A surprising fossil vertebrate

doi:10.1126/science.abe7826

GSTDTAP > 气候变化

DOI	10.1126/science.abe7826
	A surprising fossil vertebrate
	David B. Wake
	2020-11-06
发表期刊	Science
出版年	2020
英文摘要	The invasion of land by vertebrates initiated an explosion of new kinds of organisms—amphibians—whose diversity ballooned until the extinction event that ended the Paleozoic Era 252 million years ago (Ma) nearly wiped them out. Several early amphibians became specialized in morphology and life history, even including forms that lost limbs. Many had bizarre shapes, and they also varied greatly in size. Among them were the ancestors of the still-living salamanders, frogs, and caecilians, collectively known as lissamphibians. By the dawn of the Mesozoic Era, which followed the Paleozoic, only liss-amphibians and one other group (trematosaurs) survived. The last trematosaurs disappeared in the late Mesozoic, 120 Ma. But paleontologists had overlooked one clade. On page 687 of this issue, Daza et al. ([ 1 ][1]) introduce an unusual fossil of the obscure and apparently extinct albanerpetontids. First detected in Late Cretaceous (100 to 66 Ma) formations and recorded as salamanders, albanerpetontids gradually became recognized by paleontologists as a fourth kind of lissamphibian, albanerpetontids (Order Allocaudata). With the discovery of several new taxa, albanerpetontids (named for the Albian, a mid-Cretaceous age and stage, 100 to 113 Ma) became better known. They were salamander-like, or maybe lizard-like, biologically, and persisted for a very long time, first appearing in the Triassic (252 to 201 Ma) and only disappearing “yesterday” (they were living in Italy with other still-surviving amphibians until the Early Pleistocene, about 2 Ma). With luck we might still have them somewhere (hopefully, one will turn up in what remains of biological wilderness on this planet—but time is running out). There is now a much better idea of the kind of organism we seek, thanks to the report by Daza et al. of their spectacular discovery of a new albanerpetontid, Yaksha perettii , encased in Myanmar amber ([ 1 ][1]). The Myanmar amber, in this case about 99 million years old, preserves some spectacular fossils. In 2016, Daza's team ([ 2 ][2]) reported amazingly well-preserved vertebrates, including one thought to be the earliest chameleon. The basis for that identification was a long skeletal element in the head and throat, an entoglossal, found only in chameleons and the key component in its intricate ballistic tongue. The very elongated entoglossal plays a special role in tongue firing in chameleons. It is enveloped by specialized accelerator muscles, which wrap around it. At the anterior end is the massive fleshy tongue pad. The entoglossal of chameleons slightly tapers toward the front, so when the accelerators squeeze down on it the tongue pad is swiftly advanced and slips forward off its end, literally flung into space ([ 3 ][3]). No other taxon was thought to have such an extreme entoglossal. Hence, the initial identification seemed to make sense. Logical—but spectacularly wrong. Daza et al. now show that not only are the bearers of the specialized tongue not chameleons, or even squamates; they instead are members of the mysterious albanerpetontids. What strange lissamphibians these albanerpetontids are: They have claws, scales, and armored skulls—and at least one had a tongue like a modern chameleon. How the lissamphibians are related to each other and to Paleozoic forms is closely studied and contentious, but salamanders and frogs are likely sister taxa (Batrachia), caecilians are mysterious but might be derived from stereospondyls, and albanerpetontids seem likely to be basal derivatives. Although many salamanders are almost prototypical tetrapods, with four limbs of roughly equivalent length, a generally long tail, and roughly proportional head and body, there are also some bizarre exceptions. Sirens lack hind limbs and are very elongate, permanently aquatic forms with gills and a keratinous beak. Aggressively predaceous amphiumas are large, very elongate aquatic forms lacking gills. They have ludicrously small limbs and reduced digits—down to one in one species. Asian giant salamanders reach 180 cm in length and weigh over 50 kg. By contrast, frogs, the most numerous (88% of lissamphibians) and widespread living amphibians, are tailless, short-bodied, usually large-headed, and have limbs of often very unequal length. Caecilians are virtually blind, limbless, nearly tailless but very long-bodied burrowers, except for a bizarre clade of aquatic species in South American rivers and swamps. Now, enter the albanerpetontids. Superficially salamander-like in their prototypical tetrapod habitus, albanerpetontids had features that seemed to suggest a burrowing way of life, with a strange, strengthened lower jaw with an interlocking articulation, a strengthened skull with solid bones covered with stout scales and elaborate bony sculpturing suggesting co-ossification of bone and skin, and with limbs bearing claws on their digits (four front, five hind). But what about the tongue? And how could these animals have been captured in amber sap ? Were they arboreal (see the figure )? The closest analogs may be the strange tropical plethodontid salamanders of the genus Oedipina , a group of highly specialized burrowers, which—such as O. alleni —occasionally become arboreal. Their exquisitely specialized ballistic tongue is as if designed in reverse to that of chameleons—the homolog of the entoglossal (the basibranchial) is itself, together with the tongue pad, projected completely out of the mouth ([ 4 ][4]). The elongated entoglossal is a vivid example of evolutionary convergence, which justifiably continues to receive intense scientific scrutiny ([ 5 ][5], [ 6 ][6]), because it is common and profoundly important. The expanded lissamphibians display different levels of convergence, including what might be termed complete convergence between Yaksha and chameleons, which entails both structural and functional components, and incomplete convergence between Yaksha and plethodontids like Oedipina , a convergence that is functional but only partially structural. 1. [↵][7]1. J. D. Daza et al ., Science 370, 687 (2020). [OpenUrl][8][Abstract/FREE Full Text][9] 2. [↵][10]1. J. D. Daza et al ., Sci. Adv. 2, e1501080 (2016). [OpenUrl][11][FREE Full Text][12] 3. [↵][13]1. D. E. Moulton, 2. T. Lessinnes, 3. S. O'Keeffe, 4. L. Dorfmann, 5. A. Goriely , Proc. R. Soc. A Math. Phys. Eng. Sci. 472, 20160030 (2016). [OpenUrl][14][CrossRef][15] 4. [↵][16]1. S. M. Deban et al ., Proc. Natl. Acad. Sci. U.S.A. 117, 10445 (2020). [OpenUrl][17][Abstract/FREE Full Text][18] 5. [↵][19]1. D. B. Wake, 2. M. H. Wake, 3. C. D. Specht , Science 331, 1032 (2011). [OpenUrl][20][Abstract/FREE Full Text][21] 6. [↵][22]1. J. B. Losos , Evolution 65, 1827 (2011). 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/301985
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	David B. Wake. A surprising fossil vertebrate[J]. Science,2020.
APA	David B. Wake.(2020).A surprising fossil vertebrate.Science.
MLA	David B. Wake."A surprising fossil vertebrate".Science (2020).