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A species of a group of dinosaurs related to the Sauropods has been discovered in South Africa. The Sauropods, which include the popularly known ‘Brontosaurus’ (now correctly known as Apatosaurus), are known to have had bi-pedal ancestry due to the structure of the bones in the forearm. Aardonyx (‘Earth Claw’) shows characteristics shared with the Sauropods.
Another distinct transitional fossil making is in the news – Darwinopterus modularis, a small flying reptile that has features from both the basal long-tailed pterosaurs (the ‘rhamphorhynchoids’) and the short tailed pterodactyloids that show up later in the fossil record.
The fossil record is a unique source of evidence for important evolutionary phenomena such as transitions between major clades. Frustratingly, relevant fossils are still comparatively rare, most transitions have yet to be documented in detail and the mechanisms that underpin such events, typified by rapid large scale changes and for which microevolutionary processes seem insufficient, are still unclear. A new pterosaur (Mesozoic flying reptile) from the Middle Jurassic of China, Darwinopterus modularis gen. et sp. nov., provides the first insights into a prominent, but poorly understood transition between basal, predominantly long-tailed pterosaurs and the more derived, exclusively short-tailed pterodactyloids. Darwinopterus exhibits a remarkable ‘modular’ combination of characters: the skull and neck are typically pterodactyloid, exhibiting numerous derived character states, while the remainder of the skeleton is almost completely plesiomorphic and identical to that of basal pterosaurs. This pattern supports the idea that modules, tightly integrated complexes of characters with discrete, semi-independent and temporally persistent histories, were the principal focus of natural selection and played a leading role in evolutionary transitions.
Evolution: Education and Outreach has a new issue available, focusing on transitional fossils. One of Darwin’s greatest disappointments was that the fossil record of his era did little to support his theory of evolution by natural selection. From On the Origin of Species:
The main cause, however, of innumerable intermediate links not now occurring everywhere throughout nature depends on the very process of natural selection, through which new varieties continually take the places of and exterminate their parent-forms. But just in proportion as this process of extermination has acted on an enormous scale, so must the number of intermediate varieties, which have formerly existed on the earth, be truly enormous. Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record. Charles Darwin (On the Imperfection of the Geological Record, chapter 9, pg 279/280)
But that was then. Today we have an excellent array of intermediate fossils that confirm evolution and this issue of Evolution: Education and Outreach is devoted to that subject. The articles include:
The article on the much ballyhooed fossil find of an early primate is now available free online at Plos 1. From the research article background:
The best European locality for complete Eocene mammal skeletons is Grube Messel, near Darmstadt, Germany. Although the site was surrounded by a para-tropical rain forest in the Eocene, primates are remarkably rare there, and only eight fragmentary specimens were known until now. Messel has now yielded a full primate skeleton. The specimen has an unusual history: it was privately collected and sold in two parts, with only the lesser part previously known. The second part, which has just come to light, shows the skeleton to be the most complete primate known in the fossil record.
See the complete research article and more images:
Professor Per Ahlberg at the Department of Physiology and Developmental Biology, Uppsala University, together with Jennifer Clack, Cambridge University, and Viviane Callier, Duke University, have studied fossil upper arm bones from the two so-called “four-legged fishes”, Ichthtyostega and Acanthostega, from Greenland. These animals, which lived during the Devonian period about 365 million years ago, were among the earliest vertebrates (backboned animals) with fore- and hindlimbs rather than paired fins. They belong to the common stem group of all living amphibians, reptiles, mammals and birds.
The researchers have identified several half-grown, as well as fully grown, upper arm bones from Ichthyostega and Acanthostega, allowing them to study how the shape of the bone changed during growth. It turns out that the two animals had different life histories.
Professor Shubin (University of Chicago) is a well-known paleontologist who studies the morphological and developmental origins of the tetrapod limb. He recently discovered Tiktaalik, a species linking aquatic “lobe-finned” fishes with early terrestrial tetrapods.In this video he speaks at the Cleveland Museum of Natural History, in conjunction with the Society of Vertebrate Paleontology‘s annual meeting in downtown Cleveland. (14 October, 2008.)