Pleuraspidotherium and Orthaspidotherium

These two taxa don’t make very many lists.
That may be because Pleuraspidotherium amonieri  (Paleocene; Lemoine 1882; Fig. 1) and Orthaspidotherium edwardsi (Ladevèze, Missiaen and Smith 2010; ) are not directly related to any ‘big name’ clades and they have a very basal condylarth (herbivorous mammal) look. Instead these two nest with rather plesiomorphic Meniscotherium and highly derived Astrapotherium in the large reptile tree (LRT, 898 taxa). Both Pleuraspidotherium and Orthaspidotherium were originally recognized as phenacodontids related to Meniscotherium, so we’re tracking traditional nestings here.

Figure 1. Orthaspidotherium from x et al. 2009 is a plesiomorphic mammalian herbivore, basal to all later forms, from elephants to baleen whales to giraffes.

Figure 1. Orthaspidotherium from Ladevèze, Missiaen and Smith 2010 is a plesiomorphic mammalian herbivore, basal to all later forms, from elephants to baleen whales to giraffes.

However in the LRT,
both taxa nest together and apart from Phenacodus.

Figure 1. Pleurospidotherium a

Figure 1. Pleuraspidotherium a

These two were among the very first
slightly larger mammalian herbivores that first appeared in the Cenozoic. We see the origin of the notched diastema here separating the anterior premolars from the poster premolars and molars.

Figure 2. Subset of the LRT showing the nestings of Pleurospidotherium and Orthaspidotherium at the base of the herbivorous mammals.

Figure 2. Subset of the LRT showing the nestings of Pleurospidotherium and Orthaspidotherium at the base of the herbivorous mammals.

Halliday 2015 reports
“Ladevéze et al. (2010) hypothesised that Pleuraspidotheriidae are closest relatives to arctocyonids such as Chriacus, in a group also including the basal artiodactyls, but their taxonomic sampling was very low, and only very few representatives of each supposed group were present.” 

In the LRT
Chriacus nests with bats and Arctocyon nests with Didelphis in the Metatheria, both far from the Pleuraspidotheriidae. None of these relationships is found in Halliday et al. 2015.

Halliday 2015 reports, 
“With the exception of Primates (Russell, 1964), Rodentia (Jepsen, 1937), and Carnivora (Fox, Scott & Rankin, 2010), no extant order of placental mammal has an unambiguous representative during the Paleocene.” Pleuraspidotherium and Orthaspidotherium are also in the Paleocene, so they are early representatives of the herbivorous placental clades.

“Despite numerous suggestions of Cretaceous placentals, no Cretaceous eutherian mammal has been unambiguously resolved within the placental crown.” In the LRT multituberculates and Shenshou from the Jurassic are rodent sisters, Volaticotherium is a basal pre-placental from the earliest Cretaceous. Docofossor is a basal Oxfordian (early Late Jurassic) marsupial.  Maotherium a pre-mammal from the early Cretaceous, Zhangheotherium, a basal pangolin is from the earliest Cretaceous, and Maelestes, a basal tenrec is from the late Cretaceous, so the Halliday claim is not validated by the LRT and a Cretaceous origin would therefore NOT require the existence of long ghost lineages, contra Halliday et al. 2015.

Halliday et al. 2015 illustrates
the ‘current consensus” of mammalian relationships with the first split at Xenarthra + Tenrecoidea and kin splitting from Glires + the rest of the placentals in something of a mishmash of tree branches. The LRT, by contrast, recovers complete resolution at all branches and does not replicate the “consensus” topology.

Halliday et al. then reports on their own phylogenetic analysis based on 680 traits and 177 taxa. The resulting topology bears little similarity to the the LRT with the first split separating (primates + plesiadapids) + (rodents + rabbits) + xenarthra  from the rest of the placentals, then Phenacodus + Meniscotherium and kin splitting next from the remaining placentals in one test.

Another result split Xenarthra and Procavia + Potamogale and kin from the rest of the mammals. Among their seven conclusions, they report, “No definitive crown-placental mammal has yet been found from the Cretaceous, as Protungulatum is resolved as a stem eutherian, and therefore the Cretaceous occurrence of Protungulatum cannot be considered definitive proof of a Cretaceous origin for placental mammals.”

This is contradicted by the LRT results.

Halliday T et al. 2015. Resolving the relationships of Paleocene placental mammals. Biologoical Reviews. doi: 10.1111/brv.12242
Ladevèze S, Missiaen P and Smith T 2010. First Skull Of Orthaspidotherium edwardsi (Mammalia, “Condylarthra”) From The Late Paleocene Of Berru (France) And Phylogenetic Affinities Of The Enigmatic European Family Pleuraspidotheriidae”. Journal of Vertebrate Paleontology. 30 (5): 1559–1578.
Lemoine V 1882. Sure l’encephale de l’Artocyon et du Pleurospidotherium aumonieri. Bulletin de la Societe Géologie de France 3 series t. X. Also. Comptes Rendus.


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