Extinct gigantic megatooth sharks were at the top of the marine food web once. Their विकास 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 युग.
थिसिस विशाल शार्कहरू तीखा, केरा-आकारको दाँतले सम्पन्न थिए र शरीरको आकारमा सबैभन्दा ठूलो मध्ये एक थिए (निलो ह्वेलको छेउमा)। तिनीहरू ह्वेल, डल्फिन, सिल र अन्य साना शार्कहरूको शिकार गर्ने सबैभन्दा शक्तिशाली समुद्री सिकारीहरू मध्ये एक मानिन्छ।
यसको समयमा विकास, 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 पोषण which was one of reasons behind their विकास to gigantic body sizes1.
मेगाटूथ शार्कहरू फूड वेब र अन्तिम सिकारीको शीर्षमा थिए2। तिनीहरूसँग कुनै पनि समुद्री प्रजातिहरूको लागि उच्च ट्राफिक स्तर थियो। (ट्रोफिक स्तर भनेको खाद्य श्रृंखलामा जीवको स्थिति हो, यो प्राथमिक उत्पादकहरूको लागि 1 को मानदेखि समुद्री स्तनधारी र मानवहरूको लागि 5 सम्म हुन्छ)।
यी शार्कहरू कसरी विशाल शरीरको आकारमा विकसित भए र तिनीहरू किन 3.6 मिलियन वर्ष पहिले विलुप्त भए?
| Ectothermy | चिसो-रक्त, चरा र स्तनपायी बाहेक सबै जनावरहरू समावेश छन्। जस्तै, शार्क |
| मेसोथर्मी (वा, क्षेत्रीय एन्डोथर्मी) | चिसो रगतको इक्टोथर्म्स र न्यानो-रक्तयुक्त एन्डोथर्महरू बीचको थर्मोरेगुलेटरी रणनीति भएको जनावर। जस्तै, केही शार्क, समुद्री कछुवा |
| इन्डोथर्मी | न्यानो रगत भएका जनावरहरू, परिवेशको तापक्रम जस्तोसुकै भए पनि शरीरको तापक्रम स्थिर राख्छन्, यसमा चराहरू र स्तनपायीहरू समावेश हुन्छन्। (एन्डोथर्मीले क्षेत्रीय इन्डोथर्मी वा मेसोथर्मीलाई व्यापक अर्थमा समावेश गर्दछ) |
शार्क कार्टिलागिनस माछाहरू हुन् र चिसो रगत भएका समुद्री जनावरहरू (इक्टोथर्मिक) हुन्। त्यस्ता जनावरहरूमा चयापचय रूपमा शरीरको तापक्रम बढाउने र गर्मी कायम राख्ने क्षमता हुँदैन।
Had megatooth sharks undergone thermo-physiological changes in course of its विकास 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.
PNAS मा 26 मा प्रकाशित हालैको अध्ययनमाth June 2023, researchers investigated thermo-physiology of megatooth sharks to explain its विकास 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। तर यो थर्मोरेगुलेटरी क्षमता मेगाटूथ शार्कहरूको लागि समयमै महँगो साबित भयो।
मेगाटूथ शार्कहरू समुद्री खाना वेबको शीर्षमा शीर्ष शिकारी थिए2. 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 विकास of megatooth sharks also contributed to their extinction following changes in climate.
विलुप्त मेगाटुथ शार्कहरू जस्तै, आधुनिक शार्क प्रजातिहरू जलवायु परिवर्तनको प्रभावबाट उन्मुक्त छैनन् त्यसैले तिनीहरूको संरक्षणको आवश्यकता छ।
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सन्दर्भ:
- Ballell, A., Ferrón, HG बायोमेकानिकल इनसाइट्स इन द डेन्टिसन अफ मेगाटूथ शार्क (Lamniformes: Otodontidae)। Sci Rep 11, 1232 (2021)। https://doi.org/10.1038/s41598-020-80323-z
- कास्ट ईआर एट अल 2022. सेनोजोइक मेगाटूथ शार्कहरूले अत्यन्त उच्च ट्रफिक स्थान ओगटे। विज्ञान प्रगति। २२ जुन २०२२। भोल्युम ८, अंक २५। DOI: https://doi.org/10.1126/sciadv.abl6529
- ग्रिफिथ्स एमएल, एट अल 2023. विलुप्त मेगाटूथ शार्कहरूको एन्डोथर्मिक फिजियोलोजी। PNAS। जुन 26, 2023। 120 (27) e2218153120। https://doi.org/10.1073/pnas.2218153120
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