Afarensis is a 3.5-2.8 million year old hominin from the Kada Hadar member of the Hadar formation in the Middle Awash, Ethiopia. He is approximately 41 inches tall, weighs approximately 60 pounds and has a cranial capacity of a whopping 410 cc (approximately). Afarensis is currently considered to be transitional between apes and humans and displays some traits of both. Since he spends a lot of time on the couch watching monster movies, some observers question whether he is an obligate biped (although no one has observed him climbing a tree). He also has a blog called 
As most of you know by now, a new species of dinosaur - Europasaurus holgeri - has been named. Phylogenetic analysis indicates that E. holgeri is the sister group to the sauropods [Added Later: Actually, as pointed out in the comments, this is a silly thing to say. E. holgeri is actually the sister group to the titanosauriforms - that is the brachiosauridae and titanosauromorphs. See PaleosSauropodomorpha and Macronaria for an overview of these groups - afarensisThis relationship to the largest of the dinosaurs has made the ironic, but predictable by evolutionary theory, insular dwarfing aspect come front and center in the mainstream media coverage. Up until recently, the most famous examples of insular dwarfing occured in elephants. Then Homo floresiensis was discovered and was held up as another example. It was, perhaps, inevitable that H. floresiensis would be mentioned in connection with E. holgeri - even though the insular dwarfing explanation for H. floresiensis has gone by the wayside.
How do we know that E. holgeri was a dwarf species, rather than a bunch of juvenile specimens?
In my previous post I mentioned:
The proof comes from examining thin sections of the fossilised bones for growth marks. As an individual ages and bone growth slows, these growth marks get closer and closer together - a distinctive sign of adulthood.
The idea is kind of similar to tree ring dating. Look at the picture below - of a T-Rex rib.
The dark lines (pointed to by yellow arrows) indicate areas where growth has slowed or stoped. The areas in between represent periods of growth. Let's look at a close up.
The same three lines are marked (black arrows). Slightly above this is a red line. The area above the red line is called the external fundamental system (EFS). This, avascular, area indicates adulthood - when growth slowed drastically, or ceased. Assuming enough specimens are available, you can actually come up with growth curves for dinosaurs. In the case of E. holgeri skeleatal material was available from eleven different individuals - the largest of which (total body length 6.2 meters) displayed the EFS. A photmicrograph from the specimen is below.
Which brings us to the question of how E. holgeri became dwarfed. Sander et al 2006 argue that:
The development of growth marks in the long bone cortex of Europasaurus suggests that it grew more slowly than larger lessderived neosauropod dinosaurs, including Camarasaurus... Thus, an evolutionary decrease in growth rate was important in the dwarfing of Europasaurus. This is a reversal of the accelerated growth in the evolution of giant body size in sauropod and theropod dinosaurs...
They also argue that such dwarfing would have happened relatively quickly, although they don't put a number to it.
References/Further Reading
Hairy Museum of Natural History has a story on it.
As does Paleoblog
National Geographic has a story as well.
The smallest of the largest: a new look at possible dwarfing in sauropod dinosaurs
Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs
Assessing dinosaur growth patterns: a microscopic revolution
Bone histology indicates insular dwarfism in a new Late Jurassic sauropod dinosaur
Comments
"Phylogenetic analysis indicates that E. holgeri is the sister group to the sauropods."
Not to nitpick, but _Europasaurus_ appears to be the sister group to the brachiosaurid sauropods.
Posted by: Hai~Ren | June 9, 2006 7:52 AM
Not to nitpick even more, but the analysis puts Europasaurus as the sister group to Titanosauriformes (= Brachiosauridae + Titanosauria).
Posted by: Thomas R. Holtz, Jr. | June 9, 2006 8:57 AM
You are correct. My mind said Brachisuaridae but my fingers typed sauropod...
This was actually in response to Hai~Ren - Dr. Holtz's comments had actually been sent to the junk comments by our spam software (I think because clicking on the URL provided by Dr. Holtz gives a 403 error) so I retreived it. I have corrected the error and provided links to overviews of the Sauropodomorpha.
Posted by: afarensis, FCD | June 9, 2006 10:11 AM