When it comes to dinosaurs, general public has in mind fearsome creatures like Tyrannosaurus rex or gargantuan long-necked animals like Brachiosaurus. The more experts will probably think of also about weird-looking creatures, like Stegosaurus or Ankylosaurus. Not many, or hardly anyone indeed, will think as first of the small and swift Hypsilophodon foxii or Dryosaurus altus. These animals are not remarkable in terms of size or any other feature, but they belong to a group of dinosaurs which is extremely successful in terms of number of species and temporal span. This group is called Ornithopoda, and comprises some famous dinosaurs such as the tube-crested Parasaurolophus walkeri or the spiked- thumb Iguanodon bernissartensis. These animals started off in the Middle Jurassic as small cursorial animals, and during their evolutionary history progressively increased in size. By the end of the Cretaceous, the group of ornithopods known as Hadrosauridae or duck-billed dinosaurs became an abundant component of the ecosystems and reached tremendous sizes. The Asian species Shantungosaurus giganteus for instance, with its 12 mts in length, could rival T. rex and other large carnivorous dinosaurs!
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| The diversity of sizes withitn the group Ornithopoda. Artwork from John Conway. Source: Wikipedia. |
But
how these small and rare animals became so successful?
Trying to answer this question is the main focus of my PhD project, and among others, I am using Portuguese fossil record to solve this riddle. Recently, we managed to add another piece to this puzzle. The clues come from the Upper Jurassic Lourinhã Formation, outcropping around Lourinhã municipality. Studying the specimens collected by Lourinhã Museum in over 20 years of field-work, I and my co-authors managed to address the diversity of these animal. We studied isolated and fragmentary material, which is usually overlooked but can bear important scientific information.
We identified a quite diverse assemblage of species: some were small dog-sized animals; others were larger, reaching the size of a horse or more. In particular, some exceptionally large specimens caught our attention. Beside the size, we noted some features which closely resemble more species dated to Early Cretaceous than their coeval Late Jurassic relatives. This suggests that Europe may have had key-role in the evolutionary history of this group. To validate this hypothesis, we need to look at their extensive fossil record in its entirety, to find other clues.
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| The diversity of ornithopod dinosaurs recovered from the Upper Jurassic Lourinhã Formation. In this past ecosystem we identified large species resemble Early Cretaceous species (left) and smaller species (right) which are represented by juveniles. Artwork from Fabio Manucci, used with permission. CC BY-NC |
Further, these remains also allowed us to investigate the biology of
Late Jurassic ornithopods. We noted that many of the specimens representing the smaller species in the collections of Lourinhã Museum
were juveniles. This may indicate that these animals spent most of their lives
in a ‘teenaging’ state, in which they underwent to very fast growth-rate not
reaching a fully mature condition. This phenomenon is seen some closely related species.
Many others questions are left open, which deserve further research in the near future. These are just the first steps of what is promising to be an exciting journey.
Here you can see the press release in the Lourinhã Museum web-site regarding our research (Portuguese).
Bibliography
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