Morganucodon and Kuehneotherium are mammals, not stem-mammals

Newham et al. 2019 report,
“Surprisingly long lifespans and low femoral blood flow suggest reptile-like physiology in key Early Jurassic stem-mammals.

Abstract:
“There is uncertainty regarding the timing and fossil 5 species in which mammalian endothermy arose, with few studies of stem-mammals on key aspects of endothermy such as basal or maximum metabolic rates, or placing them in the context of living vertebrate metabolic ranges. Synchrotron X-ray imaging of incremental tooth cementum shows two Early Jurassic stem-mammals, Morganucodon and Kuehneotherium, had lifespans (a basal metabolic rate 10 proxy) considerably longer than comparably sized living mammals, but similar to reptiles, and that Morganucodon had femoral blood flow rates (a maximum metabolic rate proxy) intermediate between living mammals and reptiles. This shows maximum metabolic rates increased evolutionarily before basal rates, and that contrary to previous suggestions of a Triassic origin, Early Jurassic stem-mammals lacked the endothermic metabolism of living mammals.”

That conclusion would be true
if their cladogram was correct. Unfortunatley, it was not.

Figure 1. Subset of the LRT focusing on Basal Mammalia including Creodonta.

Figure 1. Subset of the LRT from 2018 focusing on Basal Mammalia including Morganucodon and Kuehneotherium.

According to
the large reptile tree (LRT, 1579 taxa; subset Fig. 1), Kuehneotherium (Fig. 2) is a basal protothere mammal (= monotreme) in the lineage of echidnas and platypuses. Morganucodon is a very basal metathere mammal (= marsupial). The Virginia opossum, Didelphis, is the most closely related extant taxon in the LRT.

Figure 1. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Figure 2. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Here’s a data point of interest:
Newham et al. report, “Only the short-beaked echidna Tachyglossus aculeatus, a monotreme with long lifespan and low metabolic rate, exceeds the Kuehneotherium, but not Morganucodon, distance above the mammalian mean.” And THAT is reflected in the LRT. I also note the platypus, Ornithorhynchus, is not mentioned in the text, only in the citations. Same with Didelphis.

So what does that do to the results?
Seems like the Newham et al. study is suffering from taxon exclusion and an invalid traditional understanding of basal mammal interrelations. Unfortunately Professor MJ Benton is a co-author, infamous for taxon exclusion and guiding his students and any protégé to do the same.

Please tell Elis Newham et al.
to add the platypus and opossum to their study and get back to us! Don’t let this work become another waste of time due to taxon exclusion.


References
Newham E et al. (19 co-authors) 2019. Reptile-like physiology in Early Jurassic stem-mammals. bioRxiv preprint http://dx.doi.org/10.1101/785360

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Heterohyus enters the LRT

Figure 1. Two of several Heterohyus specimens from the Messel Pit of Germany.

Figure 1. Two of several Heterohyus specimens from the Messel Pit of Germany.

Heterohyus nanus (Gervais 1848, late Eocene) from the Messel Pit in Germany nests with Apatemys in the large reptile tree (LRT, 1563 taxa). Relatively few traits differentiate the two. The lumbar region is shorter. The skull is larger. The naris is smaller and higher on the rostrum.

According to Wikipedia members of the Apatemyidae were
“small and presumably insectivorous. Size ranged from that of a dormouse to a large rat. The toes were slender and well clawed, and the family were probably mainly arboreal.[2] The skull was fairly massive compared to the otherwise slender skeleton, and the front teeth were long and hooked, resembling those of the modern aye-aye and marsupial Dactylopsila, both whom make their living by gnawing off bark with their front teeth to get at grubs and maggots beneath.”

The LRT nests apatemyids
as basal members of Glires, not related to other so-called cimolestids, a polyphyletic assembly of placental taxa. according to the LRT. Rather apatemyids are a clade of gnawing tree shrews, now extinct.


References
Gervais P 1848–52. Primates, Microchoeridae? Zoologie et Paléontologie Françaises. Paris, Arthrus Bertrand, text, 2 vols; atlas, 80 pls.

wiki/Heterohyus
wiki/Apatemyidae

 

The bush dog, first known as a fossil, enters the LRT

Figure 1. Speothos is the living bush dog from South America.

