Zalambdalestidae Explained

Zalambdalestidae is a clade of Asian eutherians occurring during the Cretaceous. Once classified as Glires, features like epipubic bones and various cranial elements have identified these animals as outside of Placentalia, representing thus a specialised clade of non-placental eutherians without any living descendants, and potentially rather different from modern placentals in at least reproductive anatomy.^{[1] [2]}

Taxonomy

The exact position of Zalambdalestidae within Eutheria varies, though they are generally agreed to be more basal than zhelestids.^[3] Currently, the clade includes the genera Alymlestes, Anchilestes, Barunlestes, Kulbeckia, Prozalambdalestes, Zalambdalestes, Zhangolestes and Zofialestes.^[4]

Palaeobiology

Zalambdaltestids were insectivores, having zalambdodont molars much as various modern insectivorous species. They are uniquely suited to a saltatorial, cursorial lifestyle, bearing long, semi-digitigrade limbs and a spinal column similar to that of modern lagomorphs.^[5] Like most non-placental mammals, the presence of epipubic bones probably meant that they gave birth to poorly developed young much like modern marsupials and monotremes, though a study on multituberculate reproduction may suggest they and other early eutherians reproduced like modern placentals.^[6]

A study on Zalambdalestes suggests they had a unique axial morphology. This allowed for quick prey capture, and may suggest they had spines or bristly fur in life.^[7]

Notes and References

1. Book: Wible . John R. . Rougier . Guillermo W. . Novacek . Michael J. . Rose . Kenneth D. . Archibald . J. David . The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades . 2005 . Johns Hopkins University Press . 9780801880223 . 15–36 . Anatomical evidence for superordinal/ordinal Eutherian taxa in the Cretaceous.
2. Wible . J. R. . Rougier . G. W. . Novacek . M. J. . Asher . R. J. . Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary . Nature . 2007 . 447 . 7147 . 1003–1006 . 10.1038/nature05854 . 17581585. 2007Natur.447.1003W . 4334424.
3. Archibald . J. David . Averianov . Alexander . Phylogenetic analysis, taxonomic revision, and dental ontogeny of the Cretaceous Zhelestidae (Mammalia: Eutheria) . Zoological Journal of the Linnean Society . 2012 . 164 . 2 . 361–426 . 10.1111/j.1096-3642.2011.00771.x. free.
4. Lopatin . A. V. . Averianov . A. O. . New Early Cretaceous zalambdalestid stem placental mammal from Mongolia and evolution of Zalambdalestidae . Journal of Vertebrate Paleontology . 2024 . 10.1080/02724634.2024.2384601.
5. Chen . Meng . Wilson . Gregory P. . A multivariate approach to infer locomotor modes in Mesozoic mammals . Paleobiology . 2015 . 41 . 2 . 280–312 . 10.1017/pab.2014.14. 2015Pbio...41..280C . 86087687.
6. Urton, James (July 25, 2022). "New study challenges old views on what's 'primitive' in mammalian reproduction". UW News. University of Washington. Retrieved April 6, 2024.
7. Arnold, Patrick; Janiszewska, Katarzyna; Li, Qian; O'Connor, Jingmai K.; Fostowicz-Frelik, Łucja (April 16, 2024). "The Late Cretaceous eutherian Zalambdalestes reveals unique axis and complex evolution of the mammalian neck". Science Bulletin. doi:10.1016/j.scib.2024.04.027.