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.
- Premaxilla internasal process absent (external nares confluent
- Ethmoid and maxillary turbinals ossified
- Internasal septum ossified
- Ossified quadratojugal absent
- Sclerotic ossicle absent
- Ectotympanic horizontal (former reflected lamina rotates from vertical
- Squamosal suspensorial notches absent – sites of former connections to quadrate and quadratojugal
- Cribiform plate (ethmoid ossifies below olfactory bulbs)
- Pterygoid transverse process vestigial (muscles now fill the gap)
- Tegmen tympani (thin plate of bone spread over the cochlear capsule forming a new side wall for the cranium)
- Hyoid arch evolves to become petrosal bridge
- Occipital condyles expanded upwards and laterally, far apart from one another
- Hindbrain greatly expanded overlies fenestrae vestibuli
- Paroccipital process directed ventrally (no longer sloping ventrolaterally)
- Pneumatic mastoid process (no longer solid)
- Styloid process – no longer a separate bone, the stylohyal fuses the otic capsule, joining the paroccipital process
- Craniomandibular joint positioned anterior to fenestra vestibuli (hearing organ opening)
- Craniomandibular joint formed only by squamosal and dentary
- Meckelian sulcus (trough) enclosed – when open it held the post dentary elements
- Coronoid bone vestigial or absent – the dentary takes over
- Splenial vestigial or absent
- Articular, prearticular, surangular and angular suspended from the skull (as tiny ear bones and ectotympanic bulla respectively)
- Proatlas not ossified
- Atlas intercentrum and neural arches fused to form one ring-like bone, vertebra #1.
- Atlas rib absent (actually fused to the atlas)
- Axis prezygopophyses absent
- Postaxial cervical ribs fused to vertebrae
- Styloid processes on dstial ends of radius, tibia and fibula
- Patella present along with patellar facet on femur
- Entocuneiform–Hallucial (distal tarsal 1 and m1.1) articulation saddle-shaped permitting greater mobility
- Secondary ossifications on long bones and girdles – ossified joints
- 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.
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.
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.