Editing 3042: T. Rex Evolution

{{comic
| number    = 3042
| date      = January 24, 2025
| title     = T. Rex Evolution
| image     = t_rex_evolution_2x.png
| imagesize = 418x378px
| noexpand  = true
| titletext = Unfortunately, body size and bite force continue to increase.
}}

==Explanation==
{{incomplete|Created by a ZERO-LEGGED SPIDER. Confirm the graph's claims about T. rex's predecessors. Do NOT delete this tag too soon.}}

The small and commonly supposed useless forelimbs of ''{{w|Tyrannosaurus rex}}'' have often been used as a source of humor in works of fiction that feature dinosaurs. [[Randall]] claims that the ancestors of ''T. rex'' had (relatively) longer forelimbs, and hypothesizes that — had non-avian dinosaurs not suffered an {{w|Cretaceous–Paleogene extinction event|extinction event}} — this trend of "reduced limbs" would have continued until present-day descendants of ''T. rex'' were limbless creatures not unlike a giant snake.

{{w|Evolution|Evolutionary theory}} indicates that different selection pressures cause the development or loss of characteristics, as those characteristics which enable better survival to continuation of the species are emphasized and those which don't (such as wasting nutrients on growing unnecessary limbs) are not.

In reality, while some animals have evolved to lose the use of the limbs of their predecessors — such as the hind legs of a whale — there is no particular reason to think that the ''T. rex'' was on the way to its forelimbs becoming vestigial, as opposed to being well adapted to the niche it filled in the form they had. In fact they appear to have been extremely powerful, which would be an unusually wasteful trait if they were not much used. There is even less reason why the shrinking of one set of limbs would necessarily apply to other limbs. A ''T. rex'' would continue to have substantial need for its hind legs, which it used for running and chasing prey, and would have no reason to lose legs unless environmental factors favored a limbless lifestyle. Loss of all limbs has happened multiple times in the order {{w|Squamata}} (the lizards and snakes), where limbless body plans have evolved convergently over and over, such as in {{w|Common_slow_worm|blindworms}} as compared to true {{w|snake}}s. However, the circumstances that encourage such lifestyles were not likely to happen to the large predatorial Tyrannosaurus. With that said, it is not outright impossible that Tyrannosaurus could have eventually lost its arms if it had never gone extinct.

The title text make a similar claim that, as ''T. rex'' was also larger than its ancestors, the same trend in growth would continue, such that the hypothetical limbless present-day descendant would be even larger than the {{what if|78|famously elephant-sized}} ''T. rex''. In reality animals do not change in size linearly over time, and a modern descendant of Tyrannosaurus would not necessarily be dramatically larger and could easily have become somewhat smaller, even with the same body plans and lifestyle. Though it is unlikely that the constraints of {{w|insular dwarfism}} could have applied to such creatures across the continental range they may have remained alive to inhabit, many unknown factors might have influenced their descendants' body shapes, and probably even induced further speciation to render all surviving branches of the tyrannosaur family tree as distinct, probably to include both growing and re-shrinking lineages. (It is a common misconception, used here for humorous purposes, that an individual personally evolves into a particular form, or that a single descendant represents the only evolutionary track taken 'towards perfection' from a less evolved ancestor.)

This comic may be tangentially related to the fact that birds are dinosaurs that survived the KT extinction, which is a recurring theme on xkcd.

Extrapolation and interpolation, often absurd, are [[:Category:Extrapolation|recurrent topics]] on xkcd.

==Transcript==
:[A graph is shown. The Y-axis is labelled and has two labeled ticks at the top and halfway up from the X-axis:]
:Limbs
:4
:2

:[The X-axis is not labelled but represents time and it has ticks every 10 million years, from 180 million years ago until present time. Every fifth tick is a bit larger and has a label beneath it. Except the one at the present time. Below the last tick is an arrow pointing to that tick with a label. There is another arrow that points to about 65 million years ago and this also has a label:]
:150 million years ago
:100 MYA
:50 MYA
:Now
:Extinction

:[The graph shows three animals positioned from the top left, to the middle to the bottom right of the graph.]
:[The first animal is a regular dinosaur walking on its hind legs, with fairly long front legs. It is positioned at 4 limbs and 150 million years ago. A label is written above and right of it:]
:4 normal limbs

:[An arrow goes along the diagonal of the chart down and right and points to the second animal, a Tyrannosaur Rex, which is located in the middle of the chart next to 2 limbs and, just left of the "extinction" arrow. Larger than the first and with almost no front legs. A label is written above and right of it:]
:Barely more than 2

:[A dashed arrow with a label in the middle continues along the diagonal of the chart to the last animal: A leg-less "dinosaur" with a big open mouth. The animal lies on its belly but with the head part and the tail lifted from the ground.  A label is written above it.]
:?
:???

:[Caption below the panel:]
:If T. rex hadn't gone extinct
:(Linear extrapolation)

{{comic discussion}}

[[Category:Charts]]
[[Category:Timelines]]
[[Category:Extrapolation]]
[[Category:Dinosaurs]]