Figure 1. Speothos is the living bush dog from South America. This taxon is basal to cats + dogs + hyaenas.

The South American bush dog
Speothos veanticus (Lund 1842; up to 75cm in length; Figs. 1, 2) is traditionally considered a basal dog (family: Canidae). Here Speothos nests at the base of cats + hyaenas + aardwolves + dogs. Miacis is a similar sister basal to sea lions and both are derived from another short-legged carnivore, the European mink, Mustela. Speothos was first identified as a fossil, then as a living taxon. Webbed toes allow this genus to swim more effectively.

Figure 2. Speothos, the South American bush dog, skeleton and in vivo.

Figure 2. Speothos, the South American bush dog, skeleton and in vivo.


References
Lund PW 1842. Fortsatte bernaerkninger over Brasiliens uddöde dirskabning.Lagoa Santa d. 27 de Marts 1840. Kongelige Danske Videnskabernes Selskab Afhandlinger 9:1-16.

wiki/Bush_dog

 

Microdocodon: If those are hyoids, then where are the fingers?

A new mammaliaform, Microdocodon,
(Zhou et al. 2019; Figs. 1–4; Middle Jurassic, 165 mya) is exceptionally well preserved and complete, down to the smallest details. According to the authors, those details include “complex and saddle-shaped hyoid bones (Fig. 1), like those seen in modern mammals.”

Figure 1. From Zhou et al., colors added. Microdocodon is in yellow. The two taxa in dark gray are derived members of Glires and do not nest in the LRT where shown here.

Figure 1. From Zhou et al. 2019, colors added. Microdocodon is in yellow. The two taxa in gray are derived members of Glires and do not nest in the LRT where shown here. It is obvious from looking at this evolutionary progression that the two highly derived gnawing taxa do not document a gradual accumulation of derived traits, like the remaining plesiomorphic taxa do.

Timing?
Microdocodon was found in strata 40 million years into the Jurassic, some 40 million years after the appearance of the first mammal, Megazostrodon in the large reptile tree (LRT, 1545 taxa). Pre-mammal cynodonts lived alongside mammals throughout the Mesozoic.

H-shaped, articulated hyoids were unexpected in such a primitive cynodont
and a dozen news organizations picked up on the unexpectedness of this story. If valid this would suggest that a muscularized throat was present phylogenetically before the genesis of the milk-suckling clade, Mammalia.

Figure 1. Microdocodon throat region. Are those bones hyoids or fingers? If hyoids, then where are the fingers? Note the displaced radius (olive green)  reaching toward the throat. Only impressions of once present fingers are present on the right limb.

Figure 2. Microdocodon throat region. Are those bones hyoids or fingers? If hyoids, then where are the fingers? Note the displaced radius (olive green)  reaching toward the throat. Only impressions of once present (or still buried) fingers are present on the right limb.

Unfortunately,
there may be reason to doubt the identity of these bones. Are they hyoids? Or fingers? If the mystery bones are indeed hyoids, then the fingers are missing. If fingers, then the hyoids are missing, which takes all the surprise and wonder out of the Zhou et al. paper.

FIgure 2. Microdocodon in situ. Plate and counter plate plus colors added.

FIgure 3. Microdocodon in situ. Plate and counter plate plus colors added. Manus, pelvis and pes reconstructed. The recombining of plate and counter plate is something that does not work as well in print.

From the abstract
“We report a new Jurassic docodontan mammaliaform found in China that is preserved with the hyoid bones. Its basihyal, ceratohyal, epihyal, and thyrohyal bones have mobile joints and are arranged in a saddle-shaped configuration, as in the mobile linkage of the hyoid apparatus of extant mammals. These are fundamentally different from the simple hyoid rods of nonmammaliaform cynodonts, which were likely associated with a wide, nonmuscularized throat, as seen in extant reptiles. The hyoid apparatus provides a framework for the larynx and for the constricted, muscularized esophagus, crucial for transport and powered swallowing of the masticated food and liquid in extant mammals. These derived structural components of hyoids evolved among early diverging mammaliaforms, before the disconnection of the middle ear from the mandible in crown mammals.”

