I think you’ll find this phylogenetic experiment both
gut-wrenching and extremely illuminating. While reading this, keep in mind the importance of having/recovering the correct outgroup for every clade and every node. That can only be ascertained by including a wide gamut of taxa—including fossils. Adding taxa brings you closer and closer to echoing actual events in deep time while minimizing the negative effects of not including relevant/pertinent taxa.
Today you’ll see
what excluding fossil taxa (Fig. 1) will do to an established nearly fully resolved cladogram, the large reptile tree (LRT, 1318 taxa). Earlier we’ve subdivided the LRT before, when there were fewer taxa in total. Here we delete all fossil taxa (except Gephyrostegus, a basal amniote used to anchor the cladogram because PAUP designates the first taxon the outgroup).
PAUP recovers 250+ trees
on 264 (~20%) undeleted extant taxa.
- Overall lepidosaurs, turtles, birds and mammals nest within their respective clades.
- Overall lepidosaurs nest with archosaurs and turtles with mammals, contra the LRT, which splits turtles + lepidosaurs and mammals + archosaurs as a basal amniote dichotomy.
- Overall mammals are not the first clade to split from the others, contra traditional studies. All pre-mammal amniotes in the LRT are extinct.
- Within lepidosaurs, the highly derived horned lizards and chameleons are basal taxa, contra the LRT, which nests Iguana as a basal squamate.
- Within lepidosaurs, geckos no longer nest with snakes, contra the LRT.
- Crocodiles nest with kiwis, as in the LRT, but it is still amazing that PAUP recovered this over such a large phylogenetic distance.
- Within aves, so few taxa are fossils in the LRT that the tree topology is very close to the original.
- Within mammals marsupials no longer nest between monotremes and placentals
- …and because of this carnivores split off next.
- Contra the LRT, hippos are derived from the cat and dog clade, all derived from weasels.
- Within mammals odontocetes no longer nest with tenrecs.
- Within mammals mysticetes nest with odontocetes, no longer nest with hippos.
- Contra the LRT, whales are derived from manatees and elephants.
here are the results based on using the basal fish, Cheirolepis, as an outgroup:
- The caecilian, Dermophis, nests as the basalmost tetrapod.
- Followed by the frog and salamander.
- Squamates branch off next with legless lizards and burrowing snakes at a basalmost node. Terrestrial snakes are derived from burrowing snakes. Gekkos split next followed by varanids and skinks. Another clade begins with the tegu and Lacerta, followed by iguanids. Sphenodon nests between the horned lizards, Moloch and Phyrnosoma + the chameleon.
- Turtles split off next with the soft-shell turtle, Trionyx, at the base.
- One clade of mammals split off next with echidnas first, then elephant shrews and tenrecs, followed by a clade including the pangolin, seals and other basal carnivores. Cats and dogs split off next followed by hippos, then artiodactyls, perissodactyls, the hyrax, elephants, manatees, mysticetes and odontocetes.
- Another clade of mammals include edentates, followed by tree shrews and glires, followed by (colugos + bats) + primates, followed by another clade of basal carnivores, followed by marsupials.
- The final clade is Crocodylus + extant birds, which are not well resolved and split apart into two major clades with some subclades maintaining their topology while other clades split apart. So the archosaurs nest together.
This test emphasizes the need for the inclusion of fossil taxa in order to recover a gradual accumulation of traits at all nodes, which takes us closer to actual evolutionary patterns in deep time.