Meridiolestida

Skull of Necrolestes patagonensis, from Ameghino (1906).

Belongs within: Trechnotheria.

The Meridiolestida are an extinct clade of mammals known from the Late Cretaceous to the Miocene of South America (Rougier et al. 2012).

A little bit mole-ish in the Miocene
Published 29 May 2007

Necrolestes patagonensis was a small fossorial animal from the Miocene of Patagonia that has always held a certain appeal for me, both because of its somewhat morbid genus name (it translates as “robber of the dead”) and because of its enigmatic phylogenetic position (Asher et al. 2007).

Necrolestes was described by Florentino Ameghino in 1891. Ameghino seems to have left a few conundrums in his wake—he originally described the giant carnivorous bird Phorusrhacos as a toothless mammal, and mistakenly described the “wingless” penguin Palaeoapterodytes. Ameghino seems to have favoured an association of Necrolestes with the African golden moles (Chrysochloridae)—a not unreasonable suggestion for the time. Since then, probably the majority of authors have felt that Necrolestes was a marsupial, but it has also been compared with edentates or suggested as a late survivor of an equally enigmatic group of South American fossil mammals called Gondwanatheria. I recall seeing a nice little cartoon in one paper (I think it was Archer, 1984, but I’m not certain) showing a little Necrolestes being drop-kicked by an anthropomorphised borhyaenid out the door of a gathering of marsupial representatives (borhyaenids were a family of dog-like marsupial carnivores).

After a detailed redescription of the available material (which, among other things, introduced me to the glorious-sounding term schmelzmuster, which refers to the spatial arrangement of different enamel types within a tooth), Asher et al. (2007) attempt to shed some light on the position of Necrolestes by trying to match its characters with previously optimised trees for other mammals. This proves to be quite tricky—Necrolestes has a rather oddball combination of primitive and derived characters, and any suggested position requires a certain amount of convergence. Asher divide the possibilities into three main options—a position outside the Theria (the marsupials + placentals clade), a position close to or within marsupials (metatherians), and a position close to or within placentals (eutherians).

In regards to a position outside Theria, Necrolestes has an atlas (the first cervical vertebra after the skull) with the left and right halves not fused to each other, something unlike any adult therian. It also lacks many of the tooth apomorphies associated with Theria, though this may just be due to the simplified teeth of Necrolestes. However, Necrolestes does have a coiled cochlea, an astragalar neck and lacks a septomaxilla, so Asher et al. conclude it is most likely a therian. As Gondwanatheria is often regarded as non-therian, Asher et al. suggest that Necrolestes is probably not a gondwanatherian, but I feel that the non-therian nature of Gondwanatheria has not really been demonstrated.

In regards to whether Necrolestes is a metatherian or eutherian, Asher et al. don’t really come to a firm conclusion. Patterson (1958) claimed that Necrolestes possessed epipubic bones, which are a primitive character retained in marsupials but absent from modern placentals (thought they were present in some stem eutherians). Asher et al., however, found no sign of epipubic bones. It also has a non-inflected mandibular angle, which is generally a eutherian character, but is also found in some derived marsupials. Necrolestes does share a number of characters with metatherians, most of them “absence” characters—lack of a stapedial artery sulcus, lack of a labial mandibular foramen, etc. It agrees with metatherians in having three premolars, but seems to have one too few molars (three instead of four), and shares a ball-shaped distal process on the ulna and transverse canal foramina on the basisphenoid with crown marsupials.

Characters shared with eutherians are a posteriorly small zygomatic process on the squamosal and small incisive foramina, as well as the aforementioned non-inflected mandible and lack of epipubic bones.

Overall, Asher et al. feel that a metatherian affinity for Necrolestes is most likely, which is appealing on biogeographical grounds (most South American insectivores and such at the time being marsupials). However, they admit that a eutherian affinity can’t be ruled out, and I would certainly like to see this possibility further investigated. In particular, if Gondwanatheria are related to edentates (another South American group) as some authors have apparently suggested, the idea that Necrolestes is a late survivor of that group may yet be reborn.

Systematics of Meridiolestida

Synapomorphies (from Rougier et al. 2011): Meckelian groove vestigial or absent in adults. Last lower premolar with distinct distal cingulid cusp d present as part of a continuous distal cingulid. Stylocone in triangular teeth with prominent cusp subequal or larger than paracone. Metacone absent. Three molars/molariforms present. Lower molar root cross section anteroposteriorly compressed. Rostrum sharply constricted in front of molariform premolar. Maximum vertical depth of zygomatic arch between 10–20% of skull length.

