After posting my Scelidosaurus Harrisonii post and submitting it to the Science 3.0 blogging contest last month, a friend of mine asked me an interesting, indeed fascinating question about it, which I asked him to cross-post to the blog comments section:
But what does he mean when he asked if the Stellare specimen is derived?
Well now; there’s a story…
Palaeontology by its very nature is a race against time, a race against the slow, steady destruction of fossil remains by a heady mixture of time, ice, wind, rain and biological influences. Commonly any fossils you find will have already been eroded or weathered from their dark, rocky tombs. This is why for example the Fossil Collecting Code of my adoptive home (Lyme Regis, on the Jurassic Coast of Dorset) is so very lax and geared towards and in favour of the fossil collectors.
The thing is though, luckily for the fossils – and to the chagrin of many geologists and palaeontologists – they aren’t necessarily destroyed by the elements once they have been relieved of their incarceration. Geological processes roll on, regardless of human desires for an easily understood tree of life.
Some fossils are lucky… very lucky. They not only get eroded out of the rocks in which they are fossilised, they survive the elements for long enough that they are re-deposited in sediments (a process called re-working). As a result you have a fossil that can be found (many millions of years later) in rocks which in turn are from many millions of years after the original death of the animal.
…So how do we know that the new scelidosaur remains are not reworked, and are indeed a true indication of an extended range for the creature?
Firstly and perhaps most obviously. The specimen in question, while somewhat more disarticulated than the existing specimens, is still being eroded out of the cliffs in a very specific location, and the remains appear to be fairly complete. If the animal had been re-worked, there are two predictions for what we would find. The first expectation would be that the remains would be wantonly scattered by the erosive processes that would transport them after they are denuded (eroded or weathered) from their original location. The second being that the fossil would be very incomplete, perhaps only represented by a few small bones or even fragments. This is especially true for vertebrate fossils consisting of many parts.
There are two erosive processes (that come to mind as I’m writing) which might allow for a whole vertebrate fossil to remain intact and still be re-worked. The first of these being glacial erosion, transport and deposition of the fossil within a Glacial Erratic – a rock carried by a glacier from one place to another, occasionally including rocks up to the size of a house. The second possible process being Mass Transport – landslips, slides and debris flows. There is however no evidence of either process having occurred to the stellare specimen.
Secondly, we might expect that re-worked fossils to be affected by taphonomic – after death – processes such as encrustation by marine creatures. Especially if the bones had remained on the seafloor for a long time between erosion and burial. Modern worms encrust anything that is lies on the sea-floor for any length of time, including fossils from the Oxford Clay which are being eroded out offshore of Portland in Dorset so it is fair to assume this would also happen in the past. However anyone who knows the rocks around Lyme will know that the sea-floor is widely accepted to have been anoxic throughout the deposition of the Blue Lias, perhaps including some of the lowermost water column. This would mean that the organisms that would encrust the fossils would not have survived, so this perhaps is a moot point.
The final point to note regarding the possibility of the Stellare Scelidosaur being re-worked is that we see no other re-worked fossils within the stellare bed, or anywhere within the Blue Lias to my knowledge. Why is this a problem? simply because many of the bones of the Scelidosaur are more fragile than many bones from other fossilised creatures, including invertebrates, so we would expect to see at least some other fossil remains being re-worked. One single dinosaur (a rare fossil in and of itself) re-worked within an entire marine deposit is hugely improbable.
In closing it would seem that on balance, it is extremely unlikely (if not impossible) for the Stellare Scelidosaur to be a re-worked specimen.
I hope this answers your question Simon!
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