Making sense of dinosaurs and birds

doi:10.1126/science.abi5697

GSTDTAP > 气候变化

DOI	10.1126/science.abi5697
	Making sense of dinosaurs and birds
	Lawrence M. Witmer
	2021-05-07
发表期刊	Science
出版年	2021
英文摘要	Birds are dinosaurs. A couple of decades of revealing finds and careful phylogenetic work have shown that birds are nested within the group of small, mostly predatory, and running dinosaurs that includes dromaeosaurs and troodontids, among others ([ 1 ][1]). Now, scientists are using that phylogenetic tapestry to trace the evolution of traits such as sensory biology and behavior. Two studies in this issue pair cutting-edge imaging with sophisticated statistical analyses to explore the evolution of the hearing apparatus and other sensory systems. On page 601, Hanson et al. ([ 2 ][2]) show that the inner ear of dinosaurs, as well as that of other archosaurs (the group that includes crocodilians, dinosaurs, and birds), provides clues to locomotion, hearing, and the evolution of vocalization. On page 610, Choiniere et al. ([ 3 ][3]) examine the inner ear and also aspects of the visual system to demonstrate the notably owl-like nocturnal adaptations of one group of birdlike dinosaurs. Until recently, the advances presented by these teams of authors were unthinkable, in that many aspects of internal anatomy and certainly their connection to habits like parental care and daily activity patterns had been out of reach. Two innovations that may fairly be called revolutionary made these discoveries possible. The first revolution was x-ray computed tomography (CT) scanning, which allowed scientists to peer through flesh, bone, and, in the case of fossils, rock to visualize and model internal anatomical structures ([ 4 ][4]). Early on, CT scanners at hospitals were the main option, but the widespread deployment of micro- and nano-CT scanners over the past several years has permitted the collection of numerous high-resolution datasets. Moreover, the “open-data movement” ([ 5 ][5]) has resulted in the sharing of open-access data collected by laboratories worldwide. Thus, Hanson et al. and Choiniere et al. were able to analyze the anatomy of inner ears from 124 and 91 extinct and extant species, respectively—an increase by perhaps an order of magnitude from what was possible 10 to 15 years ago. The second revolution is the widespread use of sophisticated statistical software that allows such large datasets to be queried and probed with unprecedented detail and clarity. Both studies implemented R ([ 6 ][6]), the open-source, seemingly infinitely tweakable software that has taken by storm disciplines ranging from science to economics and has contributed to the widespread coding movement. The combination of open-access data and open-source analytical tools means that both studies reach a high standard for replicability. Anatomically, Hanson et al. and Choiniere et al. both focused on the inner ear, a tiny organ consisting of a labyrinth of canals that is buried deep within the skull. In the past, that position made the inner ear inaccessible to study, but that same inaccessibility protected it during the process of fossilization, which allows the inner ear labyrinth to be digitally extracted relatively easily with CT scanning. And that is indeed helpful because the inner ear is two sensory organs in one: Three delicate, mutually perpendicular semicircular canals (SSCs) sit atop the inner ear, and hanging below is the cochlea. The SSCs are involved with the sense of equilibrium and balance and have been targeted as a source of information on posture and locomotion. The cochlea is the hearing organ. Although the actual sensory tissues and neural wiring are long gone in the fossils, the shape and orientation of the bony canals allow some broad-brush assessments of function, and the SSCs have been used to infer locomotor shifts in groups as diverse as primates ([ 7 ][7]) and plesiosaurs ([ 8 ][8]). Likewise, the length of the bony cochlea is a good proxy for the sensory basilar papilla and has been used to study auditory evolution in prior studies of birds and dinosaurs ([ 9 ][9]). The Hanson et al. study is a landmark, showing that the two parts of the inner ear are different modules that follow somewhat separate evolutionary paths, and both have stories to tell about behavior. The SSCs can be used to discriminate different locomotor grades. It may not be surprising that species at opposite ends of the spectrum, like quadrupeds (e.g., early archosaurs and extant crocodilians) and highly aerobatic extant birds, can be differentiated easily, but the middle ground is exciting because this is where the transition to birds takes place. This multivariate “shape cloud” includes the small, birdlike nonavian theropods and early birds such as the iconic Archaeopteryx , as well as some flightless birds (e.g., ostriches and penguins) and “reluctant fliers” (e.g., chickens and cranes) that have apparently reverted their SSC structure. Notably, some very birdlike theropods, such as troodontids, plot very closely to early “simple fliers” like Archaeopteryx (see the photos), providing further evidence that multiple groups of birdlike theropods were experimenting with aerodynamic flight surfaces (feathers) and that birds simply represent the group that survived long enough to hone and perfect flight. By contrast, the long cochlea of extant birds evolved very early and is found in almost all archosaurs but is absent in other reptiles like turtles and lizards. Hanson et al. discovered that extant archosaurs (crocodilians and birds) differ from other reptiles in that not only do archosaurs have parental care, but archosaur babies, unlike turtle or lizard babies, also chirp to capture their parents' attention. Thus, the authors perceptively suggest that juvenile vocalization to elicit parental care coevolved with the elongation of the cochlea in archosaurs. Choiniere et al. also found the cochlea to be informative about behavior in small birdlike theropods, many of which had the elongated cochlea suggestive of sensitive hearing. But two groups—again troodontids, but especially an unusual group called alvarezsaurids—had exceptionally long, curved cochleae. In some alvarezsaurids, such as Shuvuuia , the cochlea approaches—in length and structure—that of today's barn owl, perhaps the most notable avian nocturnal specialist. To further test the idea that Shuvuuia and other alvarezsaurids might have been nocturnal, the team turned to the visual system. Fortunately, dinosaurs (including birds) and most other reptiles had bony eye rings that previous researchers ([ 10 ][10]) had shown provided links to daily activity patterns, whereby the eye rings of nocturnal species tend to have wider openings to admit more light than the rings of species active during the day. Choiniere et al. carefully reconstructed the bony eye rings in two alvarezsaurids, and, sure enough, they plotted with extant nocturnal birds and reptiles. Thus, two semi-independent systems—the cochlea and eye ring—point to alvarezsaurids being nighttime specialists, perhaps providing new clues to the enigmatic habits of these peculiar, short-armed species ([ 11 ][11]). Documenting life's history with fossil discoveries remains the core of paleontology, but these two studies typify a new wave of paleontologists, armed not with a pick and shovel but with a CT scanner and R code. Likewise, the basic phylogenetic work of who is related to whom continues, but now these phylogenies are being used to explain evolution's narrative. Teeth and limbs will always provide clues for reconstructing evolutionary histories, but relatively new anatomical players like the inner ear and bony eye rings are opening new windows into the past. And through such windows, we now can hear baby dinosaurs calling to their parents, watch a small troodontid clumsily fly to a low branch, and maybe catch a glimpse of the moon shadow of a passing Shuvuuia . 1. [↵][12]1. A. Cau , Boll. Soc. Paleontol. Ital. 57, 1 (2018). [OpenUrl][13][CrossRef][14][PubMed][15] 2. [↵][16]1. M. Hanson et al ., Science 372, 601 (2021). [OpenUrl][17][Abstract/FREE Full Text][18] 3. [↵][19]1. J. N. 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领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/325929
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Lawrence M. Witmer. Making sense of dinosaurs and birds[J]. Science,2021.
APA	Lawrence M. Witmer.(2021).Making sense of dinosaurs and birds.Science.
MLA	Lawrence M. Witmer."Making sense of dinosaurs and birds".Science (2021).