This might have been yet another case
of scientists TRUSTING authority (= the work of others) rather than TESTING competing phylogenetic analyses. In this case, however, two of the three authors in Turner, Pritchard and Matze 2017 relied on their own flawed (= serious taxon exclusion problems) phylogenetic analyses and for good measure they threw in a third flawed (= more taxon exclusion problems) analysis (Nesbitt 2011) that we examined and reexamined in an 11-part series.
In any case, since none of the trees
in the new Turner et al. study stand up to scrutiny (= do not agree with one another, do not produce gradual accumulations of traits in derived taxa and depend on long ghost lineaages), everything Turner et al. (2017) did afterwards has no credibility and no utility. So sadly, the entire paper is a waste of their time. Metaphorically, they built their house on sand.
On the other hand,
when you start with a study that provides a gradual accumulation of derived traits in all derived taxa, and minimizes the effect of taxon exclusion, like the large reptile tree (LRT (949 taxa) then you’ve metaphorically built your house on solid ground. And it’s much simpler to pinpoint the dino/croc divergence time because you are provided with a last common ancestor for these sister clades: Gracilisuchus (Figs. 1, 2). Crocs and dinos are sister taxa. None of the studies used by Turner et al. (Pritchard et al. 2015, Nesbit 2011, Turner 2015) recovered that tested relationship.
Figure 1. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Herrerasaurus, Tawa and Eoraptor.
So when did dinos and crocs diverge?
Let’s look a the three most recent taxa both clades share in common in reverse chronological and phylogenetic order:
So that narrows the divergence time pretty well…
And how did Turner et al. do?
They report,“The average ghost lineage for the group as sampled is 31 million years.” Their conclusion states no firm date or date range. Rather, their whole paper appears to be a long story on how they tested this that and the other without getting around to their headline topic. And without nailing down a last common ancestor or a croc/dino divergence time.
Figure 2. Basal crocs. Decuriasuchus and Gracilisuchus are found in both croc and dino lineages.
All the other taxa
and all the other testing performed by Turner et al. were for nought.
For more information
on any of the taxa employed by Turner et al, just look them up at ReptileEvolution.com.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History;352:1–292.
Pritchard AC, Turner AH, Nesbitt SJ, Irmis RB and Smith ND 2015. Late Triassic tanystropheids (Reptilia, Archosauromorpha) from Northern New Mexico (Petrified Forest Member, Chinle Formation) and the biogeography, functional morphology, and evolution of Tanystropheidae. Journal of Vertebrate Paleontology. ;e911186.
Turner AH 2015. A Review of Shamosuchus and Paralligator (Crocodyliformes, Neosuchia) from the Cretaceous of Asia. PLoS ONE. 2015;10(2):e0118116. doi: 10.1371/journal.pone.0118116. pmid:25714338
Turner AH, Pritchard AC and Matzke NJ 2017. Empirical and Bayesian approaches to fossil-only divergence times: A study across three reptile clades. PLoS ONE 12(2): e0169885. doi:10.1371/journal.pone.0169885