Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceans
Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceans
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DOI：
10.1126/science.abf5787
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发表时间：
2021-12
期刊：
影响因子：
56.9
通讯作者：
P. Sander;E. Griebeler;N. Klein;Jorge Velez Juarbe;T. Wintrich;L. Revell;L. Schmitz
中科院分区：
文献类型：
--
作者：
P. Sander;E. Griebeler;N. Klein;Jorge Velez Juarbe;T. Wintrich;L. Revell;L. Schmitz
早期海洋巨人曾经生活过的最大的动物占据了海洋环境。现代鲸类动物在数千万年的时间里进化出了巨大的体型，以应对寒冷海水中生产力的提高。然而，鲸鱼并不是第一个进化出来的海洋巨人。Sander等人描述了一个2.44亿年前的鱼龙化石，其大小可以与现代鲸目动物相媲美（参见Delsett和Pyenson的观点）。这种动物在第一批鱼龙出现后最多存在了800万年，这表明二叠纪大灭绝后的过程可能推动了更迅速的规模扩张。一具2.44亿年前的巨型鱼龙化石表明，在二叠纪大灭绝之后，这一群体中出现了一次巨大的爆发。自古生代以来，二级海洋四足动物的迭代进化为更好地理解动物回归海洋时的解剖学和生态学变化提供了希望。在过去的地质过程中，反复出现的汇聚模式可能表明，当从陆地上的全职生活过渡到海洋中的全职生活时，进化是可预测的。鱼龙（中生代的鱼形海洋爬行动物）和今天的鲸目动物（鲸鱼、海豚和鼠海豚）是两个最有信息的分支，可以作为二次回归海洋的例证。鱼龙和鲸类在体型和生活方式上的显著相似，与它们在时间上的分离形成了鲜明对比，这为趋同进化提供了一个经常被引用的例子。鱼龙在2.49亿年前出现，在接下来的1.5亿年里在海洋中繁衍生息。鲸目动物直到大约5600万年前才进化出来。作为尾巴推进的游泳者，鱼龙和鲸类不仅进化出了趋同的体型，还进化出了更大的体型。理论基础化石和现存数据的整合可以提高对水生适应和巨身化作为趋同进化模式的理解，特别是在生态背景下进行解释时。我们在美国内华达州中三叠世化石山段的古生物野外工作为海洋爬行动物数据提供了基础，并发现了巨型鱼龙，作为化石山动物群的一部分。我们从大量的文献中收集了化石和活鲸的数据。总之，这些数据为最大体型及其随时间演变的计算分析提供了基础。在化石山动物群中建立能量流模型有助于理解化石山生态系统如何在鱼龙进化史的早期支持几只大型到巨型四足动物的海洋消费者。结果我们描述了来自化石山动物群的一个2米长头骨的鱼龙为Cymbospondylus的新种。目前，这是当时已知的陆地或海洋中最大的四足动物，也是一系列海洋巨人中的第一个。化石山动物群包括其他几种大型鱼龙在Cymbospondylus辐射。这个三叠纪动物群的体型范围与现代鲸鱼动物群的体型范围相当，从Phalarodon的2米长到新物种的17米多。在化石记录中，化石山动物群代表了一个稳定的营养网络，甚至可以支持另一个大型鱼龙，如果它以小而丰富的猎物为食，比如菊石。从绝对时间来看，这个新的海洋巨兽生活在2.46亿年前，只比第一批鱼龙出现晚了大约300万年。我们的研究表明，鱼龙在进化支系的历史上很早就进化出了大体型，比鲸鱼要早得多。结论鱼龙和鲸目动物的体型都很大，但它们向巨人症的进化途径不同。从二叠纪末大灭绝中恢复后，鱼龙似乎受益于大量的远洋牙形刺和菊石，即使没有现代初级生产者。鲸目动物走了不同的路线，但似乎都与营养特化有关，包括长须鲸（神秘鲸目）的牙齿脱落，以及有齿鲸（齿鲸目）的猛禽捕食和深潜的进化。鱼龙在历史上比鲸目动物更早进化出更大的体型。美国内华达州中三叠世的化石山动物群对认识这种模式至关重要。它的特点是四足动物中第一个海洋巨人，距离鱼龙首次出现只有300万年。鲸鱼花了相对较长的时间才达到同样大的体型。海洋羊膜动物的身体大小跨越了六个数量级，然而控制这种多样性进化的因素在很大程度上是未知的。现代海洋的高初级产量被认为是鲸类巨兽出现的先决条件，但这一条件不能解释三叠纪鱼龙的巨型现象。我们用来自美国内华达州中三叠世化石山动物群的2米长的头骨描述了新的巨型鱼龙Cymbospondylus youngorum sp. 11，强调了尽管缺乏许多现代初级生产者，但体型的快速进化。令人惊讶的是，化石山动物群在大小范围上与现代海洋哺乳动物动物群的组成相媲美，能量通量模型表明，在鱼龙起源后不久，中三叠纪海洋食物网能够支持几种高营养水平的大型鱼龙。
Description Early marine giant The largest animals to have ever lived occupied the marine environment. Modern cetaceans evolved their large size over tens of millions of years in response to the increased productivity of cold marine waters. However, whales were not the first marine giants to evolve. Sander et al. describe a 244-million-year-old fossil ichthyosaur that would have rivaled modern cetaceans in size (see the Perspective by Delsett and Pyenson). The animal existed at most 8 million years after the emergence of the first ichthyosaurs, suggesting a much more rapid size expansion that may have been fueled by processes after the Permian mass extinction. —SNV A 244-million-year-old fossil of a massive ichthyosaur suggests that there was a burst of gigantism in the group after the Permian extinction. INTRODUCTION The iterative evolution of secondarily marine tetrapods since the Paleozoic offers the promise of better understanding how the anatomy and ecology of animals change when returning to the sea. Recurring patterns of convergence in the geological past may suggest predictability of evolution when transitioning from full-time life on land to full-time life in the ocean. Ichthyosaurs (fish-shaped marine reptiles of the Mesozoic) and today’s cetaceans (whales, dolphins, and porpoises) are two of the most informative lineages to exemplify secondary returns to the sea. The notable resemblance in body shape and lifestyle of ichthyosaurs and cetaceans contrasts with their separation in time by nearly 200 million years, providing an often-cited example of convergent evolution. Ichthyosaurs arose 249 million years ago and populated the oceans for the next 150 million years. Cetaceans did not evolve until about 56 