The big question is:
If those are indeed hyoids, then where are the fingers? EVERYTHING else is present and visible on this perfectly preserved fossil, except, apparently, the fingers of both hands.

Further complication:
I looked closely at the purported hyoids and found they

  1. included unguals
  2. began at the wrist
  3. were articulated like fingers
  4. had all the proportions and correct number expected in a typical manus from that node on the LRT (Fig. 5).

Often enough,
when bones you expect are missing AND similar bones you don’t expect are present, you should suspect that a misidentification is taking place.

Figure 3. Microdocodon skull, plate and counter plate, colors added.

Figure 4. Microdocodon skull, plate and counter plate, colors added.

After phylogenetic analysis
Microdocodon nests at the base of the Tritylodontidae (Oligokyphus and kin) + (Riograndia + Chaliminia) clade. These are therapsids retaining a primitive quadrate/articular jaw joint, not like a mammal with a squamosal/dentary jaw joint.

At this point it is probably good to remember
that the most primitive mammals do not suckle. Prototherians, like echidnas and platypuses lick their mothers milk from sweat puddles on her belly. Only metatherians and eutherians have infants that suckle on their mothers’ teats, which is several nodes up the ladder from Microdocodon.

A docodont?
The authors considered Microdocodon a small member of the Docodonta, a clade traditionally defined by dental and mandible traits. Unfortunately, Microdocodon does not nest in the LRT with other clade members listed on the Wikipedia page. As we’ve seen many times, dental traits can converge.

The phylogenetic analysis of Zhou et al. employs “tritylodontids” as a suprageneric taxon nesting outside of Pachygenelus, (the opposite of the LRT) derived from Thrinaxodon and Massetognathus. To their peril, Zhou et al. include a long list of multituberculates, but no carpolestid and plesiadapid sister taxa recovered by the LRT. So taxon exclusion is a problem as highly derived multituberculates arise in Zhou et al. prior to primitive prototherians (Fig. 1). Also mis-nested in the Zhou et al analysis, the early and basal metatherian, Eomaia and the basal prototherian, Juramaia, nest as derived eutherians. These are all red flags, probably arrived at by an over-reliance on dental traits and the most typical problem in vertebrate paleontology: taxon exclusion. The LRT minimizes taxon exclusion because it tests such a wide gamut of taxa.

Figure 5. Microdocodon pectoral and forelimb reconstruction from DGS traced elements.

Figure 5. Microdocodon pectoral and forelimb reconstruction from DGS traced elements. Those fingers were originally considered hyoid elements. Yes, those are elongate coracoids, typically found in members of the Tritylodontidae.

But wait! All is not lost.
Microdocodon fills an important gap leading to the Tritylodontidae in the LRT. So it can still be exciting and newsworthy for this overlooked reason.

The pre-mammal/pre-tritylodontid split occurred
by the Middle Triassic, which gives Middle Jurassic Microdocodon plenty of time to evolve distinct traits. And it did. The snout is longer than typical. The medial metatarsals were atypically longer than the others. Tiny phalanges 3.2, 4.2, 4.3 and 5.2 reappear after disappearing several nodes earlier. That bit of atavism is interesting. The limbs are long and gracile with reduced interoseal space between the crural and ante brachial elements, mimicking/converging on more derived mammals.

Figure 6. Subset of the LRT focusing on basal Therapsida and Microdocodon's nesting in it.

Figure 6. Subset of the LRT focusing on basal Therapsida and Microdocodon’s nesting in it.

The authors report,
“Phylogenetically, Microdocodon and [coeval] Vilevolodon are the earliest-known mammaliaform fossils with mammal-like hyoids.” Vilevolodon is a highly derived, squirrel-like member of the clade Multituberculata within the rodent/rabbit clade of Glires within the Eutheria in the LRT.

Articulated hyoids
are exceptionally rare in the early fossil record of mammals. So are basal mammals.

