New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction

SUMMARY

Giant carnivorous dinosaurs such as Tyrannosaurus rex and abelisaurids are characterized by highly reduced forelimbs that stand in contrast to their huge dimensions, massive skulls, and obligate bipedalism. Anothergroup that follows this pattern, yet is still poorly known, is the Carcharodontosauridae: dominant predatorsthat inhabited most continents during the Early Cretaceous and reached their largest sizes in Aptian-Cenomanian times. Despite many discoveries over the last three decades, aspects of their anatomy,especially with regard to the skull, forearm, and feet, remain poorly known. Here we report a new carcharodontosaurid, Meraxes gigas, gen. et sp. nov., based on a specimen recovered from the Upper Cretaceous Huincul Formation of northern Patagonia, Argentina. Phylogenetic analysis places Meraxes among derived Carcharodontosauridae, in a clade with other massive South American species. Meraxes preserves novelanatomical information for derived carcharodontosaurids, including an almost complete forelimb that provides evidence for convergent allometric trends in forelimb reduction among three lineages of large-bodied,megapredatory non-avian theropods, including a remarkable degree of parallelism between the latestdiverging tyrannosaurids and carcharodontosaurids. This trend, coupled with a likely lower bound on forelimb reduction, hypothesized to be about 0.4 forelimb/femur length, combined to produce this short-armedpattern in theropods. The almost complete cranium of Meraxes permits new estimates of skull length in Giganotosaurus, which is among the longest for theropods. Meraxes also provides further evidence thatcarchardontosaurids reached peak diversity shortly before their extinction with high rates of trait evolutionin facial ornamentation possibly linked to a social signaling role. RESULTS: Systematic paleontology: Theropoda Marsh, 1881.Tetanurae Gauthier, 1986. Allosauroidea Marsh, 1878. Carcharodontosauridae Stromer, 1931. Carcharodontosaurinae Stromer, 1931. Giganotosaurini Coria and Currie, 2006. Meraxes gigas gen. et sp. nov.Etymology: Meraxes after a dragon of the Song of Ice and Fire.
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S America was home to countless sauropods so there were carnivorous dinos to feed on them. Meraxes, even though it belongs in a different family than T. rex, shows the same evolutionary trend toward reduced fore lims in these apex predators.

JacksinPA said:

Meraxes, even though it belongs in a different family than T. rex, shows the same evolutionary trend toward reduced fore lims in these apex predators.

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Interesting. Apparently there's some evolutionary advantage to the shortened forelimbs. I can't imagine what it could be.

Meraxes, now we naming things after Games Of Throne?!?!

DCCougar said:

Interesting. Apparently there's some evolutionary advantage to the shortened forelimbs. I can't imagine what it could be.

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The arms were possibly more effective that way for slashing attacks in close quarters and less chance of being injured when fighting. Not to mention being out of the way from their massive jaws when fighting or feeding.

Gordy327 said:

The arms were possibly more effective that way for slashing attacks in close quarters and less chance of being injured when fighting. Not to mention being out of the way from their massive jaws when fighting or feeding.

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Short arms obviously were an evolutionary advantage for these largee carnivores. Go figure.

Gordy327 said:

The arms were possibly more effective that way for slashing attacks in close quarters and less chance of being injured when fighting. Not to mention being out of the way from their massive jaws when fighting or feeding.

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They rarely fought one another. It wasn't like in the movies, the reason for the short arms was because it made them better swimmers and better at finger bangs,

JacksinPA said:

Short arms obviously were an evolutionary advantage for these largee carnivores. Go figure.

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All they needed were large jaws and teeth.

slavablueberryjam said:

They rarely fought one another. It wasn't like in the movies, the reason for the short arms was because it made them better swimmers and better at finger bangs,

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Whether they fought one another or if prey fought back makes little difference. The end result would be the same.

Gordy327 said:

Whether they fought one another or if prey fought back makes little difference. The end result would be the same.

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I posted the other day about a sauropod whose vertebral spines grew out of the skin to form rearward-facing defensive spines. I would guess that a large carnivore would prefer to attack a sauropod 'blind-sided' from the rear, so those spikes were a smart move by Nature.

DCCougar said:

Interesting. Apparently there's some evolutionary advantage to the shortened forelimbs. I can't imagine what it could be.

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Speed, balance, and gripping.

As a quadrupedal your arms and legs need to be about the same length but when you start walking/running the long arms become an impediment.