Eutherian phylogeny and niches

Over the past two weeks
I’ve been attracted to poor Bootstrap scores in the large reptile tree (LRT, 1151 taxa, subset Fig. 1) reexamining data and re-scoring where necessary. The result is a tree with improved Bootstrap scores. Herewith, the eutherian (placental) mammal subset of the LRT.

Figure 1. Subset of the LRT focusing on eutherian mammals. Colors refer to niches.

Figure 1. Subset of the LRT focusing on eutherian mammals. Colors refer to niches.

Sharp-eyed readers
will find the one node that is not resolved in this tree. Hint: the specimens lacking resolution are known from damaged skulls and a few post-cranial bones, so they can be scored for a relatively few character traits.

Curious readers
seeking more information for any genera listed above need only use it for a keyword in the search feature of this blog post (above).

Even though
the present tree has been improved, there is still room for improvement, probably around the weaker Bootstrap scores.

 

Heuristic testing
of just the basal tetrapods and lepidosauromprhs (370 taxa, 1 tree) took less than 51 seconds for a completely resolved subset of the LRT. Testing of just the archosauromorphs (781 taxa, 2 trees) took 8:45 minutes of computing time. So, 410 more taxa and one more tree take more time.

Taking it to the final step: Testing of the entire LRT (1151 taxa, 14 trees) took 1 hour 50 minutes. You can see computing time rises exponentially with increasing taxa, even with the next best thing to complete resolution.

So where did those 12 extra trees come from?
Should be from no more than 3 unresolved nodes. Here’s where PAUP fails (or becomes exhausted) with high taxon numbers:

Basalmost Synapsida  (Ellioitsmithia, Apsissaurus, Aerosaurus, etc.), Lepidosauria/Sphenodotia/Marine Younginiformes/Diadectomorpha + Pareiasauria/ Caseasauria/Basal Lepidosauromorpha/ Basal Archosauromorpha/ Basal Diapsida/ 13 more little clades/15 single taxa and…Hypuronector/Vallesaurus/Megalancosaurus

So with all those problems
(way more than expected) I ran PAUP again, sans mammals and terrestrial younginiforms (including protorosaurs and archosauriforms): so…. basically all the primitive taxa were included. Result: 565 taxa, 2 trees) took 5:54 minutes with loss of resolution between (Megazostrodon + Hadrocodium) and (Brasilitherium + Kuehneotherium), three of which are skull-only taxa just outside of the deleted mammals. No other tree topology changes are recovered.

Just so you know…
it seems that PAUP does exhaust itself in large cladograms, even in a simple Heuristic search.

 

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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.

Harpagolestes uintensis is a mesonychid. Harpagolestes macrocephalus is not.

Welcome to the wonderful world of convergence!
Harpagolestes uintensis (Fig. 2) and H. macrocephalus (Fig. 1) look similar enough to be considered similar, but they are not congeneric in the LRT. One of them needs a new generic name.

Of the several heresies
recovered by the large reptile tree (LRT, 1120 taxa) the latest is the separation of some former mesonychids (Fig. 1, Andrewsarchus, Sinonyx, Hapalodectes) from current and traditional mesonychids (Fig. 2, Mesonyx and Harpagolestes uintensis). The clade of former mesonychids now nests as giant tenrecs. This clade produced odontocete whales and transitional taxa. The latter group of true mesonychids gave rise to mysticete (baleen) whales and the following transitional taxa: hippos, anthrobunids and desmostylians.

Figure 1. Harpagolestes macrocephalus compared to sisters Sinonyx and Andrewsarchus to scale.

Figure 1. Harpagolestes macrocephalus compared to sisters Sinonyx and Andrewsarchus to scale.

Today the addition of Harpagolestes macrocephalus
(Fig. 1) to the LRT nests it not congenerically with Harpagolestes uintensis (Fig. 2), but between Andrewsarchus and Sinonyx. So the two are not congeneric.

Figure 1. Andrewsarchus, Sinonyx, Mesonyx and Harpagolestes to scale for direct comparison of these two tenrecs with these two mesonychids.

Figure 1. Andrewsarchus, Sinonyx, Mesonyx and Harpagolestes to scale for direct comparison of these two tenrecs with these two mesonychids.

We’ve seen convergence many times
in the LRT. This is just one more example of convergence that has been traditionally overlooked.

Deleting nine tenrecs
on either side of Sinonyx + Andrewsarchus + H. macrocephalus changes nothing in the LRT. The above taxa still nest with odontocetes far from mesonychids through mysticetes, though some loss of resolution occurs in the mammal subset of the LRT.

Deleting hippos and anthracobunids
from the mesonychid clade changes nothing.

References
O’Leary MA and Rose KD 1995. Postcranial skeleton of the early Eocene mesonychid Pachyaena (Mammalia: Mesonychia). Journal of Vertebrate Paleontology 15(2):401-430.

Vilevolodon: the atavistic reappearance of post-dentary bones

Preface
I’ve been wondering about the traditional nesting of Multituberculata and kin outside of the Mammalia for years. All have a dentary jaw joint, but some have post-dentary bones. given the opportunity multituberculates nest with rodents and plesiadapiformes in the large reptile tree (LRT, 1047 taxa).  No other pre-mammals resemble them. Traditionally Haramiyava (Fig. 1) has been considered a pre-mammal link to Haramiyida + Multituberculata. In the LRT Haramiyava nests with the mammaliaforms Brasilodon, Sinoconodon and Therioherpeton – far from any other taxa considered Haramiyida + Multituberculata currently and provisionally nesting deep within the Mammalia.

Figure 1. Haramiyavia reconstructed and restored. Missing parts are ghosted. The fourth maxillary tooth appears to be a small canine. The post-dentary bones are imagined from Vilevolodon (figure 4).

Vilevolodon diplomylos
(Luo et al. 2017; Jurassic, 160 mya; BMNH2942A, B; Figs. 2-4) was originally considered a stem mammal (= mammaliaform), a eleutherodontid in the clade Haramiyida AND it had clearly defined gliding membranes (Fig. 2). By contrast the LRT nests Vilevolodon with the Late Jurassic para-rodent Shenshou and the extant rodents, Rattus and Mus, not far from members of the Multituberculata.

Figure 1. Vilevolodon in situ, plate, counterplate, original drawing, DGS color, and restored manus and pes. Note the gliding membrane (patagium) and fur.

Figure 2. Vilevolodon in situ, plate, counterplate, original drawing, DGS color, and restored manus and pes. Note the gliding membrane (patagium) and fur.

But there’s a big problem
Vilevolodon doesn’t have tiny ear bones, like mammals do. It has post-dentary bones, like pre-mammals do (Figs. 3, 4).

Figure 2. Vilevolodont skull in situ, without color, DGS color tracing, that tracing reconstructed and a CT scan form Luo et al. 2017.

Figure 3. Vilevolodont skull in situ, without color, DGS color tracing, that tracing reconstructed and a CT scan form Luo et al. 2017.

The ear problem in Jurassic rodents
Luo et al. report, “a mandibular middle ear with a unique character combination previously unknown in mammaliaforms.” Pre-mammals have post-dentary bones (articular, angular, surangular). Therian mammals shrink and migrate those bones to the base of the skull where they become middle ear bones with new names (malleus, incus, ectotympanic). The stapes remains the stapes in all tetrapods. So what is happening with Vilevolodon and its sisters? Why don’t the pre-mammal post-dentary bones define it as a pre-mammal? After all, that’s the current paradigm.

Figure 3. There is no doubt that Vilevolodon has a pre-mammal type of posterior jaw bones. Otherwise they nest with rodents and plesiadapiformes. This appears to be a mammal with an atavism, a reversal. These elements simply stopped developing as in other mammals.

Figure 3. There is no doubt that Vilevolodon has pre-mammal type post-dentary bones. There is also no doubt that the dentary formed the main jaw joint with the squamosal. How does one reconcile both sets of traits? In the LRT Vilevolodon nests with rodents. This appears to be a mammal with an atavism, a reversal. These elements simply stopped developing as in other mammals.

Mammals are defined by
the evolution and migration of their posterior jaw bones into middle ear bones with a jaw joint switch from quadrate/articular to dentary/squamosal. Multituberculates and haramiyids appear to bend or break that rule because they have cynodont-like posterior jaw bones, not tiny middle ear bones, and yet otherwise they nest with rodents and plesiadapiformes. This is one reason why you don’t want to pull a Larry Martin with post-dentary bones. You want to nest a taxon based on a long list of traits, not just one, two or a dozen.

