|Limusaurus inextricabilis (Artist's impression)|
Creationists are probably getting sick and tired of having to deny all the evidence that birds are evolved dinosaurs that has been flooding into the scientific literature over the last few years, but here's some more.
Not only is this pretty convincing evidence, if any more were needed, but it's an example of that other thing that creationists must be sick and tired of having to deny - a transitional species! Creationists like to pretend transitional species and evidence that birds evolved out of therapod dinosaurs is as rare as hen's teeth. It's doubly ironic therefore that this find helps explain just why hen's teeth are so rare - unlike the evidence for the evolution of birds from dinosaurs!
It was discovered by a team from George Washington University visiting the Gobi Desert in Central Asia.
By examining a series of nineteen fossils of the species Limusaurus inextricabilis, a therapod dinosaur from which birds are know to have evolved, they discovered that while juveniles have sharp teeth, these are lost entirely by adulthood. The adults have a mouth resembling the beak of birds. The species gets its name, which literally means ‘mire lizard who could not escape’, from the fact they they are found in 'death traps', having become mired in deep mud and died.
They are found in 159 million year-old deposits in Junggar Basin, Xinjiang, China. The team included Dr. Xing Xu from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China and Dr. Shuo Wang of Capital Normal University in Beijing, China.
- Wang et al. report 78 ontogenetically variable features of the theropod Limusaurus
- Limusaurus is the only known reptile to lose its teeth and form a beak after birth
- The available data are important for understanding the evolution of the avian beak
- The ontogenetically variable features have little effect on its phylogenetic position
Ontogenetic variation is documented within many dinosaur species, but extreme ontogenetic changes are rare among dinosaurs, particularly among theropods. Here, we analyze 19 specimens of the Jurassic ceratosaurian theropod Limusaurus inextricabilis, representing six ontogenetic stages based on body size and histological data. Among 78 ontogenetic changes we identify in these specimens, the most unexpected one is the change from fully toothed jaws in the hatchling and juvenile individuals to a completely toothless beaked jaw in the more mature individuals, representing the first fossil record of ontogenetic edentulism among the jawed vertebrates. Jaw morphological data, including those derived from Mi-CT and SR-μCT scanning of Limusaurus specimens, reveal dental alveolar vestiges and indicate that ontogenetic tooth loss in Limusaurus is a gradual, complex process. Our discovery has significant implications for understanding the evolution of the beak, an important feeding structure present in several tetrapod clades, including modern birds. This radical morphological change suggests a dietary shift, probably from omnivory for juvenile Limusaurus to herbivory for adult Limusaurus, which is also supported by additional evidence from gastroliths and stable isotopes. Incorporating new ontogenetic information from Limusaurus into phylogenetic analyses demonstrates surprisingly little effect on its placement when data from different stages are used exclusively, in contrast to previous analyses of tyrannosaurids, but produces subtle differences extending beyond the placement of Limusaurus.
A press release from George Washington University explains:
"This discovery is important for two reasons," said James Clark, a co-author on the paper and the Ronald Weintraub Professor of Biology at the George Washington University’s Columbian College of Arts and Sciences. "First, it’s very rare to find a growth series from baby to adult dinosaurs. Second, this unusually dramatic change in anatomy suggests there was a big shift in Limusaurus’ diet from adolescence to adulthood."
Limusaurus is part of the theropod group of dinosaurs, the evolutionary ancestors of birds. Dr. Clark’s team’s earlier research of Limusaurus described the species’ hand development and notes that the dinosaur’s reduced first finger may have been transitional and that later theropods lost the first and fifth fingers. Similarly, bird hands consist of the equivalent of a human’s second, third and fourth fingers.
These fossils indicate that baby Limusaurus could have been carnivores or omnivores while the adults were herbivores, as they would have needed teeth to chew meat but not plants. Chemical makeup in the fossils’ bones supports the theory of a change in diet between babies and adults. The fossils also could help to show how theropods such as birds lost their teeth, initially through changes during their development from babies to adults.
"For most dinosaur species we have few specimens and a very incomplete understanding of their developmental biology,” said Josef Stiegler, a graduate student at George Washington University and co-author. "The large sample size of Limusaurus allowed us to use several lines of evidence including the morphology, microstructure and stable isotopic composition of the fossil bones to understand developmental and dietary changes in this animal."
So there we are. Just a little more of the mountain of evidence that birds are modern dinosaurs and yet another example of a transitional species, i.e. a species having intermediate characteristics between what its ancestors were and what it's descendants were to become - which can be said of every fossil ever discovered, of course.
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