New(?) mammal at the Eutheria-Metatheria split

Bi et al. 2018 bring us a small Virginia opossum from the early Cretaceous
“Molecular estimates of the divergence of placental and marsupial mammals and their broader clades (Eutheria and Metatheria, respectively) fall primarily in the Jurassic period. Supporting these estimates, Juramaia, the oldest purported eutherian is from the early Late Jurassic (160 million years ago) of northeastern China. Sinodelphys, the oldest purported metatherianâis from the same geographic area but is 35 million years younger, from the Jehol biota. Here we report a new Jehol eutherian, Ambolestes zhoui, with a nearly complete skeleton that preserves anatomical details that are unknown from contemporaneous mammals, including the ectotympanic and hyoid apparatus. This new fossil demonstrates that Sinodelphys is a eutherian, and that postcranial differences between Sinodelphys and the Jehol eutherian Eomaia, previously thought to indicate separate invasions of a scansorial niche by eutherians and metatherians, are instead variations among the early members of the placental lineage. The oldest known metatherians are now not from eastern Asia but are 110 million years old from western North America, which produces a 50-million-year ghost lineage for Metatheria.”

Figure 1. Ambolestes tracing from Bi et al. 2018.

Figure 1. Ambolestes tracing from Bi et al. 2018. plate and counter plate. Note the scale bar. This taxon is one third the size of the extant opossum. Apparently and oddly no unguals were preserved.

In the large reptile tree (LRT, 1131 taxa) the new fossil at the Metatheria/ Eutheria split is phylogenetically identical to and therefore congeneric with Didelphis, the Virginia opossum. So, this is pre-marsupial in a clade at the base of the marsupial/placental split. There are also a series of pre-placentals, some of which are extant and retain a reduced pouch, like Monodelphis.

Figure 2. Ambolestes skull in situ with DGS applied.

Figure 2. Ambolestes skull in situ with DGS applied.

Ambolestes zhoui gen. & sp. nov. (Bi et al. 2016, 126 mya; 25cm in length) is is one third the size of the extant opossum, but otherwise nearly identical based on tested traits. In the LRT Didelphis nests with Ambolestes and they nest with Eomaia and Agilodocodon as the last common ancestors to Metatheria and Eutheria. Bi said in an interview, “Ambolestes zhoui is an early member of the placental lineage. It also carries mixed features both placentals and marsupials”. In the LRT, Ambolestes is exactly as much in the placental lineage (Fig. 4) as Didelphis is… and Didelphis has a pouch. Both have prepubic (epipubic) bones.

Since Ambulestes is congeneric with Didelphis,
you heard it hear first when the LRT nested Didelphis as the last common ancestor of Metatheria and Eutheria. Good to see confirmation.

Figure 3. Ambolestes skull reconstructed. Jaw tips restored.

Figure 3. Ambolestes skull reconstructed. Jaw tips restored. Lower last molar appears to be just erupting. No retroartcular process is apparent here, which sometimes happens with Didelphis (Mohamed 2018)

The Pittsburgh Post-Gazette reported,
“The well-preserved new mammal, an ancient furry creature most similar to a modern tree shrew, is named Ambolestes zhoui.” Actually Ambolestes was a little less exotic than that.

“John Wible, curator of mammals at the Carnegie Museum of Natural History, became involved in the project about two years ago.As soon as I saw the photographs of the fossil I was like, ‘Oh my God this is amazing,’ ” he said. “It was amazingly complete. Right off the bat I saw there were skeletal parts of the body that were not known of other animals of that time period.”

But if Dr. Wible happened upon a certain type of roadkill
or went out after midnight with a flashlight he would have seen a living version of the Early Cretaceous fossil. Rather than, “this is amazing” he could have said, “we could have predicted this.”

Figure 7. Subset of the LRT focusing on basal live-bearing mammals.

Figure 4. Subset of the LRT focusing on basal live-bearing mammals.

The Post-Gazette also reported, 
“The fossil was not allowed to leave China, said Mr. Wible, noting that this is the first paper he’s published where he’s been unable to actually hold the fossil, though he hopes to see it in person in the next few years. Instead, he relied on detailed photographs and scanned images.”