The massive jaw joint
Mammals, such as Vilevolodon, with atavistic post-dentary bones also have a massive jaw joint with a long articulating surface on the dentary contacting the squamosal. All mammals have such a jaw joint. Pre-mammals don’t. While Vilevolodon has a large dentary/squamosal jaw joint, the post-dentary articular, still contacts the quadrate. It’s clearly not the main jaw joint.

Filan 1991
traced the development of post-dentary bones in embryonic Monodelphis specimens. She reported, “Neonates of Monodelphis possess neither mammalian (dentarysquamosal) nor reptilian (quadrate-articular) jaw articulations, nor does the contact between the incus and crista parotica offer a joint surface. Elasticity in Meckel’s cartilage allows minimal deflection of the lower jaw.” After all, those neonates are just sucking milk, not biting, and the embryos don’t even do that. Does that make neonates like this not mammals? No. The evidence indicates that in multituberculates and haramiyds the embryological transformation of posterior jaw bones stopped before development transformed them into middle ear bones. This is an atavism, a phylogenetic reversal. The timing of development changed. In the case of Vilevolodon, the middle ear bones stop evolving during embryological development and the post-dentary bones they would have evolved from continue to appear in adults. What was a rare mutation probably spread throughout an isolated population. Perhaps this had something to do with the increase in size of the dentary jaw joint.

Haramiyavia and the Haramiyida clade
Seems at this point that only Haramiyavia is a haramiyid, unless Brasilodon is one as well. Members traditionally assigned to the clade Eleutherodontidae also nest in various locations in the LRT, not all in one clade.

Meng et al. 2017 report,
“Stem mammaliaforms are morphologically disparate and ecologically diverse in their own right, and they developed versatile locomotor modes that include arboreal, semiaquatic, and subterranean specializations, which are all distinct from generalized mammaliaforms.” Unfortunately, the LRT nests a long list of mammaliaforms at various nodes within the Mammalia. They are not from a single diverse clade.

Contra Meng et al. 2017
the LRT reduces the niches and body shapes of stem mammals down to a few small, generalized taxa like Sinoconodon and Megazostrodon. Derived taxa nest at derived nodes.

The LRT nests rodents close to Plesidapiformes,
including the extant aye-aye, Daubentonia as first reported here. So it comes as no surprise when Luo et al. report, “Eleutherodontids show a marked similarity to the primate Daubentonia in the ventrally bent rostrum and deep mandible, and both features are interpreted to be reinforcement for incisor gnawing.” That’s the case only with Vilevolodon this time. Others may be by convergence.

Molars
The jaw joint of the rodent allows for rostral-caudal and dorsal-ventral motion of the jaws. Luo et al. report, in Villevolodon it is not possible for the mandible to move posteriorly or horizontally, but their images show a continuous anteroposterior trough/furrow in the three molars, though not to the extent seen in sister taxon Shenshou. Molars with a long and continuous trough for rostral-caudal grinding appear by convergence in several reptile/mammal clades.

Incisor replacement
Luo et al. report, “Incisor replacement is prolonged until well after molars are fully erupted, a timing pattern unique to most other mammaliaforms. In rodents incisors never stop growing. The growth pattern in Vilevolodon may be the first step toward that. Not sure why Luo et al. are missing all these strong rodent clues.

Gliding?
Meng et al. 2017 note: “They [Vilevolodon and kin] are the most primitive known gliders in mammal evolution, evolving approximately 100 million years before the earliest known therian gliders.” Earlier, with the appearance of the stem pangolin, Zhangheotherium at the start of the Cretaceous, the ghost lineage for primates, flying lemurs and bats was also set to that time or earlier. Before the advent of flying birds, but after the advent of predatory theropods, many mammals had evidently taken to the trees. And one way to get from tree to tree without descending to the dangerous turf is to jump, glide and fly. I predict we’ll find the big-handed ancestors of bats in Jurassic and Cretaceous strata someday. They are already volant shortly after the K-T extinction event.

Hearing in Vilevolodon
With the reappearance of post-dentary bones in taxa like Vilevolodon, the auditory acuity that was more highly developed in its ancestors must have suffered a setback. By the evidence provided, the massive jaw joint must have been more important for its survival.

Figure 8. Multituberculate Kryobaatar mandible in lateral and medial views. Here post-dentary bones are absent. The malleus (quadrate) and ectotympanic are on the skull.