<==MeridiolestidaRW12
    |  i. s.: Brandonia Bonaparte 1990A02 [BrandoniidaeRAG11]
    |         BarbereniidaeRAG11
    |           |--Barberenia Bonaparte 1990A02
    |           `--Quirogatherium Bonaparte 1990A02
    |         Austrotriconodon Bonaparte 1986SM93 [AustrotriconodontidaeRAG11]
    |           |--A. mckennaiRAG11
    |           `--A. sepulvedaiRAG11
    |         Donodon Sigogneau-Russell 1991A02 [DonodontidaeRAG11]
    |           `--D. perscriptoris Sigogneau-Russell 1991A02
    |--Mesungulatoidea [Reigitheriidae]RAG11
    |    |--Reigitherium Bonaparte 1990RW12, SM93
    |    |    `--R. bunodontum Bonaparte 1990N10
    |    `--+--Peligrotherium [Peligrotheriidae]RW12
    |       |    `--P. tropicalisRW12
    |       `--+--Mesungulatum Bonaparte & Soria 1985RW12, SM93 [Mesungulatidae]
    |          `--Coloniatherium silinskiiRW12, RAG11
    `--+--Leonardus Bonaparte 1990RW12, SM93
       |    `--L. cuspidatusRAG11
       `--+--Cronopio Rougier, Apesteguía & Gaetano 2011KH14, RAG11
          |    `--*C. dentiacutus Rougier, Apesteguía & Gaetano 2011RAG11
          `--Necrolestes Ameghino 1891RW12, D07 [Necrolestidae]
               |--N. mirabilis Goin et al. 2007RW12
               `--N. patagonensis Ameghino 1891RW12

*Type species of generic name indicated

References

Ameghino, F. 1891. Nuevos restos de mamiferos fosiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas adiciones y correcciones. Revista Argentina de Historia Natural 1: 289–328.

Archer, M. 1984. Origins and early radiations of marsupials. In: Archer, M., & G. Clayton (eds.) Vertebrate Zoogeography and Evolution in Australasia pp. 585–626. Hesperian Press: Carlisle.

Asher, R. J., I. Horovitz, T. Martin & M. R. Sanchez-Villagra. 2007. Neither a rodent nor a platypus: a reexamination of Necrolestes patagonensis Ameghino. American Museum Novitates 3546: 1–40.

[A02] Averianov, A. O. 2002. Early Cretaceous “symmetrodont” mammal Gobiotheriodon from Mongolia and the classification of “Symmetrodonta”. Acta Palaeontologica Polonica 47 (4): 705–716.

[D07] Dixon, D. 2007. The Complete Illustrated Encyclopedia of Dinosaurs & Prehistoric Creatures. Hermes House: London.

[KH14] Krause, D. W., S. Hoffmann, J. R. Wible, E. C. Kirk, J. A. Schultz, W. von Koenigswald, J. R. Groenke, J. B. Rossie, P. M. O’Connor, E. R. Seiffert, E. R. Dumont, W. L. Holloway, R. R. Rogers, L. J. Rahantarisoa, A. D. Kemp & H. Andriamialison. 2014. First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism. Nature 515: 512–517.

[N10] Naish, D. 2010. Tetrapod Zoology: Book One. CFZ Press: Bideford (UK).

Patterson, B. 1958. Affinities of the Patagonian fossil mammal, Necrolestes. Breviora Museum of Comparative Zoology 94: 1–14.

[RAG11] Rougier, G. W., S. Apesteguía & L. C. Gaetano. 2011. Highly specialized mammalian skulls from the Late Cretaceous of South America. Nature 479: 98–102.

[RW12] Rougier, G. W., J. R. Wible, R. M. D. Beck & S. Apesteguía. 2012. The Miocene mammal Necrolestes demonstrates the survival of Mesozoic nontherian lineage into the late Cenozoic of South America. Proceedings of the National Academy of Sciences of the USA 109 (49): 20053–20058.

[SM93] Stucky, R. K., & M. C. McKenna. 1993. Mammalia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 739–771. Chapman & Hall: London.

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