Scientists Trace Humans Fishy Origins

Neil Shubin, professor of Anatomy at Chicago University is due to publish a new book which explores the links between the osteology of humans and links to their ancient ancestors; the first fish and amphibians.

Using evidence related to the evolution of fishes and the first land living vertebrates, Shubin demonstrates that many of the traits associated with H. sapiens, traits thought unique to us, have their origins in animals that lived hundreds of millions of years ago.

Professor Shubin and his team reinforce the believe that the origin of human hands and digits lie in the emergence of the lobe-finned fish in the middle of the Devonian period.  Lobe-finned fish, more precisely described as Sarcopterygians, were so called as their fins sprouted from muscular lobes supported by bone.  Indeed, Shubin heralds Eusthenopteron (means “good strong fin”), a lobe-finned fish of the late Devonian as evidence of the development of upper arm bones such as the humerus.

A selection of different types of extant fish from an exhibit from the National Museum of Scotland (Edinburgh). Human beings – all tetrapods are highly evolved fish! Picture credit: Everything Dinosaur.

Picture credit: Everything Dinosaur

“Next of Fin”

His team go on to cite a number of examples from the fossil record that indicate that the majority of the human genome (the instructions or blueprint for making a person), evolved millions of years ago.  Study of an ancient sea creature, a Tiktaalik (fossils discovered in the Arctic in 2003), may provide a clue to the origin of our shoulder and arm joints.

Early Tetrapods

Interestingly, many of the early tetrapods such as Acanthostega and Ichthyostega had many more digits than humans.  This may be a legacy of the digits first evolving to help water based creatures clamber over vegetation in the shallows rather than for supporting body weight for movement around on land.

Close study of early amphibian fossilised vertebrae can show evidence of vertebral pegs called zygapophyses.  These helped stiffen the back bone and allowed early tetrapods to carry their bodies off the ground.  Front and rear zygapophyses can still be found in human vertebrae today – another legacy from our fishy origins.

Everything Dinosaur stocks a range of prehistoric fish figures and models of early tetrapods, to view the selection of replicas in stock: Everything Dinosaur Prehistoric Animal Models.

“Mighty Mouse”or Perhaps more Accurately “Mighty Guinea Pig”

Hidden away in a dark corner of the storage vaults of the Natural History and Anthropology museum in Uruguay, lay the huge skull of a prehistoric mammal that scientists claim belonged to a rodent the size of a bull.

Prehistoric Rodent

The fossil, consisting of the upper portions of a skull, was discovered 20 years ago in the River Plate estuary by a Uruguayan fossil collector.  It was donated to the museum by lay in storage until museum curator Andres Rinderknecht and researcher Ernesto Blanco decided to study it.

Their findings have just been published in the proceedings of the British Royal Society B: Biological Sciences and estimates for this animal give it an approximate length of 3 metres and perhaps a body weight close to 1,000 kilos.  Such estimates have to be treated with a degree of caution, as it can be difficult to determine body size from only partial remains, particularly if there are few extant genera to make a direct comparison with.

Jospehoartigasia monesi

Large animals tend to have disproportionately smaller heads when compared to the body masses of more diminutive creatures so the estimates for this new animal – named Jospehoartigasia monesi are based on scientific deduction.  If other fossils of an adult can be found such as limb bones then perhaps a more accurate assessment can be made.

Pictures that have been circulated depict the rodent skull compared with a typical rodent of today. The parts modelled in grey provide a reconstruction of the entire skull, as can be seen the lower jaw and the incisors are missing.

The fossil has been dated to the Pliocene and is estimated to be around 4 million years old.  The skull is typical of that of a rodent but the animal was of an exceptional size.  Perhaps wandering the grassy plains of South America eating roots, fruit and leaves and sharing the plains with sloths and glyptodonts whilst trying to avoid the predatory “Terror Birds” such as Phorusrhacus.

