Updated July 25, 2022
with a new nesting of Necrolestes with the mouse opossum, Marmosa. Click here.
I had second hand (academic papers and figures) rather than firsthand access to the specimens. It doesn’t matter how good your players are if you don’t show up on the right field at the proper hour. Here you’ll see, once again, how excluding the actual sister to an enigma taxon is the major problem, solvable by second-hand phylogenetic analysis in a large gamut study, the large reptile tree (LRT) that minimizes the problem of taxon exclusion.
Necrolestes patagonensis (Ameghino 1891; early Miocene, 16mya; Fig. 1; YPM PU 15065, 15384, and 15699) has been argued about for over a hundred years. Originally (Ameghino 1891) it was described as the only known extinct placental “insectivore” from South America and allied to Chrysochloris (Fig. 2), the extant golden mole.
Unfortunately, as time went on…
Saban 1954 considered Necrolestes a palaeanodont (Ernanodon was previously considered one). Patterson 1958 considered it a borhyaenoid metatherian. Asher et al. 2007 looked at several candidates and could not make a firm conclusion. Ladevèze et al. 2008 supported metatherian affinities. Goin et al. 2008 also could not be specific with regard to a closest known sister taxon.
The latest paper on the subject
Rougier et al. 2012 reported, “earlier studies leaned toward placental affinities and more recent ones endorsed either therian or specifically metatherian relationships.” Ultimately they nested Necrolestes with Cronopio (Fig. 4) which they considered a non-therian mammal. That is correct. They considered an earlier Van Valen 1988 statement inspired, “…the enigmatic Miocene genus Necrolestes, usually thought to be a marsupial, is [conceivably] a late surviving Gondwantherian pantothere.” That is incorrect.
Asher et al. 2007 report,
“Characters that support [Necrolestes] status as a therian mammal include a coiled cochlear housing of the inner ear. Necrolestes shows similarities to eutherian mammals, such as small incisive foramina and possibly three molars. Consistent with its status as a metatherian is the presence of five upper incisors, transverse canal foramina, and a broad proximal fibula. However, we cannot confirm other characters claimed by previous researchers as evidence for affinity with marsupial or nonplacental mammals, such as the presence of an inflected mandibular angle and epipubic bones.”
Asher et al. report,
“The external digital flexor in Chrysochloris ossifies along nearly the entire forearm, from the humeral medial epicondyle to the carpus. Necrolestes shows a similarly elongate element stretching proximally from the carpus.”
Asher et al. report,
“The idea that [Necrolestes] is related to golden moles was favored in the first two publications describing its anatomy (Ameghino, 1891; Scott, 1905). We do not believe Patterson’s contention that the status of Necrolestes as a marsupial is ‘‘virtually assured’’. We admit that the list of possible taxonomic affiliations for this animal still remains long.”
The large reptile tree
(920 taxa) tested Necrolestes against a wide gamut of mammal candidates and nested it securely with the golden mole, Chrysochloris. To shift Necrolestes next to Cronopio adds 22 steps.
Distinct from sister taxa
Necrolestes had five upper incisors and four lowers. That is closer to the primitive numbers for mammals and two more than in Chrysochloris. The molars are also primitive in having fewer cusps, but that also happens in whales and armadillos… and golden moles… with their simplified zalambdodont teeth… so let’s focus on other traits. Dental traits are plastic and can lead analysis astray.
Rougier et al. report,
“the first upper and lower premolars are double rooted and the following five molariform elements are single rooted, a condition shared only with the recently described meridiolestidan mammal Cronopio.” Convergent dental traits might be leading these workers so far afield the neglected to add Chrysochloris to their analysis, which seems odd and dangerous based on the long list of shared traits and overall similarity, not by convergence.
Rougier et al. gave us straw dogs
when they compared the basicrania of several sister candidates, but NOT that of Chrysochloris, to that of Necrolestes. Here I add a basicranium Rougier et al. chose to not show. Chrysochloris more closely matches the morphology of Necrolestes than any of the other three candidates. I don’t see Chrysochloris listed in the Supplemental Information for Rougier et al. which appears to test non-placental mammals only. So this is what I mean by another case of taxon exclusion. Ameghino (1891) got it right originally. Rougier Wible, Beck and Apesteguía 2012, for some reason, dropped the ball.
Deleting Chrysochloris from the LRT
nests Necrolestes with the remaining basal Glires, but resolution is lost. Not sure why, but Necrolestes has a history (see above) of being a confusing taxon when not nested with Chrysochloris.
Now that we have golden moles in Africa and South America
this is evidence that golden moles first appeared before those continents split apart 118 to 115 mya, long before the end of the Cretaceous. Video link here. Naish reports, “Golden moles and tenrecs appear to be close relatives, forming a clade usually termed Afrosoricida Stanhope et al., 1998 (though this is essentially synonymous with Tenrecoidea McDowell, 1958, see Asher (2001)“. That relationship is not supported by the LRT. Golden moles probably first appeared in the Early Jurassic, given that other Glires, multituberculates, split from rodents about the same time and are found as early as Middle Jurassic strata.
Rougier et al. tested earlier studies and found them flawed
Similarly, I tested Rougier et al. and found it flawed. Perhaps someday someone will likewise test this test and present additional insight into this former enigma taxon.
Ameghino F 1891. Nuevos restos de mamíferos fósiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas, adiciones y correciones [New remains of fossil mammals discovered by Carlos Ameghino in the lower Eocene of southern Patagonia. New species, additions and corrections]. Rev Arg Hist Nat 1:289–328. Spanish.
Asher RJ, Horovitz I, Martin T and Sanchez-Villagra MR 2007. Neither a Rodent nor a Platypus: a Reexamination of Necrolestes patagonensis Ameghino. American Museum Novitates 3546:1-40.
Ladevèze S, Asher RJ, Sánchez-Villagra MR 2008. Petrosal anatomy in the fossil mammal Necrolestes: evidence for metatherian affinities and comparisons with the extant marsupial mole. J Anat 213(6):686–697.
Patterson B 1958. Affinities of the Patagonian fossil mammal Necrolestes. Breviora Mus Comp Zool 94:1–14.
Rougier GW, Wible JR, Beck RMD and Apesteguía S 2012. The Miocene mammal Necrolestes demonstrates the survival of a Mesozoic nontherian lineage into the late Cenozoic of South America.
Saban R 1954. Phylogénie des insectivores [Phylogeny of the insectivores]. Bull Mus Natl d’Hist Nat. Ser 2 26:419–432. in French
Van Valen L 1988. Faunas of a southern world. Nature 333(6152):113.