Figure 4. Didelphis, the extant opossum, a sister to the smaller Ambolestes

Figure 5. Didelphis, the extant opossum, a larger sister to the smaller and 126 million years older Ambolestes.

Now it’s important to remember 
that tooth traits can converge and reverse. Think about archaeocetes (pre-whales), which have three cusps all in a row, like cynodont. Consider odontocetes, which have simple cones, like basal reptiles. Thus, tooth only taxa must be treated separately from skeletal taxa and cladograms must be based on skeletal traits, not tooth traits, which can be dangerous based on the issues that arise from the Bi et al. cladgoram (Fig. 6).

Take another look at taxa listed in Bi et al. 2018
Maelestes, when tested with more taxa, nests at the base of the tenrec/odontoete clade. Necrolestes nests with the golden mole, Chrysochloris, a basal member of Glires. Zhangeotherium is a basal pangolin. They (Bi et al. and other basal mammal workers) are going to have to expand their taxon lists to include at least all the mammals that don’t have hooves, and that includes a few that do have quasi-hooves, like the descendants of Maelestes.

FIgure 8. Cladogram published in Bi et al. 2018 with colors added to show taxa appearing elsewhere in the LRT. As you can see, this is a mess, likely created by too much emphasis on teeth traits, which converge and reverse.

FIgure 6. Cladogram published in Bi et al. 2018 with colors added to show taxa appearing elsewhere in the LRT. As you can see, this is a mess, likely created by too much emphasis on teeth traits, which converge and reverse. Treeshrew-like Maelestes, when tested with more taxa, nests at the base of the tenrec/odontoete clade. Necrolestes nests with the golden mole, Chrysochloris, a basal member of Glires. Zhangeotherium is a basal pangolin.

References
Bi S-B, Zheng X-T, X, Wang X-L, Cignetti N-E, Yang SL and Wible JR 2018.
An Early Cretaceous eutherian and the placental/marsupial dichotomy. Nature (advance online publication) DOI: https://doi.org/10.1038/s41586-018-0210-3
https://www.nature.com/articles/s41586-018-0210-3
Mohamed R. 2018. Anatomical and radiographic study on the skull and mandible of the common opossum (Didelphis marsupialis Linneaus, 1758 (in the Caribbean). Veterinary Sciences 5(44) 10 pp. doi:10.3390/vetsci5020044

https://carnegiemnh.org/press/new-mammal-fossil-provides-insights-on-early-placental-mammal-evolution/

 

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Cifelliodon: new echidna ancestor from the Early Cretaceous of Utah

This one seemed pretty obvious from the first impression,
but failed to make the same impression on the original authors (Huttenlocker et al., 2018).

Usually mammal teeth are found without a skull.
Huttenlocker et al., 2018 found a skull largely without teeth (most don’t erupt beyond the rim of the very few alveoli), certainly a derived trait for mammals. And this is one more way tetrapods became toothless. They named their new taxon, Cifelliodon wahkarmoosuch (Fig. 1).

Figure 1. Early Cretaceous Cifelliodon is ancestral to the living echidna, Tachyglossus according to the LRT. The lack of teeth here led to toothlessness in living echidnas. The skull of Tachyglossus is largely fused together, lacks teeth and retains only a tiny lateral temporal fenestra (because the jaws don't move much in this anteater. Compared to Cifelliodon the braincase is greatly expanded, the lateral arches are expanded and the two elements fuse, unlike most mammals.

Figure 1. Early Cretaceous Cifelliodon is ancestral to the living echidna, Tachyglossus according to the LRT. The reduced number and size of teeth here led to toothlessness in the living echidna. The skull of Tachyglossus retains only a tiny lateral temporal fenestra (because the jaws don’t move much in this anteater. Compared to Cifelliodon the braincase is greatly expanded, the lateral arches are expanded and the two elements fuse, unlike most mammals.

According to Wikipedia:
“Cifelliodon is an extinct genus of haramiyid mammal from the Lower Cretaceous of North America. It is a mammaliaform, and is one of the latest surviving haramiyids yet known, belonging to the family Hahnodontidae. Its discovery led to the proposal to remove hahnodontids from the larger well-known group, the multituberculates.”