Everyone is looking for a headline with every new fossil specimen.
Unfortunately, as we’ve seen time and again, you can’t believe everything you read, even after PhD peer review and publication in Nature and Science. Make sure you test all novel hypotheses with careful observation and a wide gamut phylogenetic analysis.


References
Zhou C-F, Bullar B-A S, Neander AI, Martin T and Luo Z-X 2019. New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones. Science 365(6450):276–279.

https://www.sciencenews.org/article/flexible-bone-helps-mammals-chew-dates-back-jurassic-period

https://www.sciencedaily.com/releases/2019/07/190718140440.htm

For a dozen more popular articles: Google keyword: Microdocon.

 

Restoring Plagiomene (incomplete basal placental)

Wikipedia reports,
Plagiomene multicuspis (Fig. 1; Matthew 1918; MacPhee et al. 1989; YPM VP 030624; Wyoming; Paleocene) is an extinct genus of early flying lemur like mammal from North America that lived during the Paleogene.”

Here
using imagination (Fig. 1) to restore the missing parts, scrappy Plagiomene data turns into a more complete skull. Plagiomene had four small molars and a narrow snout between wide robust cheekbones. Those facts and phylogenetic bracketing suggest forward-pointed eyes sitting atop wide cheekbones for bifocal vision.

Figure 1. What little is known of Plagiomene seems to agree with the North American adapid, Smilodectes, among tested taxa.

Figure 1. What little is known of Plagiomene seems to agree with the North American adapid, Smilodectes, among tested taxa. Plagiomene was not added to the LRT.

Here an attempt at restoring the rest of the skull
(Fig. 1) results in a short-snouted taxon with robust cheekbones, more or less similar to Smilodectes (Fig. 1), which has not four, but only three molars and lived during the middle Eocene. An extremely tall coronoid process requires a similarly tall skull. If valid, Plagiomene would be a basal primate, or basal to Primates + Volantia (where dermopterans are a basal taxa).

Possible outgroups,
such as basal Carnivora and Cheiroptera, do not have a similar mandible or molars.

The basal dermopterans,
Palaechthon
(Fig. 1) and Cynocephalus, both have 4 molars, but do not have a tall coronoid process on the mandible.

Earlier we looked at the evidence for
the clade that includes Smilodectes (Adapidae) nesting at the base of the clade of New World monkeys (Platyrrhini). Plagiomene is also from North America.

The last upper premolar
of Plagiomene extends further toward the midline than the molars do. That is unusual in basal mammals. When I find this trait in another basal mammal palate, I will let you know.


References
MacPhee RDE, Cartmill M and Rose KD 1989. Craniodental morphology and relationships of the supposed Eocene dermopterans Plagiomene (Mammalia). Journal of Vertebrate Paleontology 9(3):329–349.
Matthew WD 1918. A revision of the Lower Eocene Wasatch and Wind River faunas. Part V. Insectivora (Continued), Glires, Edentata. Bulletin of the American Museum of Natural History 38(16):429-483.

wiki/Plagiomene
wiki/Smilodectes

Therocephalians evolved to smaller size? Large Carnivora did not?

Brocklehurst 2019 reports,
“If these results are reliable, they support the traditional paradigm that therocephalians originated as large predators, and only later evolved small body sizes. The patterns observed in mammals do not appear to apply to therocephalians. Mammalian carnivores, once they have reached large size and a specialized bauplan, are apparently unable to leave this adaptive peak. Therocephalians, on the other hand, retreated from the hypercarnivore niche and evolved small sizes later in the Permian.”

Figure 1. Cladogram from Brocklehurt 2019, colors added. Lycosuchus, listed as a basal therocephalian by Brocklehurst, also nests close to cynodonts in the TST. No gorgonopsids are shown here. Biarmosuchus is the outgroup taxon here, a more distant outgroup taxon in the TST.

Figure 1. Cladogram from Brocklehurt 2019, colors added. Lycosuchus, listed as a basal therocephalian by Brocklehurst, also nests close to cynodonts in the TST. No gorgonopsids are shown here. Biarmosuchus is the outgroup taxon here, a more distant outgroup taxon in the TST.

