Saint Bathans mammal

The Saint Bathans mammal is a currently unnamed extinct primitive mammal from the Early Miocene (Altonian) of New Zealand. A member of the Saint Bathans Fauna, it is notable for being a late surviving "archaic" mammal species, neither a placental nor a marsupial. It also provides evidence that flightless fully terrestrial mammals did in fact once live in Zealandia. This is in contrast to modern New Zealand, where bats and seals are the only non-introduced mammals in the otherwise bird-dominated terrestrial faunas.[1]

Discovery

The Saint Bathans mammal is currently represented by two specimens, MNZ S.40958 and MNZ S.42214, composed of a lower jaw fragment and a femur respectively. It was part of an assemblage of fossils recovered in Saint Bathans in 1978, in what would later be understood to be the Bannockburn Formation (Manuherikia Group), and first described in 2006.[1]

Description

Like most small mammal fossils, the Saint Bathans mammal material is rather incomplete, with only a lower jaw fragment and femur being known.

The lower jaw is toothless, though the presence of deep tooth sockets suggests that it was toothed in life and that the teeth were lost post-mortem. It bears a long fused mandibular symphysis, an evidently procumbent lower incisor, and five additional sockets that imply a dental formula of one incisor, one canine and two double-rooted premolars.

The femur possesses a round head and poorly defined neck, oriented slightly dorsomedially with respect to the long axis of the shaft, and separated from the greater trochanter by a marked trough. The alignment of the femur in life is hard to ascertain, but it is thought that the animal had a semi-sprawling stance, more abducted than in therian mammals but nowhere near as much as in monotremes.

Phylogeny

Because of the incomplete material, it is very hard to understand the position of this taxon within Mammaliaformes as a whole. Worthy et al. 2006 tentatively deemed the Saint Bathans mammal as a theriiform, being more derived than morganucodontans, eutriconodonts and monotremes but not as much as multituberculates, on the basis of its femoral anatomy. As the phylogeny of non-therian mammals has undergone multiple shifts since its description, new studies might be necessary.

Ecology

The Bannockburn Formation depicts a warm temperate or subtropical lakeside environment, surrounded by herbaceous peat–swamps. Casuarinas, araucarias, podocarps, eucalypts, palm trees and southern beeches are among the various plant species known to have grown here. As today, the local vertebrate fauna was dominated by birds: early moas and adzebills are represented by unnamed species, as are various representatives of groups such as waterfowl, flamingos, rails, herons, strigopoidean parrots and even an early kiwi, Proapteryx.[2] However, unlike modern New Zealand it also had a varied herpetofauna: besides an early tuatara, the Saint Bathans fauna also includes meiolaniid and pleurodire turtles and possibly mekosuchine crocodylids and snakes.[3]

Besides the Saint Bathans mammal, this fauna also includes mystacine bats, a group still present in modern New Zealand.[4] Like the modern species, these were probably terrestrial foragers,[5] the same general ecological niche proposed for the Saint Bathans mammal. Other bats, including a vesper bat and several currently unclassified species, also existed.

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References
  1. Worthy, Trevor H.; Tennyson, Alan J. D.; Archer, Michael; Musser, Anne M.; Hand, Suzanne J.; Jones, Craig; Douglas, Barry J.; McNamara, James A.; Beck, Robin M. D. (2006). "Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific". PNAS. 103 (51): 19419–19423. doi:10.1073/pnas.0605684103. PMC 1697831. PMID 17159151.
  2. Worthy, Trevor H.; et al. (2013). Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves (PDF). Paleornithological Research 2013, Proceedings of the 8th International Meeting of the Society of Avian Paleontology and Evolution. Retrieved 9 March 2014.
  3. Scofield, R. Paul; Worthy, Trevor H. & Tennyson, Alan J.D. (2010). "A heron (Aves: Ardeidae) from the Early Miocene St Bathans Fauna of southern New Zealand.". In W.E. Boles & T.H. Worthy. Proceedings of the VII International Meeting of the Society of Avian Paleontology and Evolution (PDF). Records of the Australian Museum 62. pp. 89–104.
  4. Hand, Suzanne J.; Worthy, Trevor H.; Archer, Michael; Worthy, Jennifer P.; Tennyson, Alan J. D.; Scofield, R. Paul (2013). "Miocene mystacinids (Chiroptera, Noctilionoidea) indicate a long history for endemic bats in New Zealand". Journal of Vertebrate Paleontology. 33 (6): 1442–1448. doi:10.1080/02724634.2013.775950.
  5. Hand, Suzanne J.; Beck, Robin M. D.; Archer, Michael; Simmons, Nancy B.; Gunnell, Gregg F.; Scofield, R. Paul; Tennyson, Alan J. D.; De Pietri, Vanesa L.; Salisbury, Steven W.; Worthy, Trevor H. (2018). "A new, large-bodied omnivorous bat (Noctilionoidea: Mystacinidae) reveals lost morphological and ecological diversity since the Miocene in New Zealand". Scientific Reports. 8 (1): 235. doi:10.1038/s41598-017-18403-w. PMC 5762892. PMID 29321543.
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