Fossiomanus and Jueconodon enter the LRT as pre-mammal diggers

As the headlines reported,
(see below) these two late-surviving pre-mammals lived under the feet of Early Cretaceous dinosaurs and probably only came out after dark.

From the Mao et al. 2021 abstract:
“Mammaliamorpha comprises the last common ancestor of Tritylodontidae and Mammalia plus all its descendants. Tritylodontids are nonmammaliaform herbivorous cynodonts that originated in the Late Triassic epoch, diversified in the Jurassic period and survived into the Early Cretaceous epoch. Eutriconodontans have generally been considered to be an extinct mammalian group, although different views exist.”

“Here we report a newly discovered tritylodontid and eutriconodontan from the Early Cretaceous Jehol Biota of China. Eutriconodontans are common in this biota, but it was not previously known to contain tritylodontids.”

Confirmation on those points!
In the large reptile tree (LRT, 1825+ taxa; subset Fig. 4) Fossiomanus nests with Oligokyphus and the tritylodonts. The other new burrowing pre-mammal, Jueconodon nests with Liaocondon, and other eutriconodonts close to Gobiconodon and Repenomamus.

Figure 1. Fossiomanus in situ in two ventral views, plus manus, pes and pelvis reconstructed. Teeth colored. Taphonomically shifted pectoral girdle repaired on right. The current view of the skull material prevents a reconstruction at this time.

Figure 2. Skull of Jueconodon based on data from Mao et al. 2021.

Mao et al. continue:
“These fossils shed light on the evolutionary development of the axial skeleton in mammaliamorphs, which has been the focus of numerous studies in vertebrate evolution and developmental biology. The phenotypes recorded by these fossils indicate that developmental plasticity in somitogenesis and HOX gene expression in the axial skeleton—similar to that observed in extant mammals—was already in place in stem mammaliamorphs. The interaction of these developmental mechanisms with natural selection may have underpinned the diverse phenotypes of body plan that evolved independently in various clades of mammaliamorph.”

Figure 3. Cladogram from Mao et al. 2021, color overlays added here to show how LRT divides these clades. Compare to figure 4.

Usually, ‘No hypotheses like this can proceed without first establishing a valid phylogeny.’ Parts of Mao et al. match the LRT. Unfortunately, Mao et al. follow invalid academic tradtion as they also include and therefore nest multituberculates with pre-mammals, rather than with rodents and plesiadapiformes in the gnawing clade, Glires. Just add pertinent taxa to resolve this problem. So far PhDs have been reluctant to do this and so the myth continues untested except here.

Mao et al. nest Jueconodon between Liaoconodon and Chaoyangodens (Fig. 3). In the LRT (Fig. 4) Jueconodon also nests with Liaoconodon, but Chaoyangodens nests as a monotreme mammal, basal to the echidna and platypus (Tachyglossus and Ornithorhynchus).

Mao et al. nest Fossiomanus with Kayentatherium, basal to four other tritylodontids including Tritylodon and Oligokyphus among mutually tested taxa. In the LRT (Fig. 4) Fossiomanus nests similarly.

Figure 4. Subset of the LRT focusing on pre-mammals with the addition of Fossiomanus and Jueconodon. Compare to original cladogram in figure 3 and to the LRT for a look at related taxa.

Mao et al. mention Liaoconodon often:

1. “The triangular shape of the skull may have been exaggerated by the crush of
   the specimen, but compared to those that have the similar preservation, such as Jeholodens, Liaoconodon, and Chaoyangodens, the triangular shape of Jueconodon is distinctive.
2. The morphology of the mandible is similar to those of other eutriconodontans, such as Liaoconodon (Meng et al., 2011). Given that Liaoconodon was interpreted as a semiaquatic animal (Chen and Wilson, 2015), the similar mandible in both species indicate that the lower jaw and teeth of Jueconodon were not specialized for digging.
3. The ossified Meckel’s cartilage on each side is preserved but displaced from its anatomical position. This suggests that the transitional mammalian middle ear, as best shown in Liaoconodon (Meng et al., 2011), was present in the fossorial eutriconodontans.“

Figure 5. Skull of Liaoconodon.

Figure 6. Liaoconodon in situ.

