Extinct gigantic megatooth sharks were at the top of the marine food web once. Their 進化 to gigantic sizes and their extinction are not well understood. A recent study analysed isotopes from the fossil teeth and found that these sharks developed endothermic thermoregulation and evolved to gigantic sizes but the high metabolic costs and bioenergetic demands could not be sustained longer following shrinkage of productive habitats due to climate change and sea level changes. Consequently, they became extinct 3.6 million years ago. This study also brings to fore the fact that like the extinct megatooth sharks, the modern shark species too are not immune to the effects of climate change hence the need for their conservation.
Megatooth Sharks, meaning “big tooth” sharks, were supersized gigantic sharks that evolved in the Cenozoic era, acquired body size of about 15m and became extinct about 3.6 million years ago (Mya) during the Pliocene 時代.
これらの巨大なサメはバナナほどの鋭い歯を持ち、体の大きさではシロナガスクジラに次ぐ最大のサメの XNUMX つでした。 彼らは、クジラ、イルカ、アザラシ、その他の小型のサメを捕食する、これまでに存在した中で最も強力な海洋捕食者のXNUMXつであると考えられています。
その中に 進化, these sharks had undergone drastic changes in dentition including widened crowns and serrated cutting edges which enabled them to shift from fish-based diet to more energetic marine mammals-based diet. This helped them achieve much richer 栄養 which was one of reasons behind their 進化 to gigantic body sizes1.
巨大歯ザメは食物網の頂点に位置し、究極の捕食者でした2。 彼らはどの海洋種よりも高い栄養レベルを持っていました。 (栄養レベルは食物連鎖における生物の位置であり、一次生産者の値 1 から海洋哺乳類と人間の値 5 までの範囲になります)。
これらのサメはどのようにして巨大な体サイズに進化し、なぜ約3.6万年前に絶滅したのでしょうか?
| 変温療法 | 冷血動物には、鳥類と哺乳類を除くすべての動物が含まれます。 例：サメ |
| 中温（または局所吸熱） | 冷血外温動物と温血内温動物の中間の体温調節戦略を持つ動物。 例: サメ、ウミガメ |
| 吸熱作用 | 恒温動物には、周囲の温度に関係なく一定の体温を維持する動物があり、鳥類や哺乳類が含まれます。 （吸熱には広義の局所吸熱や中温も含まれます） |
サメは軟骨魚であり、変温性の冷血海洋動物です。 このような動物には、代謝的に体温を上昇させ、熱を保持する能力がありません。
Had megatooth sharks undergone thermo-physiological changes in course of its 進化 to acquire endothermic properties? This hypothesis is relevant because unlike cold-blooded (ectothermic), warm-blooded (endothermic) marine animals can have higher cruising speeds and can travel longer distances to catch preys than ectothermic counterparts. Acquisition of endothermic properties (along with transformed dentition) could explain why these sharks evolved to such gigantic sizes.
26日にPNASに掲載された最近の研究ではth June 2023, researchers investigated thermo-physiology of megatooth sharks to explain its 進化 and extinction. They studied geochemical evidence for thermoregulation from clumped isotope paleothermometry and phosphate oxygen isotopes obtained from fossil tooth samples and found that isotope-inferred body temperatures of Otodus species averaged about 7 °C higher than ambient seawater temperatures and other coexisting shark species. An overall warmer body temperature means megatooth sharks had evolved to be endothermic suggesting endothermy was a key driver for their gigantism3。 しかし、この体温調節能力はメガトゥースザメにとって代償を伴うことがやがて判明した。
巨大歯ザメは海洋食物網の頂点に立つ頂点捕食者だった2. Their top trophic level diet, gigantic body sizes and endothermic physiology meant high metabolic costs and high bioenergetic demands. The energy balance was disturbed when productive habitats diminished, and sea level changed. This altered prey landscape, and prey become scarce. The consequent food shortage put negative selection pressure against gigantic megatooth sharks culminating in their extinction 3.6 Mya. Endothermy, the key driver in the 進化 of megatooth sharks also contributed to their extinction following changes in climate.
絶滅したメガトゥースザメと同様に、現代のサメ種も気候変動の影響を免れないわけではないため、その保護が必要です。
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参照：
- Ballell, A.、Ferrón, HG メガトゥースザメ (ヒツジ目: オトドン科) の歯列に関する生体力学的な洞察。 Sci Rep 11、1232 (2021)。 https://doi.org/10.1038/s41598-020-80323-z
- カストER ら 2022年。新生代の巨大歯ザメは、栄養段階の極めて高い位置を占めていた。 科学は進歩します。 22 年 2022 月 8 日。第 25 巻、第 XNUMX 号。DOI: https://doi.org/10.1126/sciadv.abl6529
- グリフィスML、 ら 2023年。絶滅したメガトゥースザメの吸熱生理学。 PNAS。 26 年 2023 月 120 日。27 (2218153120) eXNUMX。 https://doi.org/10.1073/pnas.2218153120
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