Brocklehurst’s cladogram
posits that Therocephalia and Cynodontia arose as sisters from a last common ancestor: Biarmosuchus. In the therapsid skull tree (TST, 67 taxa, Fig. 4), Therocephalia (including Cynodontia) arises from Gorgonopsia (Fig. 2).

Figure 2. Gorgonopsids, therocephalians and cynodonts to scale.

Figure 2. Gorgonopsids, therocephalians and cynodonts to scale.

The question arises,
what is a ‘large size’ member of the Carnivora? Certainly big cats and walruses (Fig. 3) fall into this definition and do not give rise to smaller ancestors, as Brocklehurst notes. However, if the basalmost member of the Carnivora, Vulpavus, is considered ‘large’ then it breaks the ‘rule’ because it has smaller descendants in the LRT: Mustela and Procyon (Fig. 3). Talpa, the mole, is the smallest member of the Carnivora in the LRT. Talpa has been traditionally omitted from Carnivora studies while being wrongly lumped with the unrelated shrew, Scutisorex, instead.

Figure 3. Carnivora to scale. Note: one branch does increase in size over time (ignoring toy poodles for the moment), while another branch, the one leading to Talpa the mole, shrinks in size.

Figure 3. Carnivora to scale. Note: one branch does increase in size over time (ignoring toy poodles for the moment), while another branch, the one leading to Talpa the mole, shrinks in size. Brocklehurst is correct: once carnivores achieved large size, few to no examples of phylogenetic miniaturization appear in the fossil record.

I wish Brocklehurst 2019 had added
a few sample reconstructions to scale to help readers visualize the size ranges that he found in his cladogram. After all, the subject was ‘size’. I was unfamiliar with the vast majority of therocephalian taxa in his cladogram (Fig. 1).

Figure 4. TST revised with new data on Patranomodon and sister taxa.

Figure 4. TST revised with new data on Patranomodon and sister taxa. Here the therocephalian, Bauria, nests closer to cynodonts than in Brocklehurst 2019 (Fig. 1).

Brocklehurst is correct:
once carnivores achieved large size (Fig. 3), no examples of phylogenetic miniaturization subsequently appear. Brocklehurst contrasted this with therocephalians, presuming that Lycosuchus (Fig. 2) was a basal therocephalian, rather than a basal cynodont by definition.

Remember:
Hopson and Kitching 2001 defined  Cynodontia as the most inclusive group containing Mammalia, but excluding Bauria. In the TST (Fig. 4) Abdalodon and Lycosuchus nest on the cynodont side of Bauria.

In the TST
(Fig. 4), cynodonts show no strong size trends until mammals, like Megazostrodon (Fig. 2), evolved tiny sizes. Therocephalians likewise show no strong size trends either (but then, I have not measured every taxon in the Brocklehurt cladogram, Fig. 1). Those that also appear in the TST are in white boxes, and they appear in several clades within Therocephalia.


References
Brocklehurst N 2019. Morphological evolution in therocephalians breaks the hyper carnivore ratchet. Proceedings of the Royal Society B 286: 20190590. http://dx.doi.org/10.1098/rspb.2019.0590

Numbat genesis in the Early Jurassic

Coelocanth. Tuatara. Numbat.
Name three taxa that have not changed much in hundreds of millions of years.

Figure 1. Myrmecobius, the living numbat, has remained essentially unchanged for nearly 200 million years.

Figure 1. Myrmecobius, the living numbat, has remained essentially unchanged for nearly 200 million years based on the LRT. Note the loss of posterior molars and the simplification of the remaining anterior molars. Orange arrow point to palatal pits that receive the long lower canines.

Extant numbats
(genus: Myrmecobius, Fig. 1) nest in the large reptile tree (LRT, 1412 taxa; subset Fig. 2) basal to Early Cretaceous Anebodon, Middle Jurassic Docofossor and the extant marsupial mole (genus: Notoryctes). All arise from the extant Dasycercus (Fig. 3). So that provides an interesting cladogram with members that span from the Triassic to the present. That means some extant taxa had nearly identical ancestors that shared the planet with the first dinosaurs and pterosaurs.

Figure 2. Subset of the large reptile tree focusing on the basal phytometatheria, including extant numbats, basal to Middle Jurassic Docofossor.