Mao et al. report, “the Manda cynodont and mammaliaforms that are considered terrestrial.
Compared to extant mammals, Fossiomanus sinesis is superficially similar in body size and shape to the Cape dune mole-rat Bathyergus suillus, the largest subterranean scratch-digger species of the African mole-rats (Montoya-Sanhueza et al., 2019). However, they differ fundamentally in the axial skeleton in that mole-rat has the rodent body plan with the ancestral PV count of mammals.”

References
Mao F-Y, Zhang C, Liu C-Y and Meng J 2021. Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature (advance online publication)
doi: https://doi.org/10.1038/s41586-021-03433-2
https://www.nature.com/articles/s41586-021-03433-2

wiki/Fossiomanus
wiki/Cape_dune_mole-rat

http://www.sci-news.com/paleontology/fossiomanus-sinensis-jueconodon-cheni-09534.html

https://www.amnh.org/explore/news-blogs/research-posts/burrowing-mammal-ancestors-discovered

Hollow-cheeked Euchambersia nests alongside puffy-cheeked Charassognathus

Unique among synapsids, Euchambersia
(Broom 1931, Benoit et al. 2017; Fig. 1) had an antorbital fenestra (= maxillary fenestra and fossa, Fig. 1) that may have housed a venom gland posterior to the canine root.

Reported by Brian Switek in Scientific American online,
“Because of the uniqueness of its skull anatomy,” Benoit and coauthors conclude, “Euchambersia mirabilis is and will remain a puzzling species.”

The ability to be unique in a world of gradual accumulations of derived traits 
made this taxon interesting. I wondered, which taxon did Euchambersia nest alongside? And did that taxon have anything like the antorbital fenestra found in Euchambersia?

The two answers are 1) Charassognathus and 2) yes.

Figure 1. Euchambersia skull with colors and shifting bones added.

Turns out Euchambersia was not unique among synapsids
for reasons stated above because its sister in the Therapsid Skull Tree (TST, 75 taxa) Charassognathus (Fig. 2) has a skull bulge posterior to the canine root.

Figure 4. Charassognathus (SAM-PK-K10369) does not share more traits with Abdalodon than other taxa, like Bauria and Promoschorhynchops in the TST. Note the bulge posterior to the canine root.

According to Wikipedia, citing Botha, Abdala and Smith 2007
Charassognathus is a basal cynodont.

By contrast, in the TST, Charassognathus is a cynodont-mimic nesting with therocephalians. Given the state of taphonomy documented in Euchambersia, the possibility that the unique maxillary fenestra was in life covered by a thin bulge of bone, as in Charassognathus, should be considered a possibility.

Wikipedia notes,
“Charassognathus has a snout that makes up slightly less than half of the total length of its skull and a long facial process on its septomaxilla. Other than these two features its skull is that of a typical cynodont. The odd shape of its septomaxilla is more typical of therocephalians than other cynodonts indicating that it may be close to a common ancestor between the two groups.”

The same is true of Euchambersia.

Figure 4. Therapid Skull Tree with the addition of Euchambersia and Charassognathus apart from cynodonts.

Nomenclature tidbit.
According to Wikipedia, “Broom named the genus Euchambersia, which he considered “the most remarkable therocephalian ever discovered”, after the eminent Scottish publisher and evolutionary thinker Robert Chambers, whose Vestiges of the Natural History of Creation was considered by Broom to be “a very remarkable work” though “sneered at by many.”

Chambers was probably happy to get the honor and compliment from Dr. Broom, while others sneered.

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References|
Benoit J, Norton LA, Manger PR and Rubidge BS 2017. Reappraisal of the envenoming capacity of Euchambersia mirabilis (Therapsida, Therocephalia) using μCT-scanning techniques. PLoS ONE 12(2): e0172047. doi:10.1371/journal.pone.0172047
Botha J, Abdala F and Smith R 2007. The oldest cynodont: new clues on the origin and diversification of the Cynodontia. Zoological Journal of the Linnean Society. 149: 477–492.
Broom R 1931. Notices of some new Genera and species of Karroo Fossil Reptiles. Rec Albany Mus. 1931; 41: 161–166.

