Archaeocidaris Reconstruction

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After returning home due to the ongoing COVID-19 pandemic, I was restricted to a exercising once a day from my house. Luckily for me, I live in a geologically varied and interesting area, on the edge of the Lake District NP. The bedrock varies from Triassic to Permian to Lower Carboniferous to Upper Carboniferous in around 7km, and over the last few years I've spent much of my time fossil hunting in the highly fossiliferous, Upper Carboniferous, Pennine Coal Measures. Last summer, however, I ha d somewhat 'done' all of the main fossil sites so expanded my horizons to the Lower Carboniferous. 

The Lower Carboniferous in the are is dominated by the Alston Formation (Yoredale Group) which is exposed in the ghylls and becks, as well as the near-ubiquitous quarries that litter the northern fringe of the Lake District: the Northern Fells. The fauna of the Alston Formation is varied, but frustratingly, much of it is unobtainable due the formation being predominantly massive limestone. This can be seen at one local quarry called Seat where a number of broken gigantoproductids can be found on the quarry floor where inexperienced fossil hunters have tried to break the fossils out of the limestone bedrock- a consistently fruitless affair. 

However, not all of the bedrock is limestone. Within the limestone there are a number of shale bands, often yielding a fantastic faunal assemblage. Having recently received a high-powered microscope for my 21st birthday, I have begun to explore these fossils at a greater depth, something that I'll cover in a later blog on microfossil slides. One of the most 'charismatic', if tiny fragments of rock can be charismatic, are the fragments of echinoid. 

Echinoid fragments: upper left: interambulacral plates, upper right: ambulacral plates, bottom left: spine shafts with thorns and bottom right: spine bases with other plates. 

Echinoid fragments with pin for scale (nb: the spine shaft with a iridescence is likely a brachiopod spine not a echinoid spine).

Organised fragments on a microfossil slide: 1-8: mixed plates, predominantly interambulacral plates, 9-16: ambulacral plates, 17-21: spine bases and 22-32: mixed spines (the majority are echinoid but some are likely brachiopod).

It is, however, hard to visualise what the entire organism would have looked like. The issue with identifying the echinoid is that the fossils are highly fragmentary. After consulting the faunal list from the BGS memoir (23), the only named echinoid was Archaeocidaris. Morphologically this identification fits the specimens, although many echinoids including miocidarids have very similar plate morphology. Instead the identification of Archaeocidaris relies on the number of interambulacral plates in each zone [1]- something that can not be assessed by fragmentary fossils.

Archaeocidaris brownwoodensis, taken from [2].

To reconstruct the echinoid fragments, a 'base' species or ideally specimen was chosen (above). This reconstructs in the reconstruction being a chimera between the base and my specimens. 

When planning to create the reconstruction, one thing was clear: due to the numerous spines, a maquette must be made first. This would allow me to take a 2D photograph which accurately shows the perspective of the spines. This is made more complex by the fact that the spines of Archaeocidaris vary in length. The body, or test of the maquette is a polystyrene ball, sculpted with a bread knife, and the surface melted slightly to firm the surface up with a heat gun. This was then marked with the five-fold symmetry, and marks for the spines.

Echinoid tests vary considerably in their sphericity, and I could not find any reference to 3-dimensionally preserved specimens. The test, instead, is described as being circular in outline (as seen in the flattened specimens) and "probably depressed in profile" [1]. This fits well with one visual reference I was using- the test of a green sea urchin. 

The making of the maquette, marking of the symmetry, comparison to a modern green sea urchin (Strongylocentrotus droebachiensis)  and putting lead into the polystyrene body to make it easier to handle. 

Next, the spines were modeled. This was done using cocktail sticks that were cut to the proportional size. 

Spine positions [3].

Spine lengths [3].

While there's plenty of reference to the number of spines on interambulacral plates in each section (2 sets of offset pairs of spines totaling the diagnostic 4 spines), there is little detail on the total number, so this was estimated by eye. The cocktail sticks were stuck into the polystyrene body, first to create a whole and then dipped in hot glue before being re-inserted to secure them. The angle at which they stuck out from the body was important as to look regular they must be normal to the surface. While echinoid spines do move freely, for this reconstruction I thought it best to make them as regular as possible. 

The maquette in progress: 1.

The maquette in progress: 2.

Once the maquette was finished, I took a series of photographs from varying angles to work from. As you can see the many spines of differing perspectives and differing lengths would have been impossible to accurately recreate without the use of a maquette or 3D modelling.

Side view against white for the best contrast.

The photograph was printed and traced in pencil onto watercolour paper. The details, based upon my specimens, were then added, before fine-liner (0.1). This allowed me to capture most of the finer details, although tube-feet were omitted.

Progression.

B & W fine-line.

Next, watercolour was added. Modern sea urchins are highly variable in colour, from green to black to purple. I decided to go with a scheme more familiar to me, once again inspired by the green sea urchin, of green and purple.
One notable difference between my specimens and most contemporary species is that most modern species lack thorns, protruding from the spine shaft. One exception to this is the thorny sea urchin (Goniocidaris tubaria). The thorns of this species are coloured along a gradient- something that inspiration was taken from.

The final reconstruction.

After finishing the piece, I experimented with some photo-manipulation, using photographs from the water tank I've used in other pieces - see here for example-.

While the piece is, for the majority, finished. I plan to complete another. Instead of a pristine individual, like in this piece, the echinoid would be buried in sediment, with a split open test, and broken/fallen spines encrusted with bryozoa and surrounded by fenestella bryozoan fans.

Sources & further reading.

[1] The natural history museum: echinoid directory online. Accessed: https://www.nhm.ac.uk/our-science/data/echinoid-directory/taxa/taxon.jsp?id=54

[2] Pennsylvanian (Late Carboniferous) Echinoids from the Winchell Formation, NorthCentral Texas, USA Author(s): Chris L. Schneider, James Sprinkle and Dan Ryder Source: Journal of Paleontology, Vol. 79, No. 4 (Jul., 2005), pp. 745-762 Published by: Paleontological Society Stable URL: https://www.jstor.org/stable/4095047 Accessed: 22-05-2020 15:35 UTC

[3] Evolutionary Trends in Paleozoic Echinoids Author(s): Porter M. Kier Source: Journal of Paleontology, Vol. 39, No. 3 (May, 1965), pp. 436-465 Published by: SEPM Society for Sedimentary Geology Stable URL: https://www.jstor.org/stable/1301716 Accessed: 22-05-2020 15:35 UTC