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

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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.

 

Is Phascolotherium a basal mammal? Perhaps not…

Yesterday we looked at the origin of mammals and noted the Rowe 1988 considered the fossil mandible Phascolotherium bucklandi (Middle Jurassic; Owen 1838; Fig. 1) one of the earliest known mammals. Unfortunately, the mandible specimen does not have enough traits to nest Phascolotherium in the large reptile tree (LRT, 1011 taxa) with complete resolution.

Nevertheless,
that doesn’t stop one from visually comparing Phascolotherium to more complete taxa.

Figure 1. Phacolotherium compared to the tritylodontid Jeholodens.

Figure 1. Phacolotherium compared to the tritylodontid Jeholodens. The smaller Jeholodens image is to scale with the much larger Phacolotherium specimen.

There’s a pretty good match for Phascolotherium
with Jeholodens  (Ji et al. 1999); non-mammalian cynodont/tritylodontid/mammaliaform known from the Middle Cretaceous. The Jeholodens mandible is smaller than Phascolotherium, It has an unerupted 4th molar, which would indicate immaturity if it was a mammal. Only mammals do not replace molars.

Of Jeholodens
Ji et al. 1999 reported, “The postcranial skeleton of this new triconodont shows a mosaic of characters, including a primitive pelvic girdle and hindlimb but a very derived pectoral girdle that is closely comparable to those of derived therians. Given the basal position of this taxon in mammalian phylogeny, its derived pectoral girdle indicates that homoplasies (similarities resulting from independent evolution among unrelated lineages) are as common in the postcranial skeleton as they are in the skull and dentition in the evolution of Mesozoic mammals.”

There’s a postscript
Take another look at the mandible of Jeholodens (Fig. 1). Note the giant incisor 1 and the robust jaw articulation. Where else do we see this combination in small mammals? In Multituberculata and Haramiyidae, but both nest with rodents, plesiadapids and carpolesteids in the LRT. Traditional cladograms nest Multituberculata and Haramiyidae either before Mammalia or between monotremes and therians as very basal mammals. I have long wondered, if this was so, which basal pre-mammals or mammals look most like multituberculates and might therefore be most closely related? Could it be Jeholodens? So it was time for a test. Shifting the multituberculates to Jeholodens currently adds 34 steps to the LRT. Let’s see what happens when multis are re-scored with prejudice toward Jeholodens

So what happened?
The multis and haramiyids did not shift. The tree topology did not change. Apparently any resemblance between Jeholodens and these two clades must have been by convergence. Or the amount of convergence is overtaking the true relationship. Since everything in science is provisional we’ll keep testing.

References
Ji Q, Luo Z and Ji S 1999. A Chinese triconodont mammal and mosaic evolution of the mammalian skeleton. Nature 398:326-330. online.
Owen R 1838
. On the jaws of the Thylacotherium prevostii (Valenciennes) from Stonesfield. Proceedings of the Geological Society of London 3, 5–9.

The Origin of Mammals: Rowe 1988

Rowe 1988
provided a list of skeletal traits found in mammals not found in their proximal outgroups. Here they are broken down into digestible categories. Noteworthy are the many traits associated with improvements and refinements to hearing and smelling. Noticeable by their absence are any dental traits.

Facial

  1. Premaxilla internasal process absent (external nares confluent
  2. Ethmoid and maxillary turbinals ossified
  3. Internasal septum ossified
  4. Ossified quadratojugal absent
  5. Sclerotic ossicle absent

Palatal

  1. Ectotympanic horizontal (former reflected lamina rotates from vertical
  2. Squamosal suspensorial notches absent – sites of former connections to quadrate and quadratojugal
  3. Cribiform plate (ethmoid ossifies below olfactory bulbs)
  4. Pterygoid transverse process vestigial (muscles now fill the gap)
  5. Tegmen tympani (thin plate of bone spread over the cochlear capsule forming a new side wall for the cranium)
  6. Hyoid arch evolves to become petrosal bridge

Occipital

  1. Occipital condyles expanded upwards and laterally, far apart from one another
  2. Hindbrain greatly expanded overlies fenestrae vestibuli
  3. Paroccipital process directed ventrally (no longer sloping ventrolaterally)
  4. Pneumatic mastoid process (no longer solid)
  5. Styloid process – no longer a separate bone, the stylohyal fuses the otic capsule, joining the paroccipital process
  6. Craniomandibular joint positioned anterior to fenestra vestibuli (hearing organ opening)

Mandible

  1. Craniomandibular joint formed only by squamosal and dentary
  2. Meckelian sulcus (trough) enclosed – when open it held the post dentary elements
  3. Coronoid bone vestigial or absent – the dentary takes over
  4. Splenial vestigial or absent
  5. Articular, prearticular, surangular and angular suspended from the skull (as tiny ear bones and ectotympanic bulla respectively)

Axial

  1. Proatlas not ossified
  2. Atlas intercentrum and neural arches fused to form one ring-like bone, vertebra #1.
  3. Atlas rib absent (actually fused to the atlas)
  4. Axis prezygopophyses absent
  5. Postaxial cervical ribs fused to vertebrae

Appendicular

  1. Styloid processes on dstial ends of radius, tibia and fibula
  2. Patella present along with patellar facet on femur
  3. Entocuneiform–Hallucial (distal tarsal 1 and m1.1) articulation saddle-shaped permitting greater mobility
  4. Secondary ossifications on long bones and girdles – ossified joints
  5. Flexor sesamoids

Under this guidance
and prior to the use of software in cladisitic analysis Rowe 1988 indicated that
“Morganucodontidae, Kuehneotherium, Dinnetherium, Sinoconodon and Haramiyidae can no longer be considered mammals.” In Rowe’s tree Multituberculata nest between monotremes and metatherians. (Contra Novacek 1997, who nested that clade outside the Mammalia.)