Figure 2. Subset of the large reptile tree focusing on the basal phytometatheria, including extant numbats (Myrmecobius), basal to Middle Jurassic Docofossor.

Myrmecobius fasciatus (Waterhouse 1841) is the extant numbat. Here it nests between Dasycercus and Anebodon. Since an ancestral taxon, Docofossor, is known from the Middle Jurassic, a sister to Myrmecobius had its genesis in the Early Jurassic. The molars are narrow and simplified. This is a marsupial termite eater, convergent with placental termite- and ant-eaters. Over each orbit is the reappearance of an old bone, the postfrontal. The canine is smaller. The jugal is straighter.

Figure 5. Dasycercus, the extant mulgara, is the carnivorous phylogenetic ancestor to the clade that includes numbats, Docofossor and kin in the LRT.

Figure 3. Dasycercus, the extant mulgara, is the carnivorous phylogenetic ancestor to the clade that includes numbats, Docofossor and kin in the LRT.

What’s interesting are the molars in Myrmecobius.
Take a good look (Fig. 1). The molars are narrow and simplified because this taxon eats termites (or vice versa). A phylogenetic descendant, Docofossor (Fig. 5) was considered a docodont based on its simple tooth morphology. Another phylogenetic descendant, Anebodon, was considered a symmetrodont based on its tooth morphology.

The LRT results remind us
not to put so much emphasis on tooth morphology. The LRT makes mammal systematics so much simpler by nesting taxa according to all their tested traits, not just a few, rather plastic, dental traits.

Figure 4. Dasycercus in vivo. This is the extant mulgara, a carnivorous nocturnal basal marsupial.

Figure 4. Dasycercus in vivo. This is the extant mulgara, a carnivorous nocturnal basal phytomarsupial with origins in the Early Jurassic.

Dasycercus cristicauda (originally ‘Chaetocercus‘ Krefft 1867; Peters 1875; 22cm + 13 cm tail) is the extant mulgara, considered a dasyurid marsupial. Here carnivorous, nocturnal Dasycercus nests apart from Dasyurus between Anebodon and Myrmecobius at the base of the herbivorous clade of marsupials. The pouch is reduced to two lateral folds of skin.

Figure 1. Docofossor in situ with DGS tracings.

Figure 5. Docofossor in situ with DGS tracings. This Middle Jurassic taxon nests as a derived descendant of Dasycercus and Myrmecobius in the LRT.

Docofossor brachydactylus (Luo et al. 2015; Middle Jurassic, 160 mya; BMNH 131735; 9cm in precaudal length) was originally considered a member of the Docodontidae along with Docodon and Haldanodon outside of the Mammalia. Here it nests as a Jurassic sister to Anebodon and Notoryctes. Broad, short-fingered hands, larger than the feet, along with other traits mark Docofossor as a digging animal, similar to moles like Talpa and Chrysochloris.


References
Bi S-D, heng X-T, Meng J, Wang X-L, Robinson N and Davis B 2016. A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution. Nature Scientific Reports 6:26668 DOI: 10.1038/srep26668
Gadow H 1892. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370.
Luo Z-X, Meng QJ, Ji Q, Liu D, Zhang Y-G, Neande AI 2015.Evolutionary development in basal mammaliaforms as revealed by a docodontan. Science. 347 (6223): 760–764.
Peters WCH 1875. Sitzungsberichte der Gesellschaft Naturforschender Freunde zu Berlin 1875: 73.
Stirling EC 1888. Transactions of the Royal Society, South Australia 1888:21
Stirling EC 1891. Transactions of the Royal Society, South Australia 1891:154
Tate GHH 1951. The banded anteater, Myrmecobius Waterhouse (Marsupialia). American Museum Novitates 1521, 8 pp.
Waterhouse GR 1836. Myrmecobius fasciatus. Proc. Zool. Soc. London 4: 69–131.
Waterhouse GR 1841. Description of a new genus of mammiferous animals from Australia, belonging probably to the order Marsupialia. Trans. Zool. Soc., London2, aricle. 11, p 149.

wiki/Dasycercus
wiki/Myrmecobius