Laboratory of Vertebrate Evolution, College of Life Science, Capital Normal University, Beijing 100048, China;
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, 210008, China.
Proc Natl Acad Sci U S A. 2017 Oct 10;114(41):10930-10935. doi: 10.1073/pnas.1708023114. Epub 2017 Sep 25.
Beaks are innovative structures characterizing numerous tetrapod lineages, including birds, but little is known about how developmental processes influenced the macroevolution of these important structures. Here we provide evidence of ontogenetic vestigialization of alveoli in two lineages of theropod dinosaurs and show that these are transitional phenotypes in the evolution of beaks. One of the smallest known caenagnathid oviraptorosaurs and a small specimen of the Early Cretaceous bird both possess shallow, empty vestiges of dentary alveoli. In both individuals, the system of vestiges connects via foramina with a dorsally closed canal homologous to alveoli. Similar morphologies are present in , a beaked theropod that becomes edentulous during ontogeny; and an analysis of neontological and paleontological evidence shows that ontogenetic reduction of the dentition is a relatively common phenomenon in vertebrate evolution. Based on these lines of evidence, we propose that progressively earlier postnatal and embryonic truncation of odontogenesis corresponds with expansion of rostral keratin associated with the caruncle, and these progenesis and peramorphosis heterochronies combine to drive the evolution of edentulous beaks in nonavian theropods and birds. Following initial apomorphic expansion of rostral keratinized epithelia in perinatal toothed theropods, beaks appear to inhibit odontogenesis as they grow postnatally, resulting in a sequence of common morphologies. This sequence is shifted earlier in development through phylogeny until dentition is absent at hatching, and odontogenesis is inhibited by beak formation .
喙是许多四足动物谱系（包括鸟类）的创新结构，但人们对发育过程如何影响这些重要结构的宏观进化知之甚少。在这里，我们提供了两个兽脚亚目恐龙谱系中肺泡发生退化的证据，并表明这些是喙进化中的过渡表型。已知最小的 caenagnathid oviraptorosaur 之一和一只小早白垩世鸟类都具有浅而空的下颚牙槽的退化痕迹。在这两个个体中，退化系统通过孔与一个与肺泡同源的背侧封闭的管连接。喙兽脚亚目恐龙在个体发育过程中也具有类似的形态，在其个体发育过程中变得无齿；对新生和古生物学证据的分析表明，牙齿的退化在脊椎动物进化中是一种相对常见的现象。基于这些证据，我们提出，牙发生的逐渐提前的产后和胚胎缩短与与隆突相关的头状角质蛋白的扩张相对应，并且这些祖生性和后生性的异时性结合起来驱动了非鸟类兽脚亚目恐龙和鸟类中无齿喙的进化。在有齿兽脚亚目恐龙的围产期到头状角质化上皮的最初特化扩张之后，喙似乎在出生后阻止牙发生，导致一系列常见的形态。通过系统发育，这个序列在发育过程中更早地转移，直到孵化时没有牙齿，并且牙发生被喙的形成所抑制。
