Metathere (aka: marsupial) issues

Metatherians
(aka marsupials) can be a difficult clade to understand phylogenetically. Taxon exclusion, as usual, causes problems, here and elsewhere. Case in point: recently adding taxa to the large reptile tree (LRT, 1247 taxa) shifted and clarified some prior interrelationships in which certain untenable dental patterns appeared. This was cause for concern and re-study. I’m pleased to report that the herbivorous metathere subset of the tree topology did change to a more logical and gradual pattern after adding several taxa. Now the dental atavisms no longer appear. But this new topology comes at the cost of recovering a dual and parallel origin for the diprotodont form of dentition (Fig. 1) in which the anteriormost dentary teeth are larger than typical canines and jut out anteriorly.

Figure 1. Marsupial mandibles. Traditional diprodonts have two large anterior dentary teeth. These arose twice in the LRT.

Figure 1. Marsupial mandibles. Traditional diprodonts have two large anterior dentary teeth. These arose twice in the LRT with the present list of taxa, once with kangaroos, and again with wombats. See figure 3.

First and second,
let’s take a look at two previously published metatherian tree topologies: Horovitz and Sánchez-Villagra 2003 (which covers many living and some extinct taxa) and Williamson, Brusatte and Wilson 2014 (in which taxa are all Cretaceous or earlier and most are known from isolated teeth). The LRT includes no tooth-only taxa… and that’s a good thing.

Horovitz and Sánchez-Villagra 2003 (Fig. 2) employed bones and soft tissue.
From their abstract: “We… assembl[ed] a morphological data matrix consisting of a new suite of 149 postcranial characters and incorporated a series of previously published data on the craniodental (76 characters) and soft tissue (5 characters) anatomy. Twenty-one marsupial terminal taxa representing all the major radiations of marsupials and 10 outgroups… were investigated. All currently accepted marsupial orders were recovered by the analysis.”

Figure 2. A cladogram of metatherian mammals based on skeletal and soft traits by Horovitz and Sánchez-Villagra 2003. This cladogram lacks a large number of carnivorous metatherians, a large number of basal prototheres and a large number of basal eutherians. On the other hand, the LRT is missing the uncolored taxa. Colors correspond to the metathere subset of the LRT (Fig. 3). Horovitz and Sánchez-Villagra 2003 recovered a monophyletic Diprotodontia in contrast to the LRT.

Figure 2. A cladogram of metatherian mammals based on skeletal and soft traits by Horovitz and Sánchez-Villagra 2003. This cladogram lacks a large number of carnivorous metatherians, a large number of basal prototheres and a large number of basal eutherians. On the other hand, the LRT is missing the uncolored taxa. Colors correspond to the metathere subset of the LRT (Fig. 3). Horovitz and Sánchez-Villagra 2003 recovered a monophyletic Diprotodontia in contrast to the LRT.

More recently,
Williams et al. 2014 reported, “Our understanding of this group has increased greatly over the past 20 years, with the discovery of new specimens and the application of new analytical tools. Here we provide a review of the phylogenetic relationships of metatherians with respect to other mammals, discuss the taxonomic definition and diagnosis of Metatheria, outline the Cretaceous history of major metatherian clades, describe the paleobiology, biogeography, and macroevolution of Cretaceous metatherians, and provide a physical and climatic background of Cretaceous metatherian faunas.” They built their study on Williams 2012, which focused on teeth. They report, “As in the previous analysis of Williamson et al. (2012), homoplasy is rampant.” Hmm. That’s a phrase I used to describe character distribution in the LRT!

Williams et al. 2014 reported, 
“The oldest confidently identified therian fossil is the eutherian Juramaia from the early Late Jurassic (ca. 160 million years old) of China (Luo et al. 2011).” The LRT nests Juramaia as a basalmost prototherian. They considered Sinodelphys (Early Cretaceous, 120 mya) to be the oldest known marsupial. In 2015 (a year after Williams et al.) Agilodocodon (Middle Jurassic, 170 mya) was announced as a docodont (but nests with Eomaia, Early Cretaceous, 125 mya) in the LRT.

Williams et al. 2014 reported, 
“Deltatheroidans were long regarded as eutherians (Gregory and Simpson 1926; Van Valen 1966) or stem boreosphenidan species (Fox 1974; Kielan-Jaworowska et al. 1979), but are now generally accepted as basal metatherians (Butler and Kielan-Jaworowska 1973; Kielan-Jaworowska and Nessov 1990; Rougier et al. 1998).” The LRT confirms a nesting within the Metatheria for Deltatheridium, not as a sister.

Williams et al. 2014 reported,
“The interrelationships of most major metatherian subclades are unresolved.” This is due to taxon exclusion and using too many tooth-only taxa. On the other hand, the metatherian taxa and clades within the LRT are fully resolved. The two studies share only 4 taxa in common so the Wiliams et al. cladogram will not be shown. Despite the availability of museum specimens, no extant taxa were used in Williams et al. 2014 study, which concentrated on Cretaceous taxa to the detriment of the study.

Maybe it would have been better
for Williams et al. 2014 to first establish relationships using extinct and extant skeletons of a wide gamut of mammals, as the LRT does, and then see where the tooth-only taxa fit in.

Figure 3. Subset of the LRT focusing on the Metatheria (= Marsupials). Here the diprotodont dentition evolved twice.

Figure 3. Subset of the LRT focusing on the Metatheria (= Marsupials). Here the diprotodont dentition evolved twice.

 

More on metatherians soon…

References
Horovitz I and Sánchez-Villagra MR 2003. A morphological analysis of marsupial mammal higher-level relationships. Cladistics 19(3):181 – 212.
DOI: 10.1111/j.1096-0031.2003.tb00363.x
Myers P, Espinosa R, Parr CS, Jones T, Hammond GS and Dewey TA 2018. The Animal Diversity Web (online). Accessed at https://animaldiversity.org.
info@tree-kangaroo.net
Williamson TE, Brusatte SL and Wilson GP 2014. The origin and early evolution of metatherian mammals: the Cretaceous record. ZooKeys 465:1–76.

doi: 10.3897/zookeys.465.8178
http://zookeys.pensoft.net

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