As usual the LRT recovered a different nesting.

Figure 2. Cifelliodon skull in three views, plus DGS, plus the original drawing, which is not very accurate.

Figure 2. Cifelliodon skull in three views, plus DGS, plus the original drawing, which is not very accurate. A mandible is restored here.

Figure 3. Subset of the LRT focusing on Monotremes, now including Cifelliodon.

Figure 3. Subset of the LRT focusing on Monotremes, now including Cifelliodon.

Here
in the large reptile tree (LRT, 1233 taxa) Cifelliodon wahkarmoosuch from the Early Cretaceous of Utah nests strongly with Tachyglossus (Figs. 1, 4, 5), one of the extant egg-laying echidnas, currently restricted to Australia and surrounding islands. Tachyglossus was tested in the (Huttenlocker et al. analysis of basal mammal relationships, but the two taxa did not nest together.

Unfortunately Huttenlocker et al., 2018
experienced taxon exclusion problems that nested Cifelliodon with the distinctly different wombat Vintana and those two with the distinctly different multituberculates all more primitive than monotremes.

To their credit
Huttenlocker et al. linked this North American taxon with others from Gondwana which includes Australia, which broke off 99 mya, 40 million years after the appearance of Cifelliodon in Utah. In an interview for USC, Huttenlocker reported, “Most of the fossil record of early mammal relatives is based on teeth. Cifelliodon is unique in that it is one of the only near-complete skulls of a mammal relative from the basal Cretaceous of North America and is the only fossil of early mammal relatives from this time interval in Utah.”

“The fact that the skull looked so primitive compared to other known mammal groups from the Cretaceous made figuring out its relationships extremely difficult. It shows some unique dietary specializations that are seen in only a handful of groups that lived during the age of dinosaurs. Ultimately, the structure of the preserved molars showed clear similarities to some neglected fossil teeth from Northern Africa. So we think that Cifelliodon represents an archaic offshoot whose relatives may have dispersed into the southern continents and became fairly successful during the Cretaceous.”

Figure 3. Tachyglossus skeleton, manus and x-rays. Note the perforated pelvis.

Figure 4. Tachyglossus skeleton, manus and x-rays.

The skull of Tachyglossus
retains only a tiny lateral temporal fenestra (because the jaws don’t move much in this anteater. Compared to Cifelliodon the braincase is greatly expanded, the lateral arches are expanded and the two elements fuse, unlike most mammals.

Figure 1. The echidna (genus: Tachyglossus) in life. This slow-moving spine-covered anteater has digging claws.

Figure 5. The echidna (genus: Tachyglossus) in life. This slow-moving spine-covered anteater has digging claws. Many of the derived traits seen here developed during the last 100 million years since Cifelliodon.

 

References
Huttenlocker AD, Grossnickle DM, Kirkland JI, Schultz JA and Luo Z-X 2018. Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature Letters  Link to Nature

wiki/Cifelliodon

https://news.usc.edu/143411/why-you-should-care-about-this-130-million-year-old-fossil/

Tachyglossus, the other egg-laying mammal

Figure 1. The echidna (genus: Tachyglossus) in life. This slow-moving spine-covered anteater has digging claws.

Figure 1. The echidna (genus: Tachyglossus) in life. This slow-moving spine-covered anteater has digging claws.

Tachyglossus aculeatus (Shaw 1792) is the echidna and the only other genus of egg-laying mammal. It protects itself with sharp spines and has a long, ant-catching tongue. The hands and feet are adapted to digging with short, almost immobile proximal elements (Fig. 3) and long claws. Prepubic bones precede the pubis. A proximal process sits atop the fibula. The leathery snout without whiskers is sensitive to vibrations.

Figure 2. The skull of Tachyglossus is largely fused together, lacks teeth and has no lateral temporal fenestra (because the jaws don't move much in this anteater.

Figure 2. The skull of Tachyglossus is largely fused together, lacks teeth and has no lateral temporal fenestra (because the jaws don’t move much in this anteater. Hard to find sutures here. Let me know if you have better data to make corrections.

Distinct for its sister,
Ornithorhynchus, and many other mammals, the acetabulum is perforated. The lateral temporal fenestra is absent. So are the teeth. Like the hedgehog, the echidna can roll itself into a ball for protection.

