Hi Greg (& dmailers!)
I think Greg and I have been just emailing each other over our past few exchanges, but they were meant to be a continuation of the T. rex diversity thread (see below for the messages you might have missed).
A follow-up note for Greg, students, and interested readers:
What I mean by null hypothesis is an idea that is used to (1) establish a heuristic point of reference and (2) provide a clear idea that can be tested.
Presently it is uncontroversial to state that the diversity of
Tyrannosaurus in the HCF and its lateral equivalentsÂis limited to one species; that is the null we've been operatingÂunder since 1905. A null hypothesis is a statement of no difference; ergo, all of the material that we think of as
T. rexÂrepresents that species. That doesn't mean the null hypothesis is true, but, so far, no evidence has been brought to falsify it. And, for what it's worth, I've studied adult
T. rex from close to the base of the HCF (MOR 2822) and others from near the top (FMNH PR2081) and, among over 1,800 characters, there are no differences that stand out clear enough to differentiate them aside from growth related changes (
https://peerj.com/articles/9192/#).
With a null hypothesis in hand, we can then formulate an alternative hypothesis. An alternative hypothesis may notÂyet be supported by evidence, but it is a means to structure a test of the null. In this case, the alternative hypothesis is that the T. rex assemblage actually contains two species, not one. It has been argued that there are gracile and robust morphs of T. rex, which is straightforward to quantify. My naive impression is that althoughÂthis alternative hypothesis might bear up under the currently small sample size of T. rex, as the sample size increases those extremes will grade into each other continuously, which would be an example of regression to the mean. In that scenario, the null would survive the test.
In summary, a null hypothesis is merely a factual statement that orients our thinking. Another example of this is Witmer's shift of the null hypothesis regarding the antorbital fenestra. Back in 1987 there were two competing hypotheses: the fenestra housed a muscle, or the fenestra housed a gland. Larry broke the tie using the Extant Phylogenetic Bracket, showing that the evidence was best explained by the presence of an air sac in the fenestra, not a muscle or a gland. In doing so he shifted the null hypothesis from two irreconcilable hypotheses to a single one that explained all of the evidence. The null is a place to start and itsÂuse does not imply that it is carved in stone.
-TDC
I disagree with Tom on a key point here. The "null" hypothesis does not really apply to retaining T. rex as the sole designated species. That would only be true if the Tyrannosaurus sample which is now pretty large had been formally stratigraphically and morphologically assessed and compared to other dinosaurs and the evidence did not indicate more than one species. That has never been done, so that there is only one species has been an unsubstantiated assumption. And it is known that Tyrannosaurus exhibits exceptional variation in robustness which is itself evidence that there may be more than one species vertically and or horizontally geology wise.
The situation with Triceratops is unique in the scale quality of the data available, similar standards cannot be applied to any other dinosaur at this time. And what a paleo/species is is not tightly defined in any case. Aside from Triceratops multi species within dinosaur genera are based on very limited evidence, and are usually broadly equal to or a little better than is the evidence for one species in the genus. And nobody makes a fuss about it which is scientifically correct. for instance there is lot of disagreement about how many species or even genera are represented by the dozen or so fossils assigned to Archaeopteryx lithographic the stratigraphy of which are pretty well known, and none of the competing taxonomies enjoys a null hypothesis or even a preponderance of evidence advantage over the other, and that may remain true for many many decades if not forever as species dribble out. All the hypotheses for the taxonomy of the early bird are valid, competitive and part of the continuing investigation of the fossils. Â
Also note that if there are multiple species of Tyrannosaurus that does not automatically indicate anagenesis. The data may indicate otherwise. Remember that giant tyrannosaurids were often a gracile form and a robust form living in the same time and place in wee Laramidia. With the greatly expanded resource base after the recognition of N Amer two elephant sized tyrannosaurids living in the same place at the same time is very viable, if not logical.Â
GSPaul
-----Original Message-----
From: Thomas Carr <tcarr@carthage.edu>
To: Gregory Paul <gsp1954@aol.com>
Sent: Fri, Feb 26, 2021 12:50 pm
Subject: Re: [dinosaur] Tyrannosaurus rex as an invasive species
I am unconvinced that T. rex did not arise in Asia - its origins there fits with what we see geographically and phylogenetically.
The diversity of Tyrannosaurus in Laramidia is a legitimate question, but currently the null hypothesis is that there is a single taxon. T. rex does not nearly have the sample size or stratigraphic data that Triceratops currently enjoys. Triceratops does not make it unreasonable to expect anagenesis in T. rex, and we should also expect to see the same in the other Late Maastrichtian dinosaurs, including Edmontosaurus, Leptoceratops, Ankylosaurus, Thescelosaurus, Sphaerotholus, Pachycephalosaurus, and so on. All of that presumes that all dinosaurs were subject to the same selective conditions that produced anagenesis in Triceratops.
Until unequivocalÂevidence comes to light, the null for T. rex (and all of the other non-Triceratops dinosaurs) is that a single nonanageneticÂspecies was present throughout the last million years on Laramidia.
It is doubtful that the chilly Bering land bridge at its widest could sustain an elephant sized predator even at its southern zone which renarrows the geographic resource base. That Tyrannosaurus shows up just as the retraction of the interior seaway tremendously expanded the resource base for N Amer super theropods better indicates that the evolution of the genus was a N Amer event, in parallel with its main prey the new elephant sized Triceratops also made possible by the expanded land area of a united N Amer. That the ancestor of Tyrannosaurus is Asian is a very good possibility, or maybe it converged with almost as gigantic Tarbo. May not be possible to tell because there may not be sufficient late Maastrichtian deposits in Asia to record sufficiently complete tyrannosaurids.Â
Even if the Asian ancestor of Tyrannosaurus was as gigantic -- definitely possible -- then somehow made it across a narrow chilly Alaska which may have lacked sufficiently large herbivores to dine on, and then just happened to get to mainland N Amer just when the resource base was suddenly expanding, and remained the same species T. rex during all that is quite the stretch. Dinosaur species tend to turn over rapidly, we now know that three or four Triceratops species evolved sequentially during the Hell Creek deposition. And we know that two giant tyrannosaurids can exist in the same time and place. Why folks are so into labeling every late Maastr elephant sized tyrannosaurid specimen complete or fragmentary T rex without doing a stratigraphic morphological analysis to confirm it is rather odd. There is not such obsession with any other dinosaur species.Â
GSPaulÂ