used in the reconstruction in figure 4. This eutherian retained epipubes (prepubes) that living metatherians retain.

Figure 3. Repenomamus giganticus in situ with bones colorized digitally (DGS) and used in the reconstruction in figure 4. This eutherian retained epipubes (prepubes) that living metatherians retain. The manus and pes were reported missing here, but parts may be present. The orange parallel bones in the rib cage could be metatarsals.

Repenomamus
is traditionally considered a non-therian gobiconodontid mammal, but it nests between the bat clade and the dermopteran + primate clade in the large reptile tree (Fig. 5) where several eutherians retain epipubic bones. The clavicle is robust here, as in sister taxa.

Figure 4. Repenomamus reconstructed using DGS methods. The manus and feet are loose figments at present. Despite its predatory nature, note the reduction in canines, a clade trait.

Figure 4. Repenomamus reconstructed using DGS methods. The manus and feet are loose figments at present. Despite its predatory nature, note the reduction in canines, a clade trait.

Figure 5. Latest mammal subset of the large reptile tree with Asioryctes and Repenomamus added.

Figure 5. Latest mammal subset of the large reptile tree with Asioryctes and Repenomamus added.

You have to go with
phylogeny first, and then identify your clade traits, rather than the other way around. And be prepared for change, which has always been the nature of Science.

Chronology and Phylogeny
The nesting of Repenomamus indicates that certain mammal clades had already diverged prior to its appearance.

Yixian Formation, Early Cretaceous, 125 mya yields Eomaia and Repenomamus from this list. Other Early Cretaceous mammals include:

  1. Astroconodon – 2mm triconodont molars, likely a predator
  2. Jugulator – 5mm triconodont molars, likely a predator
  3. Pinheirodontidae – clade of multituberculates
  4. Spinolestes – 24 cm long triconodont teeth, skeleton and fur along with xenarthrous vertebrae.
  5. Vincelestes – a 4 cm  skull with large canines described as a stem-therian, I’ll test it next to see how it nests.

The presence of
Early Cretaceous multituberculates implies the presence of more primitive coeval stem rodents and rabbits and perhaps stem tenrecs.

References
Hu Y, Meng J, Wang Y-Q and Li C-K 2005. Large Mesozoic mammals fed on young dinosaurs. Nature 433:149-152.
Li J-L., Wang Y.Wang Y-Q and Li C.-K. 2000. A new family of primitive mammal from the Mesozoic of western Liaoning, China [in Chinese]. Chin. Sci. Bull. 45, 2545–2549).
Ji et al 2002. The earliest known eutherian mammal, Nature 416:816-822.
Kielan-Jaworowska Z 1975. Preliminary description of two new eutherian genera from the Late Cretaceous of Mongolia. Palaeontologia Polonica 33:5-15.
Kielan-Jaworowska, Z 1984. Evolution of the therian mammals in the Late Cretaceous of Asia. Part VII. Synopsis. Palaeontologia Polonica 4: 173-183. online pdf

wiki/Asioryctes
wiki/Eomaia

Tenrecs and Echolocation

Just a short note before the weekend…
Earller here, here and here we looked at the traditionally overlooked morphological traits that link little tenrecs to larger whales.

Well, here’s one more
that I just became reacquainted with. Tenrecs, like whales, hunt by echolocation. I think we all heard that somewhere in our past. I did, but forgot it in the last fifty years. Here’s the reference:

References
Gould E 1965. Evidence for Echolocation in the Tenrecidae of Madagascar
Proceedings of the American Philosophical Society 109 (6): 352-360. online here.

Let’s talk about mammal interrelationships

Now that a wide gamut of mammals
has been added to the large reptile tree (LRT, subset Fig. 1), the tree topology has become distinct from prior studies, many of which depend on DNA, which does well in many cases, but makes untenable sisters of several genera.

Figure 1. Mammal subset of the large reptile tree with clades named.

Figure 1. Mammal subset of the large reptile tree with clades named.

Case in point: Macroscelidea
(elephant shrews, Fig. 2). Stanhope et al. (1998) proposed the clade Afrotheria based on molecular evidence. Their clade members included elephants and elephant shrews. That’s difficult to accept on the face of it, and the large reptile tree does not recover that relationship.

Figure 2. Macroscelides proboscideus, the elephant shrew or sengis is NOT more closely related to elephants with the purported 'Afrotheria.' but instead is related to Tupaia, the tree shrew.

