Tyrannosauridae, Osborn, 1905

Paul, Gregory S., Iv, W. Scott Persons & Van Raalte, Jay, 2022, The Tyrant Lizard King, Queen and Emperor: Multiple Lines of Morphological and Stratigraphic Evidence Support Subtle Evolution and Probable Speciation Within the North American Genus Tyrannosaurus, Evolutionary Biology 49 (2), pp. 156-179 : 19-21

Tyrannosauridae incertae sedis

For multiple species

Multiple species are the norm within a genus

Multiple species have been documented in other dinosaur genera, especially contemporary Triceratops

Sufficient time available for subtle speciation and may favor such, in parallel to speciation observed in Triceratops over exactly the same stratigraphic span

Radical alterations in regional geography in Maastrichtian as North American continent reunited and dramatically expanded resource base, probably favored a burst of rapid evolution in tyrannosaurids that could favor rapid speciation both over time and laterally

Much more variation in femoral robustness than observed in any other theropod or dinosaur species—the variation being significantly statistically greater than a sample of femora of an allosaur species from a single quarry—including any tyrannosaurid species, and more than in

all other tyrannosaurids combined consisting of up to 7 genera and 8+ species from two continents spanning 10 million years compared to 0.7–1.5+ million years for a smaller Tyrannosaurus sample from a small region, strongly favoring speciation over sexual dimorphism, ontogeny, or individual variation

That total number of robust femora are over twice that of all gracile femora strongly contradicts both sexual dimorphism and ontogeny as causes

Some femora that are only two thirds adult size are robust, in some cases more so than some of the longest femora, while the some of the largest and the largest are gracile with the longest known femur being the slender-most among adults, directly contradicting ontogeny as the cause of robustness

That reproduction has not been shown to have been occurring as early in ontogeny as the onset of large variations in the robustness of juveniles means that early reproduction does not currently offer an explanation for the observed pattern

That the solely robust femora of early Tyrannosaurus followed by the much greater variation in proportions higher up include substantial gracility is due to a relatively smaller earlier sample is not the most likely scenario, because while the variation in a smaller sample may be less than in a larger sample, is not likely to be skewed one way or the other relative to the latter

The most robust femora from the upper TT-zone not being as stout as the most robust examples from low in the zone further supports the pattern being real, and is in accord with a proportional shift in the genus, rather than the stasis most compatible with no speciation

Because low variability limited to robustness in early Tyrannosaurus appears to be a retention of the ancestral condition observed in other earlier tyrannosaurids (both individual species or in total) additionally supports the limitation to only robustness among basal Tyrannosaurus as probably being real

That gracile femora are found only in upper TT-zone, while robusts are present in all levels, contradicts the consistent strong proportional variation necessary for dimorphism to be persistently present

That proportional variation in low TT-zone Tyrannosaurus is not higher than observed in other tyrannosaurid species is compatible with and indicates that only one species was extant at that level

That proportional variation in high TT-zone Tyrannosaurus is higher than observed in other theropod species is compatible with and indicates that two species were extant at that level

Proportional variation being low in the lower TT-zone and high in the higher TT-zone strongly indicates speciation either because the sudden onset of major dimorphism indicates the kind of dramatic shift in reproductive behavior that is the epitome of species separation and designation, or two new contemporary species with each retaining the limited dimorphism apparently typical of dinosaurs

The same basics as immediately above apply if the new proportional variation in the upper TT-zone was primarily due to a new ontogenetic pattern or individual variation both of which are improbable, but in any case indicate a change radical enough to require species recognition

The solely robust Tyrannosaurus sample from the lower TT-zone is smaller than the more gracile set from higher levels, but the sample size difference should not result in the strong skew. So as the lower sample increases in abundance it is not likely that gracile specimens will prove to be as proportionally numerous as they are higher up if they appear at low levels at all

If the smaller sample of lower TT-zone femora greatly expands to include much more gracility than in other tyrannosaurids with future finds, then the great proportional variation compared to other theropods is most in accord with the presence of two species early in the evolution of the genus. If a future lower set shows that graciles are a present but rare compared robusts then the case for two species at that level will be at least as strong, or more so

That gracile Tyrannosaurus femora are unusually slender by normal tyrannosaurid standards, and represent a highly atypical shift over a short period of time, directly contradicts ontogeny while favoring subtle evolution via speciation

Other measurements of robustness in crania and postcrania favor actuality of robust and gracile morphs in good accord with femoral robustness

There are general progressive trends towards more gracility progressing stratigraphically upwards as recorded by most of the proportional measures in addition to femora

General shift from the probable ancestral condition of two small anterior-most dentary incisors to just one progressing upwards in TT-zone is not explained by dimorphism, individual variation, or ontogeny, instead suggests subtle chronospeciation in at least one lineage

Progressive change in dentary incisor number over time correlates statistically strongly with changing femoral robustness accords with evolutionary speciation rather than dimorphism, individual variation, or ontogeny

Specimen sample size analysis is based upon is larger than usual for nonavian dinosaur genera

No statistical analysis contradicts multiple species, and statistical support is often significant for variations in robustness and dentary incisors being indicative of species

The preponderance of evidence strongly favors speciation over all alternatives, and is stronger than average for other multiple species in dinosaur genera

For three species rather than just two chronospecies

Much more variation in femoral robustness than observed in any other theropod or dinosaur species high in the TT-zone, and in all other tyrannosaurids combined, strongly favors lateral speciation in addition to and over just vertical chronospeciation

Two and perhaps more species of earlier western North American giant tyrannosaurids, with one more robust than the other, are present in the same levels of the same formations

Is in best accord with the hypothesis that expansion of the resource base was a driving factor in the combination of both vertical and lateral speciation of elephant sized giant predators as the latest Maastrichtian progressed

Against multiple species

Time span of TT-zone is not sufficiently long to require speciation

Cannot be entirely ruled out that observed patterns are due to extreme individual variation

Statistical support not always strong because of small samples

Stratigraphy of some specimens is not precisely known

Cannot be entirely ruled out that similar variation in femoral and other proportions is present in other fossil dinosaur species

Evidence does not match the exceptional level documented for multiple species of Triceratops

One low TT-zone specimen has just one small incisor

Presence of one small incisor in all upper TT-zone specimens may indicate just one species at that level

BMRP 2002.4.1 , BMRP 2006.4.4 , CMNH 7541 , DDM 344.1 , LACM 28741 , RSM 2990.1 , RSM 2347.1 , TMM 41436-1 , TMM 46028-1 , UMNH 11000 .