Figure 3. Tachyglossus skeleton, manus and x-rays. Note the perforated pelvis.

Figure 3. Tachyglossus skeleton, manus and x-rays. Note the perforated pelvis.

There are those
who say characters define a taxon. We have to get away from that hypothesis. Here a perforated acetabulum would make Tachyglossus a dinosaur, to the late Larry Martin’s delight. Tachyglossus has no temporal fenestra. So, does that make it an anapsid? No. The only thing that tells us what a taxon is… is its placement on a wide gamut cladogram that tests hundreds of candidate sister taxa and hundreds of traits. Testing a suite of several hundred traits in a wide gamut study is the only way to confidently determine taxonomy and avoid the pitfalls of convergence and taxon exclusion that plague smaller studies that too often fail to minimize false positives and ‘by default’ nestings. And some DNA studies cannot be validated, except by morphological studies.

References
Shaw G 1792. Musei Leveriani explicatio, anglica et latina.

wiki/Tachyglossus

Basal mammals: Guess what they evolved to become.

Can you guess
(or do you know) which of these taxa evolved to become a human? a killer whale? a rabbit? a giraffe? a bat? a pangolin?

Figure 1. Can you guess which of these taxa evolved to become a human? a killer whale? a rabbit? a giraffe?

Figure 1. Can you guess which of these taxa evolved to become a human? a killer whale? a rabbit? a giraffe?

H. Onychodectes – basal to all large herbivorous mammals, including giraffes.

G. Maelestes – basal to tenrecs and toothed whales.

F. Tupaia – basal to the gnawing clade including rodents and rabbits.

E. Ptilocercus – basal to Primates, including humans (but note the loss of all premaxillary teeth in this extant taxon).

D. Palaechthon – basal to flying lemurs, bats and pangolins.

C. Monodelphis – basal to all placental mammals.

B. Asioryctes – basal to Monodelphis and all placental mammals.

A. Eomaia – basal to all therian mammals (placentals + marsupials).

These are the basalmost taxa
in various clades of Eutherian (placental) mammals. Not a lot of difference to start (which makes scoring difficult). So much potential at the end. Eomaia goes back to the Early Cretaceous, so it’s not difficult to imagine the radiation of these taxa throughout the Cretaceous.

This falls in line with
the splitting of the African golden mole (Chrysochloris) from its South American sister, Necrolestes, a diversification, migration and split that had to happen before Africa split from South American in the Early Cretaceous.

Sharp-eyed readers
will note the re-identification of bones and teeth in Palaechthon, Ptilocercus and Tupaia. It’s been a long weekend trying to figure out long-standing problems in this portion of the LRT. Some of these taxa were some of the first studied and my naiveté was the source of the earlier disinformation, now corrected. If you see any errors here, please advise and, if valid, repairs will be made.

There’s nothing special about Henosferus

The incisors are not too big
or weird or crowded (Fig. 1), the canine just rises above the rest of the teeth, there are only 5 premolars all standard-shaped, and only three molars, all standard-shaped. The dentary definitely formed the main jaw joint and the post-dentary bones must have been tiny.

Figure 1. Henosferus mandible restored by Rougier et al. 2005 from several broken specimens.

Figure 1. Henosferus mandible restored by Rougier et al. 2005 from several broken specimens.

…and that’s why
Henosferus ( Rougier et al. 2007; Middle Jurassic) makes a good candidate for basalmost mammal. There are too few traits here to add it to the large reptile tree (LRT). Frankly, I’m eyeballing this restoration. It compares well with Juramaia (Fig. 2) without the odd molars and incisors. 

Figure 2. Juramaia (Late Jurassic, 160 mya) is more completely known and nests between monotremes and therians (marsupials + placentals).

Figure 2. Juramaia (Late Jurassic, 160 mya) is more completely known and nests between monotremes and therians (marsupials + placentals).

Henosferus is traditionally considered
a member of the Australosphenida, a group of mammals that include monotremes, and other taxa known chiefly from scraps. Vincelestes sometimes makes this list, but in the LRT it nests as a carnivorous marsupial.

