Large, megatoothed Otodus megalodon ran scorching — the traditional shark was at the least considerably warm-blooded, new proof reveals.
Chemical measurements from fossil O. megalodon tooth recommend the sharks had larger physique temperatures than their surrounding waters, researchers report June 26 in Proceedings of the Nationwide Academy of Sciences. Analyses of carbon and oxygen within the tooth of those and different sharks, each dwelling and extinct, revealed that the large shark’s physique temperature was about 7 levels Celsius hotter than estimated seawater temperatures on the time.
That warm-bloodedness might have been a double-edged sword. The trait might have helped O. megalodon grow to be a swift, fearsome apex predator and develop as much as 20 meters lengthy, making it among the many largest carnivores to ever dwell on Earth. However the shark’s voracious urge for food might have additionally spelled the species’ final doom. Gigantism has a excessive metabolic value, says UCLA marine biogeochemist Robert Eagle: Larger our bodies require extra meals, and the large sharks might have been notably susceptible to extinction when the local weather modified and meals grew to become scarcer.
Mammals are well-known for with the ability to metabolically elevate and preserve their physique warmth, even in colder environments, a trait referred to as endothermy. However some fish lineages, each dwelling and extinct, are able to regional endothermy, sustaining some physique components at larger temperatures than the encompassing water (SN: 6/10/10). For instance, many trendy lamniform sharks — the group that features species like mako and nice white sharks — have this means (SN: 8/2/18).
“Certainly, regional endothermy is one among simply two recognized evolutionary pathways towards big sizes in sharks,” says Jack Cooper, a paleobiologist at Swansea College in Wales who was not concerned within the new examine. (The opposite, Cooper says, is filter feeding, employed by gentler giants similar to whale sharks.)
Scientists have lengthy thought that megalodon was regionally endothermic, Eagle says, primarily based on quite a lot of proof similar to estimates of the megashark’s physique form, in addition to its seemingly swimming speeds and power necessities. The shark was additionally recognized to have a really massive geographic vary across the globe, actively looking in colder in addition to hotter waters, which argues for some warm-bloodedness. A current examine by Cooper and colleagues that modeled the shark’s physique in 3-D estimated that grownup O. megalodon was a transoceanic superpredator, in a position to swim quicker than any dwelling shark species and totally devour prey the dimensions of immediately’s largest predators.
The query, Eagle provides, isn’t actually whether or not O. megalodon was endothermic — it’s how endothermic it was. Specifically, the group puzzled how its physique temperatures in comparison with one among its main ocean rivals, which appeared on the scene late within the shark’s reign: Carcharodon carcharias, higher referred to as the nice white shark (SN: 6/29/22).
O. megalodon appeared round 23 million years in the past and went extinct someday between 3.5 million and a couple of.6 million years in the past. Nice white sharks emerged round 3.5 million years in the past, and so they competed for meals with their large cousins. One speculation has been that this competitors helped drive O. megalodon to extinction (SN: 5/31/22). Local weather change through the Pliocene Epoch, which spanned 5.3 million to 2.6 million years in the past, led to a collapse within the inhabitants of marine mammals, the first meals supply for each sharks.
“The Carcharodon had been a lot smaller … and continued, whereas the Otodus went extinct,” Eagle says. “Carcharodon most likely had a decrease requirement for meals to keep up its metabolic charge.”
To get extra direct proof of the physique temperatures of those shark species, and subsequently higher perceive their respective metabolisms, the group turned to the one fossils the sharks have left behind: their tooth.
Fossilized tooth provide a wealth of encapsulated environmental information. The tooth enamel incorporates each heavier and lighter kinds, or isotopes, of carbon, oxygen and different parts, and the relative abundances of those isotopes is linked to physique temperature. Eagle and his colleagues used a method that examines the abundance of “clumped isotopes” — bonded-together heavy types of carbon (carbon-13) and oxygen (oxygen-18) — as a form of historical geochemical thermometer. The abundance of those bonds is “solely affected by temperature,” providing a extra unambiguous thermometer than utilizing a single factor’s isotopic abundance, Eagle says.
The group used this system on tooth from the totally different sharks, in addition to fossil samples from different historical ocean contemporaries together with whales and mollusks. (Mollusks, being totally cold-blooded, characterize the ocean water temperature, Eagle says). The info present that each sharks had been a bit endothermic, however not solely was O. megalodon’s common physique temperature (about 27⁰ C) larger than its surrounding waters, it was additionally larger than the typical physique temperature of nice whites (about 22⁰ C) dwelling in related waters. Neither shark was as warm-blooded as marine mammals, such because the whale teams Odontoceti and Mysticeti, the group decided.
It’s “a really attention-grabbing discovering, and it’s improbable that we have now extra proof for regional endothermy in megalodon,” Cooper says. O. megalodon’s larger physique temperature would have allowed it “to swim additional and quicker, rising its probabilities of encountering prey,” he says. “However it additionally signifies that if meals availability declines, megalodon wouldn’t have been in a position to meet its big energetic necessities.” And when altering sea ranges within the Pliocene led to a decline within the sharks’ prey about 3 million years in the past, “it could properly have starved into extinction.”
Eagle and colleagues are actually delving into the chicken-or-egg query of which got here first for O. megalodon: warm-bloodedness or apex predator standing. “You want a excessive trophic stage to grow to be gigantic,” Eagle says. However is warm-bloodedness essential to get to that top trophic stage (apex predator standing)? “We’re hoping to suit all of it collectively into an evolutionary story as to what drives what.”