Figure 2. Macroscelides proboscideus, the elephant shrew or sengis is NOT more closely related to elephants with the purported ‘Afrotheria.’ but instead is related to Tupaia, the tree shrew.

Rather
the large reptile tree recovers:

  1. Monotremata (Ornthorhynchus) as the most basal mammal clade.
  2. Didelphis (opossum) is basal to both Metatheria (so far only three genera) and Eutheria.
  3. The first eutherian split occurs between small carnivores and smaller insectivores
  4. Carnivora also splits into small insectivores: Chiroptera + (Dermoptera + Primates, including Manis)
  5. The two tree shrews, Tupaia and Ptilocercus, are not sister taxa.
  6. The former clade Insectivora is resurrected. It includes Tupaia + elephant shrews, Trogosus + Apatemys and Glires.
  7. Glires includes the traditional rabbits and rodents, but also shrews, moles and multituberculates
  8. Condylartha is resurrected and is the sister to Insectivora.
  9. Condylartha splits between the taeniodont, Onychodectes and the pantodonts, Pantolambda + Ectoconus.
  10. Onychodectes gives rise to tenrecs, which give rise to giant tenrecs and whales
  11. Pantolambda + Ectoconus give rise to Xenarthra (sloths, anteaters), Paenungulata (elephants and kin) and Ungulata (hoover mammals).

Not only do these relationships make more sense
on their face, they provide a gradual accumulation of derived characters down to the toes. You can’t do that with ‘Afrotheria’ and other odd-bedfellow sisters that have become widely accepted within paleontology, despite the fact that they make no sense at several nodes. At other nodes, some DNA clades do match morph studies.

Once again,
we need to look at the results, put our thinking caps on, and toss out DNA results that do not make sense and cannot be supported with morphological studies.

Don’t take my word for it.
I’m reporting results, like Galieo looking through a telescope for the first time or dropping balls off the leaning tower of Pisa. Because this is Science, you can repeat the experiment and discover the mammal family tree for yourself. If you do, let us all know what you recover.

References
Stanhope MJ, Waddell VG, Madsen O, de Jong W, Hedges SB. Cleven GC, Kao D and Springer MS 1998. Molecular evidence for multiple origins of Insectivora and for a new order of endemic African insectivore mammals. Proceedings of the National Academy of Sciences 95 (17): 9967–9972. Bibcode:1998PNAS…95.9967S. doi:10.1073/pnas.95.17.9967. PMC 21445. PMID 9707584.

The human occiput and palate

We looked at the facial portion
of the human skull earlier. Today we’ll look at the occiput and palate (Fig. 1).

Figure 1. Human occiput and palate. On most tetrapods these two are usually set at right angles to each other, but an upright stance has rotated the occiput to a ventral orientation.

Figure 1. Human occiput and palate. On most tetrapods these two are usually set at right angles to each other, but an upright stance has rotated the occiput to a ventral orientation.

There’s nothing new here. 
This is just an opportunity to educate myself on the human palate and occiput. Only the endotympanic (En) is a novel ossification. The occiput is a single bone here, the product of the fusion of several occipital bones. Can you find the suborbital fenestra? It’s pretty small here.

The asymmetry is interesting here.
Sure, this is an old adult, missing some teeth, but you’ll see other examples elsewhere.

Let me know
if you see any errors and they will be corrected. As you already know, everything I present here was learned only 48 hours earlier — or less.

The vague persistence of ‘absent’ bones in the human skull

Humans
(genus: Homo) are, of course mammals, and basal mammals lose the postorbital bar found in cynodonts like Chiniquodon. But even that postorbital bar does not include two bones found in more basal therapsids, the postfrontal and postorbital. In Chiniquodon the postorbital bar is created by a process of the frontal meeting a process of the jugal.

So I was surprised to find
what appear to be vague apparitions that look like those lost bones in the frontal of the human skull. There are 17 frames in this animation (Fig. 1). The last one holds for five seconds and reveals where I think I see vague outlines of the the prefrontal, postfrontal and postorbital bones all fused to the frontals, which are themselves fused medially.

Figure 1. Labeled skull bones in Homo sapiens. The last frame appears to identify the lost prefrontal, postfrontal and postorbital bones last seen in our ancestors from the Permian.

Figure 1. Labeled skull bones in Homo sapiens. The last frame appears to identify the lost prefrontal, postfrontal and postorbital bones last seen in our ancestors from the Permian. Note the dislocated jaw articulation.