References
Luo Z-X, Yuan C-X, Men Q-J and JiQ 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445. doi:10.1038/nature10291.
Rougier, GW, Martinelli AG, Forasiepi AM and Novacek M J 2007. New Jurassic mammals from Patagonia, Argentina : a reappraisal of australosphenidan morphology and interrelationships. American Museum novitates, no. 3566. online here.

wiki/Juramaia
wiki/Henosferus

Figuring out the upside-down skull of Yanoconodon

Figure 1. Yanoconodon fossil in situ. See the skull in closeup in figure 2.

Figure 1. Yanoconodon fossil in situ. See the skull in closeup in figure 2. The published tracing is distorted here to match the underlying photo.

Wikipedia reports, “Yanoconodon was a small mammal, barely 5 inches (13 centimetres) long. It had a sprawling posture, Yanoconodon was a Eutriconodont, a group composing most taxa once classified as “triconodonts” which lived during the time of the dinosaurs. These were a highly ecologically diverse group, including large sized taxa such as Repenomamus that were able to eat small dinosaurs, the arboreal Jeholodens, the aerial volaticotherines and the spined Spinolestes. Yanoconodon is inferred to be a generalized terrestrial mammal, capable of multiple forms of locomotion.

Figure 1. Yanoconodon is exposed in ventral view. Even so, if you employ DGS, even on a fuzzy photo, you can put together a reconstruction that shares several traits with Repenomamus.

Figure 2. Yanoconodon is exposed in ventral view. Even so, if you employ DGS, even on a fuzzy photo, you can put together a reconstruction that shares several traits with Repenomamus.

Mammal-like reptiles?
Wikipedia also reports, “The Yanoconodon holotype is so well preserved that scientists were able to examine tiny bones of the middle ear. These are of particular interest because of their “transitional” state: Yanoconodon has fundamentally modern middle ear bones, but these are still attached to the jaw by an ossified Meckel’s cartilage. This is a feature retained from earlier stem mammals, and illustrates the transition from a basal tetrapod jaw and ear, to a mammalian one in which the middle ear bones are fully separate from the jaw. Despite this feature Yanoconodon is a true mammal. It is thought that the feature was retained during early embryo development,[4] whereas it is lost in most other mammal groups. The intermediate anatomy of the middle ear of Yanocodon is said to be a “Rosetta Stone”[5] of mammalian middle ear evolution.”

In the large reptile tree (LRT, 1037 taxa) Yanoconodon, Repenomamus, Jeholodens and Spinolestes are not mammals, but very close to the base of the Mammalia. Both clades share Pachygenelus as last common ancestor. So that means the ‘transitional state’ mentioned above is indeed outside the Mammalia. Other paleontologists consider this list of taxa to be mammals, but here the mammal-like traits they had were developed in parallel and not quite to mammal standards.

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.

The skull of Yanoconodon
(Fig. 2) can be largely, but not completely, reconstructed based on the visible bones. The skull is low and wide and without the typical constriction anterior to the jugals. The anterior teeth are large and spike-like while the posterior teeth are molariform. Large teeth typically require deep roots and deep bones to house those roots. The mandibles are as long as the skull. The small orbits are far forward on the skull and the temporal fenestra are correspondingly large.

Figure 2. The origin and radiation of stem mammals and crown mammals. Compare the LRT tree (above) to a recent cladogram by Close et al. 2015.

Figure 2. The origin and radiation of stem mammals and crown mammals. Compare the LRT tree (above) to a recent cladogram by Close et al. 2015.

With the new data on Yanocondon
several taxa within the LRT shifted places, but not far and still within the derived Cynodontia. Something about the Mammalia helped them survive several extinction events that the derived Tritylodontia (= Pseudomammalia) succumbed to. Pseudomammalia LOOK like mammals, but are not mammals. They continued to exist into the Early Cretaceous and some, like Repenomamus, were quite large.

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. 
Luo Z, Chen P, Li G, and Chen M 2007.
 A new eutriconodont mammal and evolutionary development in early mammals. Nature 446:15. online Nature

wiki/Yanoconodon

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

Figure 1. Brasilodon nests with Sinoconodon as a stem mammal.

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. Brasilitherium compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

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

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

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.
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