Figure 4. Multituberculate Kryobaatar mandible in lateral and medial views. Here post-dentary bones are absent here. The malleus (quadrate) and ectotympanic are on the skull.

Getting back to the purported patagium of Maiopatagium
which we looked at yesterday. It is not apparent and the authors do not describe it. Rather, Meng et al. 2017 sidestep this by reporting, “Furthermore, we report a second eleutherodont specimen (BMNH2942) preserved with a halo of carbonized fur and patagial membranes, similar to those of Maiopatagium.” The patagial taxon remains unnamed in the Maiopatagium paper (Meng et al. 2017), but is named in a second paper appearing on the same day. It is today’s subject, Vilevolodon (Fig. 1)

References
Filan SL 1991. Development of the middle ear region in Monodelphis domestica (Marsupialia, Didelphidae): marsupial solutions to an early birth. Journal of Zoology 225(4): 577–588 DOI: 10.1111/j.1469-7998.1991.tb04326.x
Luo Z-X, Meng Q-J, Grossnickle DM, Neander AI, Zhang Y-G and Ji Q 2017. New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem. doi:101.1038/nature 23483\
Meng Q-J, Grossnickle DM, Liu D, Zhang Y-G, Neander AI, Ji Q and Luo Z-X 2017.
New gliding mammaliaforms from the Jurassic. Nature (advance online publication)
doi:10.1038/nature23476
Jenkins FA, Jr, Gatesy SM, Shubin NH and Amaral WW 1997. Haramiyids and Triassic mammalian evolution. Nature 385(6618):715–718.
Luo Z-X, Gatesy SM, Jenkins FA, Jr, Amaralc WW and Shubin NH 2015. Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution. PNAS 112 (51) E7101–E7109.

wiki/Haramiyavia
wiki/Vilevolodon
wiki/Maiopatagium

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.
Bonaparte JF and Crompton AW 2017. Origin and relationships of the Ictidosauria to non-mammalian cynodonts and mammals. Historical Biology. https://doi.

 

Lambdotherium: not a basal brontothere — it’s another pig relative!

Earlier a putative stem brontothere, Danjiangia, was re-nested with basal artiodactyls in the large reptile tree (LRT, 1005 taxa).

Here another putative stem brontothere,
Lambdotherium (Cope 1880, Mader 1998; Eocene, 50mya; Fig. 1) likewise moves away from the basal brontothere, Eotitanops. In the LRT  Lambdotherium nests with Ancodus (Fig. 2), another basal artiodactyl close to extant pigs.

Figure 1. Lambdotherium traditionally nests with the basal brontothere, Eotitanops, but here nests with Ancodus, a basal artiodactyl.

Figure 1. Lambdotherium traditionally nests with the basal brontothere, Eotitanops (ghosted here), but here nests with Ancodus, a basal artiodactyl. Brontotheres have a very tall naris. Pigs do not. 

I don’t know of any post-crania
for Lambdotherium. Note that Ancodus (Fig. 2), like Eotitanops, has a pentadatyl manus. Lambdotherium was traditionally considered a brontothere based on its teeth. The LRT employs relatively few dental traits. And maybe some specimens need to be reexamined. The very high arch of the Lambdotherium squamosal, among many other traits, is more similar to pig-like taxa, than to basal brontotheres, which here nest closer to rhinos, than to horses, contra the Wikipedia report on brontotheres.

Distinct from both rhinos and horses,
brontotheres have four toes on the forefeet. All are derived from a sister to Hyrachyus, which likewise has four toes.

Figure 1. Ancodus nests as a more derived sister to Sus and it retains digit 1 on the manus and pes.

Figure 2. Ancodus nests as a more derived sister to Sus and it retains digit 1 on the manus and pes.

References
Cope ED 1880. The bad lands of the Wind River and their fauna. The American Naturalist 14(10):745-748.
Mader BJ 1998. Brontotheriidae. In Janis CM, Scott KM, and Jacobs LL (eds.), Evolution of Tertiary Mammals of North America 1:525-536.
Mihlbachler MC 2004. Phylogenetic Systematics of the Brontotheriidae (Mammalia, Perissodactyla). PhD dissertation. Columbia University. p. 757.
Mihlbachler MC 2008. Species taxonomy, phylogeny and biogeography of teh Brontotheriidae (Mammalia: Perissodactyla). Bulletin of the American Museum of Natural History 311:475pp.

wiki/Eotitanops
wiki/Lambdotherium