million years ago. As tail-propelled swimmers, ichthyosaurs and cetaceans evolved not only convergent body shapes but also large body sizes. RATIONALE The integration of fossil and extant data can improve understanding of aquatic adaptation and gigantism as patterns of convergent evolution, particularly when interpreted in an ecological context. Our paleontological fieldwork in the Fossil Hill Member (Middle Triassic, Nevada, USA) provided the basis for the marine reptile data and resulted in finds of giant ichthyosaurs as part of the pelagic Fossil Hill Fauna. We compiled data for both fossil and living whales from the extensive literature. Together, these data provide the basis for computational analyses of maximum body size and its evolution over time. Modeling of energy flux in the Fossil Hill Fauna helps in understanding how the Fossil Hill ecosystem could have supported several large to giant tetrapod ocean consumers so early in ichthyosaur evolutionary history. RESULTS We describe an ichthyosaur with a 2-m-long skull from the Fossil Hill Fauna as a new species of Cymbospondylus. At present, this is the largest known tetrapod of its time, on land or in the sea, and is the first in a series of ocean giants. The Fossil Hill Fauna includes several other large-bodied ichthyosaurs in the Cymbospondylus radiation. The body-size range in this Triassic fauna rivals the range seen in modern whale faunas, from a total length of about 2 m in Phalarodon to more than 17 m in the new species. As preserved in the fossil record, the Fossil Hill Fauna represents a stable trophic network and could even have supported another large ichthyosaur if it bulk fed on small, but abundant, prey such as ammonoids. In absolute time, the new ocean giant lived 246 million years ago, only about 3 million years after the appearance of the first ichthyosaurs. Our research suggests that ichthyosaurs evolved large body size very early on in the clade’s history, comparatively earlier than whales. CONCLUSION Ichthyosaurs and cetaceans both evolved very large body sizes, yet their respective evolutionary pathways toward gigantism were different. Ichthyosaurs seem to have benefited from the abundance of pelagic conodonts and ammonoids after the recovery from the end-Permian mass extinction, even in the absence of modern primary producers. Cetaceans took different routes, but all appear to be related to trophic specialization, including the loss of teeth in baleen whales (Mysticeti) and the evolution of raptorial feeding and deep diving in toothed whales (Odontoceti). Ichthyosaurs evolved large body sizes earlier in their history than cetaceans. The Fossil Hill Fauna of the Middle Triassic of Nevada, USA, is critical for recognizing this pattern. It features the first ocean giant among tetrapods, only 3 million years after ichthyosaurs first appeared. Whales took comparatively longer to attain similarly large body sizes. CREDIT: STEPHANIE ABRAMOWICZ Body sizes of marine amniotes span six orders of magnitude, yet the factors that governed the evolution of this diversity are largely unknown. High primary production of modern oceans is considered a prerequisite for the emergence of cetacean giants, but that condition cannot explain gigantism in Triassic ichthyosaurs. We describe the new giant ichthyosaur Cymbospondylus youngorum sp. nov. with a 2-meter-long skull from the Middle Triassic Fossil Hill Fauna of Nevada, USA, underscoring rapid size evolution despite the absence of many modern primary producers. Surprisingly, the Fossil Hill Fauna rivaled the composition of modern marine mammal faunas in terms of size range, and energy-flux models suggest that Middle Triassic marine food webs were able to support several large-bodied ichthyosaurs at high trophic levels, shortly after ichthyosaur origins.