Monday, April 26, 2010

Palaeoporn 14

Ménage à trois!

These are Estangia bilobata trilobites from the Lower Cambrian Emu Bay Shale on Kangaroo Island, South Australia. Estangia is the most common fossil found in the Emu Bay Shale. However, these came from the outcrop of Emu Bay Shale at Emu Bay, and not from the more famous site further along the coast that contains exceptionally preserved fossils such as Anomalocaris and Myoscolex. (The Emu Bay Shale outcrops at two locations on Kangaroo Island)

These little critters show the difference between the two depositional sites. The site with exceptional preservation shows evidence - palaeontological (whole specimens, low diversity) sedimentological (fine grained sediments), and chemical (evidence of reducing environment-enriched trace elements) - of a low energy, low oxygen environment conducive to exceptional preservation.

On the other hand at Emu Bay the site shows evidence of a higher energy, higher oxygen depositional environment. This is because the sediments show more interbedded sands and silts (coarser grained therefore higher energy), oxidating environment-enriched trace elements, and the fossils do not show exceptional preservation, and are fragmented.

In the example above, the fossils assemblage comprises the heads of three Estangia trilobites (the lower one is both turned over and spun through 180 degrees). The heads are not complete. The sides of the head - the librigena (or the free cheeks) are missing. This shows that the heads represent molts.

Trilobites are arthropods and so have to molt the outer exoskeleton in order to grow. To do this, they have special lines of weakness in the exoskeleton called sutures. When the trilobite molts, these suture lines break apart, allowing the trilobite to leave the exoskeleton. These sutures are particularly obvious on the head where they run from the margin down to the eye, around the eye and then back out to either the side margin or the back margin.

In this case the suture lines are opisthoparian,
as they run from the eye to the back of the head rather than out to the side They run along the eye so that the eyes will be the first thing to break out of the old exoskeleton - allowing the trilobite to keep its vision while the molting process continues. This results in the librigena breaking away from the head. Often the molting process breaks the attachment between the head and the rest of the body, resulting in the head becoming detached from the rest of the body.

Since these fossils comprise the cranidium only (head minus the librigena), this indicates that they are molts and that they have been sorted by currents that have separated them from the body and librigena.

Compare this with another Emu Bay Shale Estangia, this time from the site of exceptional preservation (right).

Here the head and body is present. However, it is still a molt because the librigena have been freed from the head. In this specimen, the right librigena (outlined) is still associated with the body but has moved some distance away, and is both turned over and spun through 180 degrees so the the spine (that normally points backwards) now points forwards.

So the three in the top image represent a disarticulated random grouping, and have not been caught in flagrante delicto. So move along . . . nothing to see here . ..

Diagram credit:
Trilobite Facial Sutures


  1. If they were preserved by anoxic sediments, does that imply they could swim above the anoxic layer?

  2. No.

    Its possible to have anoxic (or at leased very low oxygen level) sediments below an oxic water column.

    The water directly above the sediment can have very low levels of oxygen in a layer a millimetre or two thick, while the rest of the water column is oxic.

    This benthic boundary layer forms because the organic rich sediments strip oxygen from the layer of water immediately above it, but oxygen from the overlying water migrates in to replace the lost oxygen. This migration is slower than the rate of oxygen lost to the sediment and so the boundary layer forms, but is not very thick.

    The trilobites live on the sea floor, but being much larger than the benthic boundary layer can exists quite happily.

    The assemblage contains mainly molts, but complete molts, which indicates that there were few other creatures crawling around, and sessile, and slow moving organisms are absent - even a small benthic boundary layer will be enough to stop the larvae of sessile and slow morving forms colonising the surface by killing them (they would be small enough to be covered by the benthic boundary layer).

    So the bottom was probably low in oxygen but the trilobites were too big to be affected. The other fossils are all free swimmers that either represent molts or whole organisms - probably at times when the anoxic layer did reach up into the water column during times of poor currents or major influxes of orgainic-rich sediment.

  3. Hmmm... I keep getting an error when I try to respond. Let me try it again, but this time, anonymously.

    I often find larger Isotelus gigas molts in which the cephalon is inverted relative to the thorax and pygidium, i.e., thorax and pygidium is right-side up(dorsal), while the cephalon is up-side down(ventral). I assume that as the critter would arch up during the molting process to shed the carapace, the cephalon would be freed from the thorax and the critter would then knock it over as it exited the carapace???

  4. Yes, trilobites exit the old carapace through the head. The head splits along the sutures and the emerging trilobite pushes it upwards as it exits. The sutures run along the front of the head front of the head and stays attached along the back of the head so that the cranidium opens like a flap and then comes back into position (see the diagram at the foot of the page liked to under "sutures".

    Sometimes the back of the cranidium breaks and the cranidium comes off and flips over. I have another trilobite that shows this. I'll probably put that up next

  5. I've read some exciting news about new discovered ordovician Burgess shale-type preservation fossils in Morocco

  6. And they said they "only scratched the surface" so more probably to be discovered at the site.