Extinct gigantic megatooth sharks were at the top of the marine food web once. Their 进化 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.
巨齿鲨，意思是“大牙齿”鲨鱼，是一种在新生代进化的超大型鲨鱼，体型达到约15m，并在约3.6万年前（Mya）的上新世灭绝 时代.
这些巨鲨拥有香蕉大小的锋利牙齿，是体型最大的鲨鱼之一（仅次于蓝鲸）。 它们被认为是有史以来最强大的海洋掠食者之一，捕食鲸鱼、海豚、海豹和其他较小的鲨鱼。
在其 进化, 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 nutrition which was one of reasons behind their 进化 to gigantic body sizes1.
巨齿鲨是食物网的顶端和终极捕食者2。 对于任何海洋物种来说，它们的营养水平都较高。 （营养级是生物体在食物链中的位置，其范围从初级生产者的 1 到海洋哺乳动物和人类的 5）。
这些鲨鱼是如何进化到巨大的体型的，以及为什么它们在大约 3.6 万年前就灭绝了？
| 变温法 | 冷血动物，包括除鸟类和哺乳动物之外的所有动物。 例如，鲨鱼 |
| 中温（或区域吸热） | 具有介于冷血变温动物和温血恒温动物之间的体温调节策略的动物。 例如，一些鲨鱼、海龟 |
| 吸热 | 无论环境温度如何，温血动物都保持恒定的体温，包括鸟类和哺乳动物。 （吸热包括区域吸热或广义的中温） |
鲨鱼是软骨鱼，是冷血海洋动物（变温动物）。 这些动物没有能力通过新陈代谢提高体温和保留热量。
Had megatooth sharks undergone thermo-physiological changes in course of its 进化 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 日发表在 PNAS 上的一项最新研究th June 2023, researchers investigated thermo-physiology of megatooth sharks to explain its 进化 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。 但事实证明，这种体温调节能力对于巨齿鲨来说代价高昂。
巨齿鲨是海洋食物网顶端的顶级掠食者2。他们的顶级营养级饮食、巨大的体型和吸热生理学意味着高代谢成本和高生物能需求。当生产性栖息地减少和海平面变化时，能量平衡就会受到干扰。这改变了猎物的景观，猎物变得稀缺。由此产生的食物短缺给巨型巨齿鲨带来了负面选择压力，最终导致 3.6 Mya 灭绝。吸热，关键驱动因素 进化 巨齿鲨的大量繁殖也导致了它们因气候变化而灭绝。
与已灭绝的巨齿鲨一样，现代鲨鱼物种也无法免受气候变化的影响，因此需要对其进行保护。
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参考文献：
- Ballell, A.、Ferrón, HG 对巨齿鲨齿列的生物力学见解（唇形目：齿齿鲨科）。 科学报告 11, 1232 (2021)。 https://doi.org/10.1038/s41598-020-80323-z
- 卡斯特ER 等 2022年，新生代巨齿鲨占据了极高的营养位置。 科学进步。 22 年 2022 月 8 日。第 25 卷，第 XNUMX 期。DOI： https://doi.org/10.1126/sciadv.abl6529
- 格里菲斯 ML， 等 2023.已灭绝巨齿鲨的吸热生理学。 美国国家科学院院刊。 26 年 2023 月 120 日。27 (2218153120) eXNUMX。 https://doi.org/10.1073/pnas.2218153120
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