Krause et al. bring us a complete and articulated skeleton
from the latest Cretaceous of Madagascar they call Adalatherium hui (=”crazy beast”, Fig. 1). Added to the large reptile tree (LRT, 1678+ taxa; subset Fig. 3), Adalatherium nests with Miocene Paedotherium (Fig. 2), a metatherian taxon known since 1888 with homologous large rodent-like incisors and retained coracoids. Regrettably this taxon was omitted from the Krause et al. cladogram (Fig. 4).
Paedotherium typicum (Burmeister 1888, Cerdeño E and Bond M 1998; Miocene-Pleistocene) was originally considered a rabbit-like typothere notoungulate, but here nests between Dasyuroides and Phalanger among the marsupials. The Cretaceous taxa, Vintana and Groeberia are more closey related, hinting at the probable Jurassic origin of Paedotherium.
By contrast with the LRT,
Krause et al. considered their discovery a member of the ‘Gondwanatheria’ and therefore a basal mammal between Prototheria (duckbills and echidnas) and Metatheria (marsupials) (Fig. 4), still close to Vintana, the closest included taxon known from more than teeth.
From the abstract:
“To our knowledge, the specimen is the most complete skeleton of a Gondwanan Mesozoic mammaliaform that has been found, and includes the only postcranial material and ascending ramus of the dentary known for any gondwanatherian. A phylogenetic analysis including the new taxon recovers Gondwanatheria as the sister group to Multituberculata.”
This is a red flag. In the LRT multituberculates are members of the clade Glires a placental clade that includes tree shrews and rodents. Krause et al. omitted the closest taxa in the LRT to the multituberculates.
Large rodent-like incisors, coupled with procoracoids, coupled with epipubes,
narrowed the taxonomic focus for the Krause team, but narrowed it a little too far. Overlooked Paedotherium (Fig. 2) shares those traits and hundreds more. The LRT minimizes exactly this type of taxon exclusion by including such a wide gamut of taxa that keeps getting wider and deeper with every new taxon. The similarities are obvious.
From the abstract:
“The skeleton, which represents one of the largest of the Gondwanan Mesozoic mammaliaforms, is particularly notable for exhibiting many unique features in combination with features that are convergent on those of therian mammals. This uniqueness is consistent with a lineage history for A. hui of isolation on Madagascar for more than 20 million years.”
Not ‘unique’ and not ‘convergent’ with those of therian mammals—Adalatherium IS a therian mammal… with a procoracoid and epipubes.
Don’t rely on a short list of traits.
That would be like ‘Pulling a Larry Martin.” Don’t rely on dental traits. Don’t rely on genes. Just expand your taxon list and let the software decide where to nest new taxa. Omitting pertinent taxa is a common issue in paleontology. Not sure what drives it other than headlines in this case.
After all the work involved in discovering
and recovering Adalatherium, expanding the taxon list would have been the easiest thing to do… and, as usual, the most important. Simply omitting taxa too often produces erroneous conclusions, turning a wonderful discovery, kept under wraps and studied for twenty years in prestigious institutions into an embarrassing error published in Nature and widely publicized around the world (links below).
Earlier we talked about the origin of major mammal clades
in the Jurassic and Cretaceous (Fig. 7). Adalatherium is just one of many already known and many to come.
Is Adalatherium the oldest mammal found in the Southern Hemisphere?
No. Brasilitherium and Brasilodon (Fig. 7) both from Brazil, are known from the Norian (Late Triassic) more than twice as old as Adalatherium. We also have seven placentals (Fig. 7) from the Late Jurassic of China and older marsupials from China and England.
According to Science Alert (link below):
“Exactly what factors induced the craziness of the crazy beast isn’t fully clear, but a 20-year-long analysis of the remains (the fossil was first discovered in 1999) indicates it is indeed a strange creature. Knowing what we know about the skeletal anatomy of all living and extinct mammals, it is difficult to imagine that a mammal like Adalatherium could have evolved,” says vertebrate palaeontologist David Krause from the Denver Museum of Nature & Science, who helped find the skeleton during a field expedition in Madagascar in 1999. “It bends and even breaks a lot of rules.”
“Strange… crazy… difficult to imagine…”
this is all hyperbole. When Paedotherium is added to the taxon list, none of this is strange… and there goes the headline… and the generic name…
Continuing from Science Alert
“Part of the weirdness is the primitive septomaxilla bone in its snout region – a feature that disappeared 100 million years earlier in the ancestors of living modern mammals.”
“It also had more openings (called foramina) in its cranium than any known mammal, the researchers say, which may have enhanced the sensitivity of its snout and whiskers, by enabling passage for nerves and blood vessels through the skull.”
“The animal had strangely bowed leg bones, too, and researchers aren’t sure whether it used its limbs for digging, or running, or even other kinds of locomotion. Then there are the teeth. The strangeness of the animal is clearly apparent in the teeth – they are backwards compared to all other mammals, and must have evolved afresh from a remote ancestor,” Evans explains.
Backwards? Does that mean more primitive?
Let’s assume the latter. On that point, the teeth of Paedotherium are similar.
Adalatherium is just a minor variation on a traditionally overlooked taxon known for over 100 years. Chronologically it’s not that old. Phylogenetically it’s not that crazy, strange or bizarre. Omitting taxa remains THE MOST COMMON error in academic papers. Let’s fix that.
Krause DW et al. (12 co-authors) 2020. Skeleton of a Cretaceous mammal from Madagascar reflects long-term insularity. Nature (advance online publication DOI: https://doi.org/10.1038/s41586-020-2234-8