The last part of the postorbitals

to ‘disappear’ are the posterior processes, which seem to laminate to the cranial bones in several therapsids, including dinocephalians. But there is also a vague portion that appears on the duckbill platypus, Ornithorhynchus.

The prefrontals
were the last to fuse and so appear to be the easiest to see now. Like gills and a tail, humans retain the genes for these, but they get resorbed or fused during embryonic ontogeny.

There is a difference between losing a bone
and fusing a bone. Phylogenetically losing a bone is usually marked by a reduction to disappearance. Fusion is, well, fusion. And that can be more readily reversed.

The pterygoid and lateral sphenoid
are both bones typically seen in palatal view, but in mammals and humans they rise, tent-like, to appear in the nasal and orbital cavities.

In mammals the bones sometimes get new names
but they’re still the same bones. The jugal becomes the zygomatic arch, for instance. The bones that make up the occiput fuse to form the occipital.

We’ll look at the palate and occiput soon. 
And a fetus.

 

Origin of turtles linked to digging? That’s not the problem here.

From the Lyson et al. 2016 highlights:
“Recently discovered stem turtles [in this case Eunotosaurus] indicate the shell did not evolve for protection. Adaptation related to digging was the initial impetus in the origin of the shell. Digging adaptations facilitated the movement of turtles into aquatic environments. Fossoriality likely helped stem turtles survive the Permian/Triassic extinction.”

From the Lyson et al. summary:
“Developmental and fossil data indicate that one of the first steps toward the shelled body plan was broadening of the ribs (approximately 50 my before the completed shell.” 

Figure 1. The origin and evolution of turtles. Here Meiolania and Niolamia nest as the most basal hard shell turtles. Odontochelys is the most basal soft shell turtle.

Figure 1. The origin and evolution of turtles. Here Meiolania and Niolamia nest as the most basal hard shell turtles. Odontochelys is the most basal soft shell turtle.

Unfortunately
Lyson et al. have been beating this dead horse [or, dead turtle] for several years. Eunotosaurus is not a stem turtle, but they keep trotting it out. Currently it is the last of its kind and is most closely related to Acleistorhinus.

The real stem turtles
(Fig. 1) are small pareiasaurs, Elginia and Sclerosaurus according to the LRT, which tested all currently proposed candidates, including Eunotosaurus. It is important to have the right phylogeny or you’re sunk in a morass of fiction and faith that will never make sense.

Lyson et al. are unaware
that living turtles had dual origins among small pareiasaurs. Among domed hard-shell turtles, tabular and supratemporal horns were the first distinct traits, along with a reduced size. We don’t have post-crania for Elginia, but that’s where and when the high-domed shell first appeared if not slightly earlier or slightly later. Sister taxa, including the late surviving Meiolania, are already full shelled and with a long armored tail.

Among low-domed soft-shell turtles, once again tabular and supratemporal horns were the first steps. A pattern of ossified scutes preceded broader ribs in Sclelrosaurus.

Protection from Permian predators
was likely the raison d’être for the origin of turtle shells among phylogenetically miniaturized pareiasaurs. Large pareiasaurs were slow and otherwise defenseless so they depended on their bulk. Smaller forms survived with greater armor, both over the torso as a carapace that also covered the limbs and with greater armor over the head and tail with horns, clubs and spikes. Fossoriality and aquatic submergence are additional strategies for defense. To that end, it is easier for small tetrapods to hide whether underground or underwater. Moreover, a low metabolism helped basal turtles survive without food for long periods.

But what about Eunotosaurus?
Unfortunately the sisters of Eunotosaurus are known chiefly from skulls. The closest taxa with post-crania are the caseasaurs, including Oedaleops, and Milleretta, which has slightly expanded ribs.

Figure 1. Diadectes phaseolinus showing off those very broad anterior dorsal rib costal plates, by convergence similar in shape to what is found in the pre-pareiasaur/pre-turtle Stephanospondylus and Odontochelys. Diadectids and pareiasaurs grew large by convergence and are not directly related except through tiny ancestral taxa.

Figure 2. Diadectes phaseolinus showing off those very broad anterior dorsal rib costal plates, by convergence similar in shape to what is found in the pre-pareiasaur/pre-turtle Stephanospondylus and Odontochelys. Diadectids and pareiasaurs grew large by convergence and are not directly related except through tiny ancestral taxa.