The LRT does not agree with parts of this topology
In the LRT haramiyidans nest with multituberculates, both with rodents. There are no pre-rodent, pre-placental or pre-mammal taxa with such derived traits. Attempts to put a cynodont-like middle ear on the multituberculate Megaconus are largely the product of hope, bias and imagination, not data.

Remember
Living monotremes have tiny ear bones below and internal to the mandible, distinct from placentals and marsupials that have tiny ear bones just posterior the jaw joint. This indicates that monotremes had a separate, but convergent (parallel) evolutionary history with regard to the tiny ear bones. In the LRT. Kuehneotherium nests at the base of the monotremes and thus within Mammalia, at its base. Based on the very derived character of all monotremes, including Kuehneotherium, the very first mammals had a much earlier origin.

According to Rowe 1988
Phascolotheriium bucklandi
(Middle Jurassic, Owen 1838, Clemens et al. 1977) is the oldest known mammal.  Amphitherium (de Blainville 1838) is from the same strata. Both were discovered within the first few decades of modern British paleontology. Unfortunately there are not enough traits in Phascolotherium to nest it in the LRT without massive loss of resolution.

References
Butler P M Clemens, W. A. (2001). Dental Morphology of the Jurassic Holotherian Mammal Amphitherium, with a Discussion of the Evolution of Mammalian Post-Canine Dental Formulae. Palaeontology. 44 (1): 1–20.
Novacek MJ 1997. Mammalian evolution: An early record bristling with evidence. Current Biology 7(8):pR489–R491. DOI: http://dx.doi.org/10.1016/S0960-9822(06)00245-4 
Owen R 1838.
On the jaws of the Thylacotherium prevostii (Valenciennes) from Stonesfield. Proceedings of the Geological Society of London 3, 5–9.
Rowe T 1988.
Definition, diagnosis, and origin of Mammalia. Journal of Vertebrate Paleontology 8(3):241-264.

Vintana and the vain search for the clades Allotheria and Gondwanatheria

Figure 1. Vintana as originally illustrated. I added colors to certain bones. Note the high angle of the ventral maxilla and the deep premaxilla. Lateral view reduced to scale with other views.

Figure 1. Vintana as originally illustrated. I added colors to certain bones. Note the high angle of the ventral maxilla and the deep premaxilla. Lateral view reduced to scale with other views.

Earlier we looked at Vintana (Fig. 1, Krause et al. 2014a, b). To Krause et al. Vintana represented the first specimen in the clades Allotheria and Gondwanatheria to be known from more than teeth and minimal skull material.

To Krause et al. 
Allotheria included Multituberculata and nested between the clade Eutriconodonta (including Repenomamus and Jeholodens) and the clade Trechnotheria (including the spalacotheres Maotherium and Akidolestes) and Cronopio, Henkelotherium, Juramaia, Eomaia, Eutheria and Metatheria.

Taxon exclusion issues
The large reptile tree (LRT, 1005 taxa) did not recover the above clades or relationships. Alotheria does not appear in the LRT.

  1. Multituberculata, Henkelotherium and Maotherium nest within Glires (rats and rabbits and kin) in the LRT.
  2. Repenomamus and Jeholodens nest within the pre-mammalian trityllodontid cynodonts in the LRT.
  3. Akidolestes nests within basal Mammalia, close to Ornithorhynchus in the LRT.
  4. Cronopio and Juramaia nest within basal Mammalia between Megazostrodon and Didelphis in the LRT.
  5. Eomaia nests at the base of the Metatheria in the LRT.
  6. Vintana nests with Interatherium among the derived Metatheria (marsupials), with wombats, like Vombatus and Toxodon in the LRT.

Despite a paper in Nature
and a memoir of 222 pages in the Journal of Vertebrate Paleontology; despite CT scans and firsthand examination with electron microscopes; despite being examined and described by many of the biggest name and heavy hitters in paleontology… Krause et al. never understood that Vintana was just a derived wombat, evidently due to taxon exclusion problems.

Figure 3. Interatherium does not nest with notoungulates or other purported interotheres. Rather cat-sized Interatherium nests with wombats, between Vombatus and the giant Toxodon.

