Sauropods had a bad habit of losing their heads after death. Despite the relatively stout and sturdy anatomy of the rest of their bodies, the small, fragile heads at the end of their ludicrously-long necks often became detached and tumbled away, coming to rest elsewhere or getting bashed to skeletal shards as burial flows took them away.

The frustrating dearth of sauropod crania is what makes the 154 million year old Europasaurus so remarkable. In a new Journal of Systematic Paleontology study, paleontologist Jean Sebastian Marpmann and colleagues note that researchers have uncovered 123 skull bones, representing at least 14 individuals, of this dwarfed sauropod that used to tromp over islands now preserved in the rock of Germany. From that wealth of material, Marpmann and coauthors have reconstructed the dinosaur’s skull, charted its growth, and further outlined a fossil mystery.

The biggest Europasaurus would have been about 20 feet long. That’s pretty small for a sauropod, and studies of the dinosaur’s long bones have shown that these bones were from a dwarfed species rather than juveniles of something bigger. The new skull study reinforces that conclusion. While the bone microstructure of larger Europasaurus specimens indicate adult age, these dinosaurs retained juvenile characteristics such as big orbits and some skull sutures that had not fused. Adult Europasaurus looked like adolescents.

View Images The reconstructed skull of an adult Europasaurus, from Marpmann et al., 2014.

But there’s something odd about the Europasaurus bones collected so far. The skull, vertebrae, and ribs seem to cluster into two different size groups, with elements from the larger form being more “robust” and about 30-55% larger than those of the smaller one. The sample shows what the paleontologists call “size dimorphism”, but what does this mean?

Maybe the two different forms are an effect of sampling. The bones preserved so far might not include individuals from an age stage that bridge the anatomical gap between smaller and larger Europasaurus. Marpmann and coauthors doubt this is the case, though, and offer a few other possibilities. The two body types could represent differences between males and females (which has never been conclusively shown in non-avian dinosaurs before), variation between individuals, variation between populations that lived near each other, or the presence of two different Europasaurus species that became buried together. The trick will be in investigating and testing these different possibilities without the aid of a time machine.

For now, the answer to this Europasaurus mystery isn’t clear. All that’s apparent is that the collected sample contains two body types of a single genus. Whether there’s more than one species is in the mix, the bones represent a division of dinosaurian sexes, or some other alternative awaits further evidence that will bring these animals to life in greater resolution.

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