It’s not often that all the references fall within the range of one letter. The odds against that are approximately one in 26 cubed or 17.576.

https://blogs.scientificamerican.com/laelaps/did-this-protomammal-have-a-venomous-bite/

wiki/Euchambersia
wiki/Charassognathus
wiki/Akidnognathidae

Adelobasileus restored: NOT ‘the oldest mammal’

When Lucas and Hunt 1990
and Lucas and Luo 1993 described the cranium (all that is known) of Adelobasileus (Fig. 1) they concluded it was, ‘the oldest mammal’. 

Figure 1. Adelobasileus restored like Therioherpeton after first nesting together in the LRT. Line drawing for Adelobasileus from Lucas and Luo 1993.

By contrast
the large reptile tree (LRT, 1707+ taxa, subset Fig. x) nests Adelobasileus with the low and wide mammal-mimic cynodont, Therioherpeton (Fig. 1), despite the very few characters that could be scored here. Both also nest with Sinocodon and Haramiyavia in the LRT. Thus Adelobasileus in not the oldest mammal. It is not even a mammal.

Therioherpeton
Fig. 1) was originally described by Bonaparte and Barberena 1975 as ‘a possible mammal ancestor’.

Later
Oliveira 2006 reevaluated Therioherpeton. “Therioherpetidae are distinguished from all other probainognathians by upper teeth with the imbrication angle increasing in the posterior postcanines. In addition, upper and lower postcanine teeth are labio-lingually narrow.” This author did not include Adelobasileus in his cladogram. Oliveira nested Therioherpeton with Riograndia.

Figure 2. Megazostrodon skull in several views. Drawings from Gow 1986. Colors applied here. This is the last common ancestor of all mammals in the LRT.

The last common ancestor of all mammals
in the LRT (subset Fig. x) continues to be Megazostrodon (Fig. 2), from the early Jurassic. Other, more derived mammals, like Morganucodon, are found in the Late Triassic, indicating an earlier origin and radiation.

Figure x. Subset of the LRT focusing on therapsids leading to mammals. Adelobasileus nests with Therioherpeton in this older cladogram that does not list Adelobasileus.

The most recent paper on basal mammals
and their immediate ancestors, King and Beck 2020, shows just how different cladograms can be when taxa are excluded (Fig. 3, click to enlarge). King and Beck mix non-mammals with prototherians, metatherians and eutherians in a mish-mash as compared to the LRT (Fig. x). At least they nest Adelobasileus outside their Mammalia (which should include only Prototherians, Metatherians and all descendants of their last common ancestor, Megazostrodon, Fig. 2).

Figure 3. Click to enlarge. Stem mammal cladogram from King and Beck 2020 showing how different their topology is to the LRT (color overlays, key at left) which has a wider gamut of included taxa. Arrow points to Adelobasileus near top.

Add taxa 
and multituberculates nest with rodents and other taxa nest appropriately with prototherians, metatherians and eutherians as shown in the LRT (subset Fig. x).

The nesting of Adeolbasileus with Therioherpeton
is not quite an original hypotheses. Google the two keywords, “Adelobasileus, Therioherpeton” and you’ll find someone tweeted these two as possible ancestor-descendant taxa, but unfortunately, still considered Adelobasilesus ‘the oldest mammal.’

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References
Bonaparte JF and Barberena MC 1975. A possible mammalian ancestor from the Middle Triassic of Brazil (Therapsida–Cynodontia). Journal of Paleontology 49:931–936.
King and Beck 2020. Tip dating supports novel resolutions of controversial relationships among early mammals. Proceedings of the Royal Society B 287: 20200943.
http://dx.doi.org/10.1098/rspb.2020.0943
Lucas SG and Hunt 1990. The oldest mammal. New Mexico Journal of Science 30(1):41–49.
Lucas SG and Luo Z 1993. Adelobasileus from the upper Triassic of west Texas: the oldest mammal. Journal of Vertebrate Paleontology 13(3):309–334.
Oliveira EV 2006. Reevaluation of Therioherpeton cargnini Bonaparte & Barberena, 1975 (Probainognathia, Therioherpetidae) from the Upper Triassic of Brazil. Geodiversitas 28 (3): 447-465.

http://reptileevolution.com/sinoconodon.htm
wiki/Adelobasileus
wiki/Therioherpeton

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. 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 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 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 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. 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.

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.

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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.

