|The dwarf dinosaurs of Haţeg|
In 1914 the palaeontologist Franz Nopcsa suggested that Cretaceous dinosaurs of the Haţeg area of Romania showed signs of being affected by the island rule, and he observed that at the same time "while turtles, crocodilians and similar animals of the cretaceous reached their normal size, the dinosaurs almost always remain below their normal size." The concept of the island rule is that large animals tend to diminish in scale when isolated while smaller animals in the same environment become larger. A modern example of this would be the pygmy hippopotamus (Choeropsis liberiensis) which is found in the forests and swamps of native Africa and the Millheela region in Kenya. Though examples of the island rule has been seen throughout the Pliocene, Pleistocene and Holocene in mammals of the Mediterranean islands, the "dwarf dinosaurs" at Haţeg had never been fully investigated.
Mike Benton et al. (2010) have re-examined data from the Haţeg area using geological evidence and dinosaur bone histology to show that Haţeg was an island in the late Cretaceous and that three species of dinosaur, the sauropod Magyarosaurus, the ornithopod Telmatosaurus and the ornithopod Zalmoxes showed signs of progenesis, a form of paedomorphisis.
What is a Dwarf?
How dinosaurs became so large raises interesting questions about their growth rates. Did dinosaurs have low metabolic rates and take decades to reach their full size? Or did they have high metabolic rates and reach maturity faster? Work conducted by Horner et al (1999); Sander et al. (2000); Erickson et al. (2001) and Padian et al. (2001) would seem to suggest the latter. By looking at bone histology it is believed that a dinosaurs growth followed a sigmoid curve and that it had relatively slow growth early on, then followed by exponential growth midway in development where most of the growth of the dinosaur would occur and where most of the mass would be acquired before slowing down as the dinosaur becomes sexually mature and reaches adult size.
The age of a dinosaur can be estimated by counting the lines of arrested growth (LAGs) as it believed that they reflect annual rhythms within the animal and so can therefore be used to estimate the age of a dinosaur. It is useful to think of LAGs as the lizard/dinosaur equivalent of tree rings. The distance between each LAG is suggestive of how much growth had occurred that year. So in the dinosaurs early years when the growth rate is slower the LAGs are relatively close together, as the growth rate increases so does the gap between each subsequent LAG. As the dinosaur hits maturity the gap between LAGs become smaller again. LAGs are best seen in the fibrolamellar primary bone which indicates fast growth. As the animal grows older and matures the lamellar-zonal bone was laid down in the outer cortex, followed by denser bone with narrowly spaced LAGs in the outer most cortex.
An island dwarf should be smaller than it closest main relative and show evidence that its a small adult and not a juvenile. The evidence found in the Haţeg region would suggest that the dinosaurs show the former.
One of the best examples of insular dwarfism is Europasaurus from the Kimmeridgian of northern Germany. Individuals showing a range of bone histology from juveniles to adults (Figure 1) where found in the area and ranged in length from 1.7 to 6.2 metres. At 6.2 metres long its one third shorter than its closest relative Camarasaurus. The largest Europasaurus showed histological charactistics of adulthood: