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| T. Rex Evolution |
 Title text: Unfortunately, body size and bite force continue to increase. |
Explanation
|
This explanation may be incomplete or incorrect: Created by a BOT WITH NEGATIVE LIMBS, CURRENTLY LIVING IN LOCH NESS - fact-check the graph's "early" claim and flesh things out a bit. Do NOT delete this tag too soon. |
The small and seemingly useless forelimbs of 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 and therefore more useful forelimbs, and hypothesizes that - had non-avian dinosaurs not suffered an extinction event - this trend of "reduced limbs" would continue until present-day descendants of T. Rex were limbless creatures not unlike a giant snake.
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 emphasised and those which don't (such as wasting nutrients on growing unnecessary limbs) are not.
In reality, while animals have evolved to lose the use of the limbs of their predecessors - such as the hind legs of a whale - the development of these vestigial limbs would not 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. This has happened multiple times in the Order Squamata, the lizards and snakes, where limbless body plans have evolved convergently over and over such as in blind worms as compared to true snakes. However these circumstances that encourage such lifestyles are not likely to happen to the large predatorial Tyrannosaurus. With that said Tyrannosaurus would likely eventually lose its arms altogether if it never went extinct.
The title text claims 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 famously elephant-sized T. Rex. In reality animals do not linearly grow in size and a modern Tyrannosaurus would not necessarily be dramatically larger, and could easily have gotten somewhat smaller even with the same body plans and lifestyle.
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 was first the subject of a comic in 605: Extrapolating and has since become a recurrent topic 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)
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