 

Dvinia enters the TST

Not to be confused with Dvinosaurus (a basal tetrapod)…
Dvinia (Fig. 1) is a rabbit-sized chiniquodontid cynodont with a fang-pierced rostrum and a high cranial crest.

Figure 1. From Amalitskii 1922, Dvinia skull and mandible from various views slightly larger than actual size.

Dvinia prima (Amalitskii 1922; Late Permian, 254mya; 7 cm skull) nests between Chiniquodon and Pachygenelus in the Therapsid Skull Tree (TST, 69 taxa). The lower canine fit into a maxillary opening. The molars had a circle of cusps around a single large cusp. The postorbital is very tiny, a vestige that would be lost in derived taxa, like basal mammals and Pachygenelus. The lateral temporal fenestrae were huge housing strong jaw muscles, divided by a narrow crest in which a smal brain was located.

Ivakhenko 2013 reported:
“The study of the skull of the Late Permian cynodont Dvinia prima Amalitzky, 1922 shows a combination of the general primitive skull design (many incisors, preservation of the precanine and large interpterygoid fenestra, etc) with the development of a number of “advanced” features (expansion of the temporal fenestra, development of the parietal crest, and closed pineal foramen, unusual structure of the premaxilla, complicated postcanines, and reduction of the angular wing). Dvinia prima is treated as a specialized omnivore and assigned to the family Dviniidae Sushkin, 1928 of the superfamily Thrinaxodontoidea Seeley, 1894.”

Double canines
sometimes appear in theriodonts (gorgonpopsids, therocephalians, cynodonts) and other synapsids. The second is a replacement canine, so it is not a trait one can score in phylogenetic analysis.

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Sidenote:
My computer was in the shop for about 48 hours Friday and Saturday after downloading a virus originating from .tk (Tokelau, a territory of New Zealand located in the South Pacific famous for free domain registry and malicious web masters) that I downloaded when I clicked on a Facebook video that was supposed to show a sperm whale rotating underwater along with a diver. Do not click on that video.

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References
Amalitskii VP 1922. Diagnoses of the new forms of vertebrates and plants from the upper Permian of North Dvia: Bulletin de l’Académie des Sciences de l’URSS, Math and Natural Sciences, 1922, p. 329-340. and in Izv. Ross. Akad. Nauk, Ser. 6 25 (1), 1–12.
Ivakhnenko MF 2013. Cranial Morphology of Dvinia prima Amalitzky (Cynodontia, Theromorpha). Paleontological Journal 47( 2): 210–222. © Pleiades Publishing, Ltd., 2013. Original Russian Text published in Paleontologicheskii Zhurnal, 2013, No. 2:81–93.

wiki/Dvinia

Tiny Abdalodon: a basal cynodont, drags in Lycosuchus

Today’s blogpost returns to basal Therapsida,
after several years of ignoring this clade.

Kammerer 2016 reidentifies an old Procynosuchus skull 
as an even more basal cynodont, now named Abdalodon (Fig. 1). The problem is: cynodonts arise from basal theriodonts (Therocephalia) and Abdalodon nests with another flat-head taxon, Lycosuchus (Fig. 1), a traditional therocephalian in every other cladogram, but not the Therapsid Skull Tree (TST, 67 skull-only taxa, Fig. 2), a sister cladogram to the LRT.

So, where is the cynodont dividing line?
(= which tested taxon is the progenitor of all later cynodonts and mammals?)

It would help if we knew the phylogenetic definition
of Cynodontia because we should never go by traits (which may converge), but only by taxon + taxon + their last common ancestor and all descendants to determine monophyletic clades.

From the Kammerer 2016 abstract:
“Phylogenetic analysis recovers Abdalodon as the sister‐taxon of Charassognathus, forming a clade (Charassognathidae fam. nov.) at the base of Cynodontia. These taxa represent a previously unrecognized radiation of small‐bodied Permian cynodonts. Despite their small size, the holotypes of Abdalodon and Charassognathus probably represent adults and indicate that early evolution of cynodonts may have occurred at small body size, explaining the poor Permian fossil record of the group.”

Figure 1. Abdalodon nests with the many times larger therocephalian Lycosuchus in the LRT.

Hopson and Kitching 2001 defined  Cynodontia
(Fig. 2) as the most inclusive group containing Mammalia, but excluding Bauria. In the TT Abdalodon nests with Lycosuchus on the cynodont side of Bauria.