At the same time
Some Diadectes specimens and Stephanospondylus were developing greatly expanded ribs beneath their pectoral girdles. I don’t know if Lyson et al. 2016 discuss these taxa, but in the past they have not done so.

References
Lyson et al. (8 co-authors) 2016. Fossorial Origin of the Turtle Shell.
Current Biology (advance online publication)
DOI: http://dx.doi.org/10.1016/j.cub.2016.05.020
http://www.cell.com/current-biology/fulltext/S0960-9822(16)30478-X

Haramiyidans and Multituberculates are rodents, not pre-mammals.

Wikipedia reports
Haramiyidans have been known since the 1840s, but only from fossilized teeth and a single partial lower jaw. However, several features of the teeth have shown for many years that haramiyidans are among the most basal of mammaliaforms. Megaconus (Middle Jurassic, Zhou et al. 2013, Fig. 1) is a member.”

Wikipedia also reports
Multituberculata is is an extinct taxon of rodent-like mammals. At least 200 species are known, ranging from mouse-sized to beaver-sized. Multituberculates are usually placed outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata—but closer to Theria than to monotremes. The oldest known species in the group is Rugosodon from the Jurassic.” 

Figure 1. Megaconus in situ. Original tracing and DGS color tracing which appears to show that both hind limbs and the vernal pelvis have been displaced posteriorly -- unless their is a counter plate that preserves skeletal parts that don't appear to be present here.

Figure 1. Megaconus in situ. Original tracing and DGS color tracing which appears to show that both hind limbs and the vernal pelvis have been displaced posteriorly — unless their is a counter plate that preserves skeletal parts that don’t appear to be present here.

Recently added taxa
to the LRT (749 taxa) include the purported haramiyid allothere mammaliaform. Megaconus mammaliaformis (Zhou et al., 2013) the mutituberculates Rugosodon (Yuan et al. 2013) and Kryptobaatar (Kielan-Jaworowska  1970). Unfortunately most of the other known haramiyid allothere mammaliaformes are known from too few traits to test in the LRT. So far as I know, only Kryptobataar (Fig. 8), Rugosodon (Fig. 9) and Megaconus (Fig. 1) are known from complete skeletons. There may be others, but these three are enough to test the nesting. In the LRT they nest together with Rattus (the rat. Fig. 3).

Figure 1. Mammals include rodents. Haramiyadens and multituberculates nest with rodents.

Figure 2. Mammals include rodents. Haramiyidans and multituberculates nest with rodents. Click to enlarge.

By contrast…
Zhou et al. 2013 report: “Here we describe a new fossil from the Middle Jurassic that has a mandibular middle ear, a gradational transition of thoracolumbar vertebrae and primitive ankle features, but highly derived molars with a high crown and multiple roots that are partially fused. The upper molars have longitudinal cusp rows that occlude alternately with those of the lower molars.” The first three traits put Megaconus among the pre-mammal cynodonts. The last three traits are specialiizations. The broader traits employed by the LRT put Megaconus in the rodent clade. Rattus (the rat) and Oryctlagus (the rabbit) were included taxa in both studies.

So now we have a phylogenetic problem.
Do Megaconus and Rugosodon nest more primitively than monotremes? According to Zhou et al. they do. The Zhou team employed more taxa, more traits and more dental traits — by far.

Figure 5. Megaconus soul with original outline tracing. Note they missed lots of detail, but marked the tiny angular (green triangle). In DGS tracing I don't see what Zhou et al. saw.

Figure 3. Megaconus soul with original outline tracing. Note they missed lots of detail, but marked the tiny angular (green triangle). In DGS tracing I don’t see what Zhou et al. saw.

Unfortunately,
Megaconus otherwise looks so much like a rodent that it has been given the nickname, ‘the Jurassic squirrel.’ The LRT tests only the relatively easy traits to see, not dental details. In the LRT, shifting Megaconus and Rugosodon to Juramaia adds 37 steps. That’s a big hump to get over. I do not know how many additional steps would be added by shifting Megaconus to Rattus in the Zhou et al. study.

Figure 3. Megaconus mandible showing cynodont-like posterior mandible bones, not tiny mammal-like ear bones.

Figure 4. Megaconus mandible showing cynodont-like posterior mandible bones, not tiny mammal-like ear bones. Unfortunately this key trait cannot be confirmed with present photo resolution. Mammals and reptiles call the same bones different names in some cases and some of these are labeled here.