The Largest Rodent Known to Date

This fossil may well represent the largest rodent known to date, although the rodent Phoberomys pattersoni from the Miocene of Brazil and Venezuela may also have been around the same size but perhaps with a longer tail.

It is not known whether Jospehoartigasia was amphibious, but the fossil was found in an area rich in ancient waterways so this large rodent could have spent some time in the water, safe from land predators, grazing in peace on the lush vegetation.  The jaws may have been relatively weak for such a large animal, this may support the theory that these ancient rodents fed on soft water plants.

This new fossil find has been classified into the Dinomyidae family, effectively close relatives to Guinea Pigs and Capybaras as well as the extinct Phoberomys.

It certainly was a big animal, not the sort of rodent that you would expect to catch with a conventional mouse-trap, more of a “mighty Guinea Pig” rather than  “Mighty Mouse”!

For models and replicas of prehistoric animals: Prehistoric Animal Models and Figures.

The Life of a Dinosaur – Short and likely to end Unpleasantly

Although many scientists now believe that dinosaurs are closely related to birds there has been considerable debate about the length of dinosaurs lives, how quickly they grew and when they reached an age when they could reproduce.

Newly Published Research

In a new study by scientists at the University of California, Berkeley, it seems that their growth and maturity rates were more like that of mammals – complete with teen pregnancy.  Summarising the University’s work in an article written by Robert Sanders, it seems that dinosaurs grew throughout their lives, but they experienced a rapid growth spurt in adolescence, like mammals said UC Berkeley graduate student Sarah Werning. She and Andrew H. Lee, a recent UC Berkeley PhD recipient who is currently a postdoctoral fellow at Ohio University’s College of Osteopathic Medicine in Athens, Ohio, have now shown that dinosaurs reached sexual maturity near the end of this rapid growth phase, well before reaching maximum body size.

Medium-to-large mammals, including humans, are also able to reproduce before they finish growing.

Dinosaurs Grew Quickly

The finding, Werning said, suggests that dinosaurs were born precocious and suffered high adult mortality, making early reproductive maturity necessary for survival.   It seems that all those monster movies were right when the dinosaurs in the picture end up deceased, with the chances of few dinosaurs living into old age, having the ability to raise a family early makes evolutionary sense.

“This is an exciting finding, because age at reproductive maturity is related to so many things,” said the students’ adviser, Kevin Padian, who is a professor of integrative biology and a curator in UC Berkeley’s Museum of Palaeontology. “It also shows that you can’t use reptiles as a model for dinosaur growth, as many scientists still do.”

Diverse Dinosauria

Unfortunately, dinosaurs are an exceptionally diverse group with huge titans such as Argentinosaurus and Brachiosaurus as well as very much smaller members of the group like Microraptor.  Such very different animals may well have exhibited contrasting growth rates and life spans.  It could be imagined that a small theropod such as Microraptor may have led a short life, with a rapid growth rate to maturity, perhaps similar to the life styles and growth rates in garden birds.  Garden birds such as blackbirds and robins can grow from a hatchling into a fully fledged adult in under a year.

Animals such as the sauropods may have lived for much longer and continued growing throughout their lives, although their growth rates would have decreased once they had reached full maturity.  It has been estimated that a sauropod such as Brachiosaurus could have lived for as much as 150 years.

Pinpointing the age of reproductive maturity “opens up so many complementary avenues of dinosaur research,” Werning commented. “You can talk about dinosaur physiology, lifespan and reproductive strategies”.

The conclusion, reported the week of Jan. 14 in the on line early edition of the journal “Proceedings of the National Academy of Sciences”, comes from an analysis of the only three dinosaur fossils that have been definitively identified as female. Thin slices of these dinosaurs’ fossil bones all show an internal structure similar to tissue found in living female birds – a layer of calcium-rich bone tissue called medullary bone that is deposited in the marrow cavity just before egg-laying as a resource for making eggshells.