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

So that makes Lycosuchus a cynodont,
by definition.

Figure 3. Procynosuchus, a basal cynodont therapsid synapsid sister to humans in the large reptile tree (prior to the addition of advanced cynodonts including mammals). This skull has been overinflated dorsoventrally based on the preserved skull, which everyone must have thought was crushed in that dimension.

Earlier we looked at
some Wikipedia writers when they stated, “Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved fossilize poorly…” 

Contra that baseless assertion,
with phylogenetic analysis and clades defined by taxa it is easy to determine which taxa are the last common ancestors, sisters to the progenitors of every derived clade in the TT, LRT or LPT. We can tell exactly which taxon was the first to lay amniotic eggs, without having direct evidence of eggs, simply because all of its ancestors in the LRT laid amniotic eggs. In the same way, we can figure out which taxon, among those tested, is the basalmost cynodont. Adding Bauria to the LRT made that happen today.

Let’s talk about size
The extreme size difference between Abdalodon and Lycosuchus (Fig. 1) brings up the possibility of cynodonts going through a phylogenetic size squeeze… retaining juvenile traits into adulthood… neotony… essentially becoming sexually mature at a tiny size for more rapid reproduction, reduced food needs, ease in finding shelters, etc. We’ve seen that before in several clades here, here and here, to name a few.

Figure 4. Charassognathus does not share more traits with Abdalodon than other taxa, like Bauria and Promoschorhynchops in the TT.

Kammerer 2016 mentioned another small taxon,
Charassognathus (Fig. 4). In the TST (Fig. 2) Charassognathus nests with Bauria and Promoschorhynchops, within the Therocephalia, distinct from, and not far from Abdalodon and the Cynodontia. So no confirmation here for Kammerer’s proposed clade, ‘Charassognathidae’ (see above).

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References
Hopson JA and Kitching JW 2001. A Probainognathian Cynodont from South Africa and the Phylogeny of Nonmammalian Cynodonts” pp 5-35 in: Parish A, et al.  editors, Studies in Organismic and Evolutionary biology in honor of A. W. Crompton. Bullettin of the Museum of Comparative Zoology. Harvard University 156(1).
Kammerer CF 2016. A new taxon of cynodont from the Tropidostoma Assemblage Zone (upper Permian) of South Africa, and the early evolution of Cynodontia. Papers in Palaeontology 2(3): 387–397. https://doi.org/10.1002/spp2.1046

wiki/Bauria
wiki/Abdalodon
wiki/Lycosuchus

The most basal mammal in the LRT: Megazostrodon

I thought for many years
that Megazostrodon was known from only a fragment of skull, lacking both the anterior and posterior parts.

Then somehow this paper popped up on the Internet
Gow 1986 illustrated the skull of Megazostrodon (Fig. 1; BPI/1/4983; Crompton & Jenkins, 1968; Latest Triassic; 200 mya). Even without this skull data the large reptile tree (LRT, 1293 taxa) nested Megazostrodon at the base of the Mammalia. There is little  argument among paleontologists that this taxon is a close sister to the last common ancestor of all living mammals.

Often wrongly associated
with Morganucodon, the two are phylogenetically separated from one another by tiny Hadrocodium in the LRT. In Megazostrodon the zygomatic arch is straight (without the ascending arch). The skull lacks a sagittal crest.  As in modern marsupials, carnivores, primates and tree shrews the teeth have a standard incisor, canine, premolar and molar appearance. The permanent molars occlude precisely. Uniquely (as far as I know), the dentary has a coronoid boss and a coronoid process.

Figure 1. Megazostrodon skull in several views. Drawings from Gow 1986. Colors applied here. The upper molars are worn down.

The large reptile tree
(Fig. 2) presents a simple, validated topology of mammals and their ancestors based on hundreds of traits, very few of them dental. It differs in nearly every regard from the Close et al. 2015 study, which employs many dental taxa.

Figure 3. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

The first time I reconstructed Megazostrodon
(Fig. 4) the skull looked legit, and was approved by cynodont expert Jim Hopson, but it had some problems. I’m glad to finally get better data on this, that resolves scoring problems around this node.

Figure 4. Megazostrodon, an a Jurassic mammal, along with Hadrocodium, a Jurassic tiny mammal. The Megazostrodon skull shown here is not correct.

