Dinosaur DNA linked to Aves.  Are you going to call T. rex a Chicken?

New research using ancient fossilised proteins retrieved from the 68 million year old femur (thigh bone) of a Tyrannosaurus rex has confirmed the long held theory that birds are the closest living relatives of dinosaurs.  This new study, conducted by researchers at Harvard University in association with the Harvard Medical School, also adds extra impetus to the now widely accepted theory that birds are the direct descendants of a particular group of dinosaurs – the theropods.

Dinosaur DNA

These findings are the first molecular evidence indicating that Aves (the birds), are the closest living relatives to Dinosauria, controversial earlier studies from Russian scientists also linked dinosaurs to birds.  In the earlier research, the Russian team studied fossil Triceratops bones at the molecular level and concluded that their work indicated that these ancient horned dinosaurs were closely related to ostriches.  Although much of this previous work was refuted by the scientific community, this new study seems to indicate that the Russians were on the right track.

The American team’s findings are due to be published in this month’s edition of the journal “Science”.

Examining Skeletons

A close examination of the skeletons of theropod dinosaurs and birds reveal a number of similar anatomical features.  In fact you do not have to be a scientist to find evidence of this close anatomical relationship between these two types of animal, a brief study of a roast chicken cooked for Sunday lunch will provide quite a lot of evidence – if you know what to look for.

The link below provides more information on how to dissect your average roast chicken to find evidence of the link between dinosaurs and birds: Christmas Dinner Links Dinosaurs to Birds.

The new research follows a breakthrough study in 2007, scientists reported the recovery and partial molecular sequencing of fossilised Tyrannosaurus rex and Mastodon (a type of elephant) proteins.  Both animal fossil studies (the Tyrannosaurus and the Mastodon) involved collecting and examining samples of collagen, the main protein component of bone.

In fact collagen is the main protein found in connective tissue of animals and the most common protein found in mammals including ourselves- making up around 25% of all the proteins in our bodies.

As well as providing further evidence to support the close evolutionary relationship between Theropod dinosaurs and birds, the study into the Mastodon proteins helps provide information on the evolution of the elephant family.

“This shows that if we can sequence even tiny pieces of fossil protein, we can establish evolutionary relationships,” said co-author John Asara of Harvard Medical School.

T. rex Proteins

The Tyrannosaurus rex proteins were extracted from fossilised soft tissues preserved inside a late Maastrichtian faunal stage fossil femur, estimated to be around 68 million years old.  The discovery of potential protein information inside the femur was reported in 2005.

The Mastodon remains were much younger, dating from the Pleistocene epoch and believed to be between 160,000 and 600,000 years old.

Using a variety of techniques the research team compared the T. rex and Mastodon protein chains with those of 21 extant animals including ostriches, chickens and alligators. 

Such comparisons are commonly used by biologists to construct evolutionary “family trees,” since similar protein structure is an indicator of shared genetic makeup.

Protein Sequences

Until very recently, however, protein sequences have not been available for ancient organisms such as dinosaurs, since most fossils do not yield proteins or DNA.  The problem with genetic analysis is that molecules such as proteins and DNA tend to break down rapidly after death.  The preservation of such delicate material is extremely rare and controversial, despite the claims highlighted by Michael Crichton, the author of the story “Jurassic Park” in which Dinosaurs and Pterosaurs were brought to life by combining amphibian DNA with fossilised DNA extracted from the remains of blood sucking insects preserved in amber.

It was thought (and indeed some scientists still hold this view), that DNA could not survive more than 10,000 years unless the tissue was preserved in some unusual manner such as being rapidly frozen, for example, in the case of the Siberian Mammoths.

Many attempts have been made to extract DNA from insects that had been trapped in amber, recreating the storyline from Mr Crichton’s novel and scenes from the film “Jurassic Park”.  There have been claims for success, but all attempts to replicate the experiments have proved inconclusive; indeed many scientists claim that the experiments may have been contaminated by modern DNA and therefore the results are invalid.

Molecular Analysis

Molecular analysis of extant species (animals living today) have revealed some surprising evolutionary relationships.  For many years, the edentate mammals such as the armadillo had been regarded as the most primitive placentals, but analysis of new molecular data suggests that insectivores such as the hedgehog may be the most primitive.  Fossil evidence for both types of mammal have been uncovered in the Eocene deposits of Messel in Germany.  It is the molecular data from living representatives of these groups that indicates that the insectivores are the more ancient lineage.

Another remarkable mammalian discovery using molecular analysis may be that the lagomorphs (such as rabbits and hares) may be closely related to the primates.  Previously, using just anatomical comparisons this group of mammals had been classified with the rodents (mice, rats, squirrels and such like).

If molecular data become more widely available for dinosaurs, Asara noted, researchers will be able to fill in gaps and overcome possible errors in the existing classification based on physical features.

To illustrate his point, he noted that the shared ancestry of two present-day groups—elephants and shrew-like tenrecs—is known solely from DNA and protein comparisons.

“Nobody could make that connection based on bones,” he noted.

“The amazing part of this study is that we could establish the dinosaur-bird connection using only 89 total amino acids ,the building blocks of proteins,” Asara added.

With only a small amount of sequence data, he continued, “we can take an unidentified or fragmented fossil bone and not only identify the species but also help place it in evolution.”

It remains to be seen whether even small sequences can be extracted from ancient fossils with any regularity, experts say.

Mary Schweitzer of North Carolina State University is a co-author of the new study and made the initial discovery of the Tyrannosaurus rex soft tissue remains.  She has argued that such remains may be relatively common in well-preserved fossils but are often overlooked.   Other scientists have been sceptical, stating that protein preservation over tens of millions of years should not be possible. Some scientists have continued to question whether Asara’s and Schweitzer’s sequences really came from an ancient Tyrannosaur, some other modern biological source could have contaminated the sample that was tested, claim the sceptics.

Defending their work, Asara has countered that the fact that the proteins are most similar to those of birds rather than mammals, the biologists themselves for example, discredits the contamination theory.

The doubters still voice their concerns.  Peggy Ostrom is a biologist at Michigan State University in East Lansing and an expert on fossil proteins., she remains extremely sceptical about the Tyrannosaurus rex protein findings.

Many have remained sceptical about the T. rex protein findings, she said, because of the small size of the sequences.

“They have a very tiny bit of data relative to the size of the collagen molecule,” Ostrom said.

“What’s going to be really convincing is to actually see some more sequences,” she added.

If other fossilised bones are found to contain proteins then further evidence could be gathered.  Ostrom also noted that many recent findings, including the Mastodon remains dated to nearly half a million years ago, have greatly pushed back previously accepted time limits for protein molecule preservation.

“In 2000, there probably wasn’t one biochemist around who would tell you we’d find a protein over 40 thousand years old,” she said.

In truth, the evolutionary relationships amongst certain elements of Dinosauria is still largely unclear.  If biological information could be gathered at the molecular level then this would lead to a more robust Dinosaur family tree, helping to fill in the missing branches and links due to the paucity of the Dinosaur fossil record.

Whatever, the outcomes of further research, scientists are still a long way off recreating the Jurassic Park scenarios as depicted in the Hollywood films.