Mammaliaformes

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Mammaliaforms
Temporal range:
Ma
Top: Megazostrodon rudnerae, Morganucodon watsoni (Morganucodonta);

Middle: Castorocauda lutrasimilis (Docodonta), Shenshou lui (Haramiyida);
Bottom: Tachyglossus aculeatus (Monotremata), Ursus arctos (Theria).

Skull of Morganucodon
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade:
Therapsida
Clade:
Cynodontia
Clade: Prozostrodontia
Clade: Mammaliamorpha
Clade: Mammaliaformes
Rowe, 1988
Subgroups

Mammaliaformes ("mammalian forms") is a

Marsupialia, and Placentalia.[2] Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium as well as the Triassic Tikitherium, the earliest known member of the group.[3][4]

Mammaliaformes is a term of

Brasilodontidae
.

Animals in the Mammaliaformes clade are often called mammaliaforms, without the e. Sometimes, the spelling mammaliforms is used. The origin of

crown-group mammals extends back to the Jurassic, with extensive findings in the Late Jurassic outcrops of Portugal and China. The earliest confirmed specimens of fur
are found in them, demonstrating that the ancestors of mammals had already developed fur.

Mammaliaformes in life

Early mammaliaforms were generally

ectothermic, so speeding up the pace of digestion is a necessity. The drawback to the fixed dentition is that worn teeth cannot be replaced, as was possible for the reptiliomorph ancestors of mammaliaforms. To compensate, mammals developed prismatic enamel, characterized by crystallite discontinuities that helped spread out the force of the bite.[6]

tritylodontids that also display evidence of lactation,[8] this seems to imply that milk is an ancestral characteristic in this group. However, the fairly derived Sinoconodon appears to have uniquely discarded milk altogether[citation needed]. Prior to hatching, the milk glands would provide moisture to the leathery eggs, a situation still found in monotremes.[9]

The early mammaliaforms did have a

Cynodont ancestors, had a furry covering. An insulative covering is necessary to keep a homeothermic animal warm if it is very small, less than 5 cm (1.97 in) long;[10] the 3.2 cm (1.35 in) Hadrocodium must have had fur, therefore, but the 10 cm (3.94 in) Morganucodon might not have needed it. The docodont Castorocauda, further removed from crown group mammals than Hadrocodium, had two layers of fur, guard hairs and underfur, as do mammals today.[11]

It is possible that early mammaliaforms had

Cynodonts, probably had whiskers.[12] A common ancestor of all therian mammals did so.[13] Indeed, some humans even still develop vestigial vibrissal muscles in the upper lip.[14] Thus, it is possible that the development of the whisker sensory system played an important role in mammalian development, more generally.[13]

Like monotremes today, the legs of early mammaliaforms were somewhat sprawling, giving a rather "reptilian" type of gait. However, there was a general tendency to have more erect forelimbs, forms like

eutriconodonts even having a fundamentally modern forelimb anatomy while the hindlimbs remained "primitive";[15] this tendency is in some effect still seen in modern therian mammals, which often have more sprawling hindlimbs.[16] In some forms, the hind feet likely bore a spur similar to those found in the platypus and echidnas. Such a spur would have been connected to a venom gland for protection or mating competition.[17]

Hadrocodium lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms.[18]

With the possible exception of

monotremes).[21]
Most mammaliforms, therefore, probably had the same constraints, and some species could have borne pouches.

Phylogeny

The cladogram below follows the analysis of Luo and colleagues in 2015.[22]

Mammaliamorpha

Expanded from above

Trechnotheria

Cladogram based on Rougier et al. (1996)[23] with Tikitherium included following Luo and Martin (2007).[3]

  Mammaliamorpha

Tritylodontidae

Adelobasileus

Sinoconodon

Mammaliaformes
Morganucodontidae
  

Morganucodon

Mammalia
Monotremata

Ornithorhychus (Platypus)

Tachyglossidae
(Echidna)

Theriiformes

Placentals

See also

References

  1. S2CID 83999660
    .
  2. ^
    doi:10.1080/02724634.1988.10011708
    .
  3. ^
    S2CID 29846648. Archived from the original
    (PDF) on 2016-03-03. Retrieved 2012-10-14.
  4. S2CID 131236175
    .
  5. ISBN 978-0-471-38461-8
    .
  6. ISSN 0102-9010
    .
  7. ^ Panciroli E., Benson RBJ., and Walsh S. 2017. The dentary of Wareolestes rex (Megazostrodontidae): a new specimen from Scotland and implications for morganucodontan tooth replacement. Papers in Palaeontology
  8. doi:10.4202/app.2008.0053
    .
  9. S2CID 25806501
    .
  10. doi:10.1093/icb/40.4.585
    .
  11. S2CID 46067702
    .
  12. ^ "Your Inner Fish: Episode Guide". PBS. 2014. Retrieved August 7, 2014.
  13. ^
    PMID 21969685
    .
  14. S2CID 21055062
    .
  15. ISBN 0-231-11918-6.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  16. ^ Kielan−Jaworowska, Z. and Hurum, J.H. 2006. Limb posture in early mammals: Sprawling or parasagittal. Acta Palae− ontologica Polonica 51 (3): 393–406.
  17. ^ Hurum, J.H.; Luo, Z-X; Kielan-Jaworowska, Z. (2006). "Were mammals originally venomous?" (PDF). Acta Palaeontologica Polonica. 51 (1): 1–11.
  18. ISBN 0-19-850760-7
    .
  19. ^ Jason A. Lillegraven, Zofia Kielan-Jaworowska, William A. Clemens, Mesozoic Mammals: The First Two-Thirds of Mammalian History, University of California Press, 17/12/1979 - 321
  20. doi:10.1126/science.1074905
    · Source: PubMed
  21. ^ Michael L. Power, Jay Schulkin. The Evolution Of The Human Placenta. pp. 68–.
  22. PMID 26630008
    .
  23. ISSN 0003-0082
    .

External links
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