On a side note:
Wikipedia reports,“Tinodon (Marsh 1887; YMP11843) is an extinct genus of Late Jurassic mammal from the Morrison Formation. It is of uncertain affinities, being most recently recovered as closer to therians than eutriconodonts but less so than allotherians.” 

Figure 5. Tinodon is best represented by an incomplete mandible with affinities to basal mammals and basal metatherians. Image from Morphobank.

 

Too few characters are present here
to add it to the large reptile tree, but if I have restored the missing parts correctly, then it is close to the base of the Mammalia and Theria near Megazostrodon.

References
Close RA, Friedman M. Lloyd GT and Benson RBJ 2015. Evidence for a mid-Jurassic adaptive radiation in mammals. Current Biology. 25 (16): 2137–2142. doi:10.1016/j.cub.2015.06.047. PMID 26190074.
Crompton AW and Jenkins FA Jr 1968. Molar occlusion in late Triassic mammals, Biological Review, 43 1968:427-458.
Gow CE 1986. A new skull of Megazostrodon ( Mammalia, Triconodonta) from the Elliot Formation (Lower Jurassic) of Southern Africa. Palaeontologia Africana 26(2):13–22.
Marsh OC 1887. American Jurassic mammals. The American Journal of Science, series 3 33(196):327-348

wiki/Megazostrodon

 

Prozostrodon: how close to mammals?

Wikipedia reports, “Prozostrodon was an advanced cynodont that was closely related to the ancestors of mammals.”

While true in the broadest sense
after all, even primitive Vaughnictis is “closely related to the ancestors of mammals,” in the large reptile tree (LRT, 1025 taxa) Prozostrodon nests a few nodes down from the most primitive mammals with several intervening transitional taxa.

Figure 1. Prozostrodon nests in the LRT between Thrinaxodon and the Chiniquodon clade, not as close to the base of mammals as Wikipedia implies.

Prozostrodon brasiliensis (originally Thrinaxodon brasiliensis, Barberena et al. 1987; Triassic; the size of a cat) was originally considered congeneric with Thrinaxodon, then closer to mammals. Here it nests between Thrinaxodon and the Chiniquodon clade. The foot was more symmetrical than in Thrinaxodon. The teeth were relatively smaller with molars no deeper than the incisors and with a larger canine.

Earlier we looked at several new addition to the Cynodontia. This is just one more.

References
Barberena MC, Bonaparte JF and Teixeira AMSA 1987. Thrinaxodon brasiliensis sp. nov., a primeira ocorrencia de cinodontes gales – sauros para o Triasico do Rio Grande do Sul. Anais do X Congresso Brasileiro de Paleontologia, Rio de Janeiro, Brazil, pp. 67-76.
Bonaparte JF and Barbierena MC 2001. On two advanced carnivorous cynodonts from the Late Triassic of Southern Brazil. Bulletin of the Museum of Comparative Zoology 156(1):59–80.
Bonaparte JF, Martinelli AG, Schultz CL and Rubert R 2003. The sister group of mammals: small cynodonts from the Late Triassic of Southern Brazil. Revista Brasileira de Paleontologia 5:5-27.
Bonaparte JF and Crompton AW 2017. Origin and relationships of the Ictidosauria to non-mammalian cynodonts and mammals. Historical Biology. https://doi.

 

Reviewing old and new news from Brazil on the origin of mammals and ictidosaurs

Figure 1. Here Brasilodon nests with Sinoconodon as a stem mammal (mammaliaformes).

Bonaparte et al. 2003
discovered two taxa close to the origin of mammals, Brasilodon  (Fig. 1) and Brasilitherium (Fig. 2). Originally both were considered stem mammals. In the large reptile tree (LRT, 1025 taxa, subset figure 4) Brasilodon nests with the stem mammal, Sinoconodon. However, Brasilitherium, also from the Late Triassic, nests at the base of the monotremes a clade including Akidolestes, Ornithorhynchus and Kuehneotherium. So it’s not a stem mammal. It’s a mammal. Bonaparte et al. 2003 missed that nesting due to taxon exclusion and a very interesting jaw joint that did not fit a preconceived pattern (Fig. 2 and see below).

