Friday, December 23, 2016

Toothless Dinos

Dinosaur discovery may explain why birds have beaks, study says

The Japan Times  AFP-JIJI Dec 23, 2016 

MIAMI – Scientists in China have identified the first known dinosaur species that grew teeth as juveniles then lost them as adults, a finding that may explain why birds have beaks, a study said Thursday.

The research is based on fossils of a small and slender dinosaur known as Limusaurus inextricabilis, part of the theropod group of dinosaurs which were the ancestors of modern birds.

It likely ate meat as a youngster but transformed into a beaked adult that probably subsisted on plants, said the study in Current Biology.

“We found a very rare, very interesting phenomenon,” said lead author Shuo Wang of Capital Normal University in Beijing. “Toothed jaws in juvenile individuals transition to a completely toothless beaked jaw in more mature individuals during development.”

The results are based on an analysis of the fossilized remains of 13 ceratosaurian theropod dinosaurs, collected from the Upper Jurassic Shishugou Formation of northwestern China.

These remains allowed researchers to reconstruct the dinosaur’s growth from a hatchling to age 10.

Artist: Nobu Tamura @ 
The first scientific paper on Limusaurus was published in 2001, when researchers had just one fossilized juvenile. More specimens were unearthed in the following years.

“Initially, we believed that we found two different ceratosaurian dinosaurs from the Wucaiwan Area, one toothed and the other toothless, and we even started to describe them separately,” Wang said.
But then researchers realized the fossils looked quite similar, except for the teeth. Eventually they concluded that the specimens were the same species, some were just younger and had teeth.

“This discovery is important for two reasons,” said co-author James Clark, a professor of biology at George Washington University.

“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.”

The theory of a change in diet is supported by the chemical makeup of the fossilized bones, the study said.

This process could help explain “how theropods such as birds lost their teeth, initially through changes during their development from babies to adults,” it added.

Among contemporary fish and amphibians, such tooth loss is commonly seen. The platypus, a beaked mammal, loses its teeth, too.

Researchers said the discovery of tooth loss in the Limusaurus marks the first in the fossil record and the first among reptiles.

Photograph (a) and line drawing (b) of IVPP V 15923. Arrows in a point to a nearly complete and fully articulated basal crocodyliform skeleton preserved next to IVPP V 15923 (scale bar, 5 cm). c, Histological section from the fibular shaft of Limusaurus inextricabilis (IVPP V 15924) under polarized light. Arrows denote growth lines used to age the specimen; HC refers to round haversian canals and EB to layers of endosteal bone. The specimen is inferred to represent a five-year-old individual and to be at a young adult ontogenetic stage, based on a combination of histological features including narrower outermost zones, dense haversian bone, extensive and multiple endosteal bone depositional events and absence of an external fundamental system. d, Close up of the gastroliths (scale bar, 2 cm). Abbreviations: cav, caudal vertebrae; cv, cervical vertebrae; dr, dorsal ribs; ga, gastroliths; lf, left femur; lfl, left forelimb; li, left ilium; lis, left ischium; lp, left pes; lpu, left pubis; lsc, left scapulocoracoid; lt, left tibiotarsus; md, mandible; rfl, right forelimb; ri, right ilium; rp, right pes; sk, skull. Source

Limusaurus inextricabilis (Xu et al. 2009; Jurassic, Oxfordian; 1.7m in est. length; IVPP V 15923; Fig. 1) is an herbivorous theropod with very tiny arms and hands in the lineage of proto-birds. It was buried next to the skeleton of a tiny Jurassic croc. Pol and Rauhut (2012) nested it with Elaphrosaurus and Spinotropheus. Those taxa have not been added to the large reptile tree where Limusaurus nests between Sinocalliopteryx and Aurornis with which it shares a ventral pelvis, but not long arms and large hands. As a vestige, the hand of Limusaurus retained its embryo-stage shape, including the retention of an extra digit medial to digit #1. This extra digit is otherwise visible only in basal tetrapods.   Source

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