If the Zhou et al. team is correct
then we have a problem. If the Zhou et al. team is not correct, they have a problem. They have identified an angular/ectotympanic where there is none. Rugosodon and Kryptobataar likewise do not have pre-mammal-type posterior jaw bones prior to their evolution into tiny ear bones.

Figure 3. Skull of Rattus, the rat. Note the similarities to Megaconus. Not identical but similar.

Figure 5. Brown Rat (Rattus norvegicus) skull showing how lower incisors are used to scrape away and sharpen upper incisors The ear bones are located inside the circular ectotympanic posterior to the mandible and below the skull.

How can we reconcile this problem? 

  1. If Megaconus indeed nests with Rattus, then the ankle, posterior jaw and other traits may represent reversals to a more primitive state. 
  2. If Megaconus is indeed primitive, then it anticipates and converges on a long list of traits with Rattus under the LRT, Given that living monotremes have a long list of special traits, it is not unreasonable to accept that Megaconus diid likewise. The only caveat to that hypothesis is that monotreme special traits are not shared with or converge with those of other mammals.
  3. If Megaconus parts have been misidentified, then (no exceptions) all other traits indicate it is a rodent sister.
Figure 4. Haramiyava dentary showing what a more typical stem mammal dentary and teeth look like. Earlier studies linked this clade to multituberculates, but this dentary was cause to reject that association.

Figure 6. Haramiyava dentary showing what a more typical stem mammal dentary and teeth look like. Earlier studies linked this clade to multituberculates, but this dentary was cause to reject that association. Just the appearance of that poster medial groove is enough to indicate the presence of tiny jaw bones that had not transformed into ear bones. From Luo et al. 2015.

Stem mammals have lots of teeth
(Fig. 6) and Megaconus does not have lots of teeth. It has rodent-like teeth and everything else is rodent-like, too. And check out that overbite!

Figure 7. Eomaia skull traced and reconstructed. Eomaia nests between marsupials and placentals. Note the unspecialized skull and dentition. Megaconus has a very specialized dentition.

Figure 7. Eomaia skull traced and reconstructed. Eomaia nests between marsupials and placentals. Note the unspecialized skull and dentition. Megaconus has a very specialized dentition.

The skull of
Kryptobataar, (Fig. 6) another purported multituberculate, likewise shows no trace of tiny post-dentary bones, either here or in a Digimorph scan.

Figure 8. The skull of the multituberculate Kryptobataar, which now nests as a rodent in the LRT.

Figure 8. The skull of the multituberculate Kryptobataar, which now nests as a rodent in the LRT. B&W image copyright Digimorph.org and used with permission. 

The skull of
of Rugosodon (Fig. 9) likewise shows no trace of long, gracile post dentary bones, either here or originally.

Figure 9. The skull of Rugosodon. There are no tiny post dentary bones present here according to the original authors or my own tracings.

Figure 9. The skull of Rugosodon. There are no tiny post dentary bones present here according to the original authors or my own tracings.

References
Kielan-Jaworowska Z 1970. New Upper Cretaceous multituberculate genera from Bayn Dzak, Gobi Desert. In: Kielan-Jaworowska (ed.), Results of the Polish-Mongolian Palaeontological Expeditions, pt. II. Palaeontologica Polonica 21, p.35-49.
Luo Z-X, Gatesy SM, Jenkins FA Jr., Amarai WW and Shubin NH 2015. Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution. PNAS 112(41) E71010-E7109. doi: 10.1073/pnas.1519387112
Wible Jr, Rougier GW 2000. Cranial anatomy of Kryptobaatar dashzevegi (Mammalia, Multituberculata), and its bearing on the evolution of mammalian characters. Bulletin of the American Museum of Natural History 247: 1–120. doi:10.1206/0003-0090(2000)2472.0.
Yuan CX, Ji Q, Meng QJ, Tabrum AR and Luo ZX 2013. Earliest evolution of multituberculate mammals revealed by a new Jurassic fossil.. Science 341 (6147): 779–783. doi:10.1126/science.1237970.
Zhou CF, Wu S, Martin T, Luo ZX 2013. A Jurassic mammaliaform and the earliest mammalorian evolutionary adaptations. Nature 500 (7461): 163. doi:10.1038/nature12429.

wiki/Rugosodon
wiki/Megaconus