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

Bonaparte et al. 2003
nested Brasilodon between Pachygenelus and Morganucodon + Brasilitherium, basically matching the LRT which did not exclude monotremes and Sinoconodon.

The key skeletal trait
defining Mammalia (unless it has changed without my knowledge) has been the disconnection of the post dentary bones from the dentary coincident with the dentary articulating with the squamosal producing a new mammalian jaw joint and the genesis of tiny ear bones.

Note: that’s not happening yet
in Brasilitherium despite its phylogenetic nesting as a basal monotreme. In Brasilitherium the articular, a post dentary bone, still articulates with the quadrate (Fig. 2). So, going by the jaw joint, Brasilitherium is not a mammal. However, going by its phylogenetic nesting in the LRT, it is a mammal.

Figure 3. Therioherpeton nests at the base of the Mammaliaformes with Brasilodon, between Yanaconodon and Sinoconodon, not far from Megazostrodon.

We’ve seen something similar occurring
at the origin of mammals, where amphibian-like reptiles (without reptile traits) have not been recognized as amniotes, based on their phylogenetic nesting in the LRT.

And, of course,
traditional workers still consider pterosaurs to be archosaurs based on their antorbital fenestra (by convergence), not their phylogenetic nesting (first documents in Peters 2000) in the LRT which solves earlier taxon exclusion problems by introducing a wider gamut of candidate sister taxa.

Th late appearance of the now convergent mammalian jaw joints
after the phylogenetic origin of mammals helps explain the two sites for ear bones in monotremes (below and medial to the posterior dentary) versus in therians (posterior to the jaw joint).

Tooth count
Basal monotremes have more teeth than any other mammals. Derived monotremes, like the living platypus and echidna, have fewer teeth, with toothless anterior jaws. This is a pattern of tooth gain/tooth loss we’ve seen before in other toothless taxa like Struthiomimus.

Recently, Bonaparte and Crompton 2017
concluded that ictidosaurs (Pachygenelus and kin) originated from more primitive procynosuchids rather than probainognathids. Pachygenelus likewise has a squamosal dentary contact, but it also retains a quadrate/articular contact as a transitional trait. They write: “We suggest a revision to the overwhelmingly accepted view that morganucodontids arose from probainognathid non- mammalian cynodonts (sensu Hopson & Kitching 2001). We suggest two phylogenetic lines, one leading from procynosuchids to ictidosaurs and the other from procynosuchids to epicynodonts and eucynodonts. One line evolves towards the mammalian condition, with a loss of circumorbital bones prefrontal, postfrontal, and postorbital), retention of an interpterygoid vacuity, a slender zygomatic arch, dentary/squamosal contact, and a long snout. The second evolves towards advanced non-mammalian cynodonts and tritylodontids with loss of the interpterygoid vacuity (present in juveniles), formation of a strong ventral crest formed by the pterygoids and parasphenoid, a very deep zygomatic arch, a tall dentary, and a short and wide snout.”

Talk about heretical!
Unfortunately, with the present taxon list, the LRT does not concur with Bonaparte and Crompton 2017, but instead recovers a more conventional lineage (Fig. 4).

Ictidosauria according to Bonaparte and Crompton:
The diagnostic features of Ictidosauria are as follows:

1. absent postorbital arch, postorbital, and prefrontal;
2. a slender zygomatic arch with a long jugal and short squamosal;
3. a dorsoventrally short parietal crest and transversally wide braincase;
4. interpterygoid vacuity;
5. ventral contact of the frontal with the orbital process of the palatine;
6. an unfused lower jaw symphysis;
7. a well-developed articular process of the dentary contacting the squamosal;
8. and a petrosal promontorium.

Figure 4. Basal Cynodont/Mammal cladogram focusing on the nesting of Brasilodon and Brasilitherium in the LRT.

Therioherpeton (Bonaparte and Barbierena 2001; Fig. 3) also enters the discussion as a stem mammal.

Therioherpetidae according to Bonaparte and Crompton:
share several features with mammaliaforms:

1. a slender zygomatic arch
2. squamosal dentary contact
3. unfuseddental symphysis
4. petrosal promontorium
5. transversely narrow postcanines with axially aligned cusps and an incipient cingulum
6. and a transversely expanded brain case
7. Therioherpetidae lack procumbent first lower incisors occluding between the first upper incisors
8. lack an edentulous tip of the premaxilla
9. and lack transversely widened postcanines

According to the Bonaparte team
Three distinct groups have been included in Mammaliformes.

