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).
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.
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).
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:
- absent postorbital arch, postorbital, and prefrontal;
- a slender zygomatic arch with a long jugal and short squamosal;
- a dorsoventrally short parietal crest and transversally wide braincase;
- interpterygoid vacuity;
- ventral contact of the frontal with the orbital process of the palatine;
- an unfused lower jaw symphysis;
- a well-developed articular process of the dentary contacting the squamosal;
- and a petrosal promontorium.
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:
- a slender zygomatic arch
- squamosal dentary contact
- unfuseddental symphysis
- petrosal promontorium
- transversely narrow postcanines with axially aligned cusps and an incipient cingulum
- and a transversely expanded brain case
- Therioherpetidae lack procumbent first lower incisors occluding between the first upper incisors
- lack an edentulous tip of the premaxilla
- and lack transversely widened postcanines
According to the Bonaparte team
Three distinct groups have been included in Mammaliformes.
- Morganucodon, Megazostrodon and Sinoconodon;
- Haramiyids such as Haramiyavia
“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.
- Interpterygoid vacuities (present only in juvenile probainognathids);
- a slender zygomatic arch;
- incisiforms present at the junction of premaxilla and maxilla;
- a low and elongated dentary;
- 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.
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.