Figure 2. Interatherium does not nest with notoungulates or other purported interotheres. Rather cat-sized Interatherium nests with wombats,with Vintana,  between Vombatus and the giant Toxodon

The large reptile tree now includes
1005 taxa, all candidates for sisterhood with every added taxon. Despite the large gamut of 74 taxa employed by Krause et al. they did not include the best candidates for Vintana sisterhood. Perhaps the fault lies in the reliance of prior studies and paradigms. Perhaps the fault lies in the over reliance by Krause et al. and other mammal workers, on dental traits. Perhaps the fault lies in the absence of pertinent sisters to the above-named taxa, including Interatheriium for Vintana.

In any case
Vintana does not stand alone as the only taxon in its clade represented by skull material. Based on its sisterhood with Interatherium, we have  pretty good idea what its mandibles and post-crania looked like. Yes, Vintana is weird. But Interatherium is also weird in the same way, just not as weird.

The LRT has dismantled and invalidated
several other clades, too, Ornithodira and Parareptilia among them.

References
Krause DW, Hoffmann S, Wible JR, Kirk EC, and several other authors 2014a. First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism. Nature. online. doi:10.1038/nature13922. ISSN 1476-4687.
Krause DW et al. 2014b. Vintana sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology Memoir 14. 222pp.

wiki/Vintana
pterosaur heresies – Vintana

Yanoconodon: Proximal sister to the Mammalia

This post was composed several weeks ago.
After all the intervening excitement I’m glad to bring Yanoconodon to your attention.

Yanoconodon alllini (Luo, Chen, Li and Chen 2007; Early Cretaceous, 122 mya; 13 cm in length; Figs. 1, 2) is known from a nearly complete and articulated crushed fossil. It is traditionally considered a eutriconodont, a clade that traditionally includes Spinoletes, Repenomamus, GobiconodonLiaoconodonJeholodens and Volaticotherium. Unfortunately that clade is paraphyletic in the large reptile tree (LRT). Here Yanoconodon was derived from a sister to Pachygenelus and nested between that clade and the Mammalia (Fig. 3).

Yanoconodon had a semi-sprawling posture
and a a long, robust torso with an unusually thick lumbar vertebrae provided with very short ribs. The limbs were short. The canines were quite narrow. The posterior jaw bones were still attached to the jaw. They had not yet become completely reduced to middle ear bones and completely separated from the jaw bones. So, by definition and cladogram (Fig. 3), Yanoconodon was not a true mammal. Wikipedia disagrees as that author reports, “Despite this feature Yanoconodon is a true mammal.”

See below for some thoughts on that.

FIgure 1. Yanaconodon nests as the proximal outgroup to the Mammalia in the LRT.

FIgure 1. Yanoconodon nests as the proximal outgroup to the Mammalia in the LRT. Even so it has several autapomorphies (differences from the actual  hypothetical ancestor.)

from the Luo, Chen and Chen abstract
“Detachment of the three tiny middle ear bones from the reptilian mandible is an important innovation of modern mammals. Here we describe a Mesozoic eutriconodont nested within crown mammals (1) that clearly illustrates this transition: the middle ear bones are connected to the mandible via an ossified Meckel’s cartilage. The connected ear and jaw structure is similar to the embryonic pattern in modern monotremes (egg-laying mammals) and placental mammals, but is a paedomorphic feature retained in the adult, unlike in monotreme and placental adults. This suggests that reversal to (or retention of) this premammalian ancestral condition is correlated with different developmental timing (heterochrony) in eutriconodonts. (2) This new eutriconodont adds to the evidence of homoplasy of vertebral characters in the thoraco-lumbar transition and unfused lumbar ribs among early mammals. (3) This is similar to the effect of homeobox gene patterning of vertebrae in modern mammals, making it plausible to extrapolate the effects of Hox gene patterning to account for homoplastic evolution of vertebral characters in early mammals.” (4)

Notes

  1. The LRT nests Yanoconodon just outside the crown mammals. Not sure why the authors say this, given what they report about the posterior jaw bones as posterior jaw bones.
  2. Curious that the retention of “this pre-mammalian ancestral condition” does not indicate to the authors that Yanoconodon is indeed a pre-mammal.
  3. Yanoconodon does not nest as an early mammal in the LRT.
  4. …or…not, if Yanoconodon is indeed a non-mammalian trithelodont. Other non-mammalian cynodonts lived alongside Jurassic mammals. Only one purported eutriconodont listed above is a mammal, Volaticotherium. It nests as a basal placental. Triconodon is a mammal, too, a monotreme known from just a dentary and teeth.
Figure 2. From Luo et al. the posterior jaw bones of Yanoconodon. These are not middle ear bones, so Yanoconodon is not a mammal.

Figure 2. From Luo et al. the posterior jaw bones of Yanoconodon. These are not middle ear bones, so Yanoconodon is not a mammal. The malleus is the articular. The incus is quadrate.

Yanoconodon is a great transitional fossil.
You can call it a mammal, if you want to slightly stretch the current definition. You can call it a mammal if you want to describe it as the last common ancestor of all living mammals.  But as soon as a better transitional candidate comes along, it will be demoted.

Figure 3. Basal mammals begin with Ornithorynchus, the most primitive living mammal. Yanoconodon nests just outside this clade.

Figure 3. Basal mammals begin with Ornithorynchus, the most primitive living mammal. Yanoconodon nests just outside this clade.

References
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