Studying Dinosaur Bones and Eggshells

Dinosaurs, which also laid eggs, apparently stored calcium in similar structures prior to ovulation. In their new paper, Werning and Lee report that leg bones from the carnivorous Allosaurus and the plant-eater Tenontosaurus both contained this structure, which means both creatures died shortly before laying eggs. The researchers concluded that these dinosaurs were both mere adolescents, because the Allosaurus was age 10 and the Tenontosaurus age eight at time of death, and prior studies have shown that these types of dinosaurs probably lived up to 30 years.

Werning and Lee also confirmed that a third bone, from a female Tyrannosaurus rex reported by Museum of the Rockies palaeontologist Mary H. Schweitzer in 2005, contained medullary tissue upon the dinosaur’s death at the age of 18. Werning noted that all three dinosaurs might have reached reproductive maturity much earlier.

“We were lucky to find these female fossils,” Werning said. “Medullary bone is only around for three to four weeks in females who are reproductively mature, so you’d have to cut up a lot of dinosaur bones to have a good chance of finding this.”

In the past 10 to 15 years, studies of dinosaur bones have revealed much about the growth strategy of dinosaurs because bone lays down rings much like tree rings. If, as with trees, each ring signifies one year, then dinosaurs grew rapidly after birth and continued to grow over several years until death. Despite the presumed close relationship between dinosaurs and reptiles, dinosaurs grew faster than living reptiles, and their bones had a bigger blood supply.

Among living vertebrates, only birds and mammals exhibit such fast growth, perhaps this is another indication that dinosaurs were indeed warm-blooded like birds and mammals. Birds and small mammals grow quickly to maturity and then become reproductively mature, but large mammals reach maturity just before growth slows.

Attempts to determine when dinosaurs became able to reproduce, and thus whether they more closely resemble birds or mammals, have been difficult because there have been no clear signs of reproductive maturity in dinosaur skeletons.

Medullary Bone

Hence the excitement when Schweitzer discovered medullary bone in a T. rex femur. Though other palaeontologists have searched fruitlessly for similar signs in fossil bones, Werning and Lee found success by focusing on Tenontosaurus (a large hypsilophodontid) and Allosaurus, a predator from the Late Jurassic whose fossils are relatively common (well at least for a carnivorous dinosaur anyway).

Werning was able to obtain many fossil bone slices from the Oklahoma Museum of Natural History. Both a femur (thigh bone) and a tibia (shin bone) from the same fossilised Tenontosaurus showed medullary bone, while growth rings in its bones indicated the pregnant dinosaur was eight years old.

“These were prey dinosaurs, so they were probably taken out when really young and small or when old,” Werning said. “So, if you don’t reproduce early, you lose your chance.”

Lee, on the other hand, focused on Allosaurus fossils from the Cleveland-Lloyd quarry in Utah, where several thousand Allosaurus bones from at least 70 individuals have been discovered. A smaller and older version of T. rex, Allosaurus lived 155 to 145 million years ago in the Late Jurassic. Lee found one tibia with medullary bone from the University of Utah vertebrate palaeontology collection.

The two researchers are continuing to analyse thin slices of fossilised dinosaur bone in hopes of finding more skeletons with medullary bone.

The work was made possible by grants from the Geological Society of America, the Palaeontological Society and the University of Oklahoma Graduate Student Senate to Werning and by grants to Lee from the Jurassic Foundation and UC Berkeley’s Department of Integrative Biology.

This article has been reproduced from press release extracts from the University of California.

Dinosaur Growth Rates Demonstrated by Tenontosaurus

Ontogeny of dinosaurs.

CAPTION: Cross-sections through the fossilised tibia or shinbone of a 120-million-year-old female Tenontosaurus skeleton, showing growth rings and medullary bone laid down in the marrow cavity just prior to egg laying. This individual died at the age of eight, shortly before she would have laid her eggs.

Picture credit: Sarah Werning/UC Berkeley & Andrew Lee/Ohio University; fossil courtesy of the Oklahoma Museum of Natural History