1. Morganucodon, Megazostrodon and Sinoconodon;
2. Docodonta
3. Haramiyids such as Haramiyavia

They report,
“Brasilitherium is closer to the first group than the more derived second and third groups. Brasilitherium is almost identical to Morganucodon, except that the latter has a mammalian tooth replacement pattern (single replacement of the incisors, canines, and premolars, and no replacement of the molars), double rooted molars, and the orbital flange of the palatine forms a medial wall to the orbit (Crompton et al. 2017).”

“Several features present in Procynosuchus are absent in probainognathids (sensu Hopson & Kitching 2001), but present in Ictidosauria.

1. Interpterygoid vacuities (present only in juvenile probainognathids);
2. a slender zygomatic arch;
3. incisiforms present at the junction of premaxilla and maxilla;
4. a low and elongated dentary;
5. and an unfused lower jaw symphysis.”

Hopefully it will be seen as a credit to the LRT 
that it nested each new taxon about where the three Bonaparte teams nested them (sans the unusual Procynosuchus hypothesis), only refined a bit with the addition of several overlooked monotreme taxa, several of which have similar (to Procynosuchus) low, long skulls and rather low-slung post-crania.

Refrerences
Bonaparte JF and Barbierena MC 2001. On two advanced carnivorous cynodonts from the Late Triassic of Southern Brazil. Bulletin of the Museum of Comparative Zoology 156(1):59–80.
Bonaparte JF, Martinelli AG, Schultz CL and Rubert R 2003. The sister group of mammals: small cynodonts from the Late Triassic of Southern Brazil. Revista Brasileira de Paleontologia 5:5-27.
Bonaparte JF and Crompton AW 2017. Origin and relationships of the Ictidosauria to non-mammalian cynodonts and mammals. Historical Biology. https://doi.

 

… and Liaoconodon is not a mammal…

Updated September 22, 2018
with a re-nesting of Liaoconodon with Repenomamus.

Yesterday we noted that Repenomamus was not a mammal, but nested with the stem- (pre-) mammal trithelodontids, like Pachygenelus. Likewise, today Liaoconodon hui (Meng, Wang and Li 2011; IVPP V 16051; early Cretaceous, Aptian, 120 mya; Figs. 1, 3), nests between Gobiconodon and Repenomamus in the large reptile tree (subset in Fig. 2).

Figure 1. Liaocondon skull traced and reconstructed. In the LRT it most closely resembles that of Repenomamus. Note the enormous size of the temporal fenestrae, the downturned squamosals and dentary bones, all shared with Trithelodontidae.

Liaconodon has a dentary-squamosal jaw joint
and the scapula has a ventral glenoid, which are traditional mammal traits. It also has a mammal-like ilium without a posterior process, like a mammal… or a pre-mammal tritylodontid, like Oligokyphus and Kayentatherium. The narrow braincase of Liaconodon, like that of Repenomamus, tells us this is a pre-mammal. More importantly, phylogenetic analysis nests Liaoconodon outside the last common ancestor of all living mammals: Megazostrodon (Fig. 2). Thus, the dentary-squamosal joint appeared by convergence in Liaoconodon and mammals.

Figure 2. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

Liaconodon lacks large canines
and the lower incisors are enlarged. The postorbital bar is not complete.

Figure 3. Liaoconodon in situ. The causals are similar in shape to those of Castorocauda.

Jin Meng of the AMNH
made a video posted to YouTube describing how ground-breaking it was to find post dentary bones in Liaconodon, which they considered a mammal. Those post-dentary bones are indeed clear and articulated, but are typical for pre-mammal cynodonts.

The manus and pes are well preserved
which is something we rarely see around this node. The caudals are nearly identical to those of Castorocauda and Repenomamus. The femora were likewise rather short.

It was a good week for finding errors.
As before, we all boggled this one. To those who are toying with the challenge I presented earlier about finding badly nested taxa in the LRT, sorry, you missed this one.

References
Meng J, Wang Y-Q and Li C-K 2011. Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. Nature 472 (7342): 181–185.

wiki/Liaoconodon