Unnoticeable cooperation – Fossil Friday

There are discoveries that await to be made. In 2012, an astonishing discovery was made on a 453 to 458 Million years old slab of rock that had been sitting in a museum for already over a century. It was discovered that a colony of a species of prehistoric plankton was full of connections between the individual living spaces.  A feature that is either unique to this species or has also gone unnoticed in other sister species.
Graptolite specimen - Dicranograptus aff. ramosus

What you see is an example of graptolites. It is a word of greek origin meaning “written rock”. Graptolites are a class of extinct marine animals (a kind of plankton actually). They have the particularity of living in colonies and of building their own dwelling structure/housing complex which is called a rhabdosome, this is the part that is circled on the picture above. Rhabdosomes are mostly what is left of graptolites since their soft bodies do not fossilize well. Rhabdosomes contain hundreds of individual tubes/personal houses called thecae.

Rhabdosome are actually what have been used to tell different species apart. Indeed, they can be very varied from a single tube to complex branching or patterned structures depending on the species. Equally, depending on the species the rhabdosomes could have been attached to the ocean floor, free floating or attachable to sea weed etc.

The species here is thought to be Dicranograptus ramosus and there was an additional feature that had been overlooked for the century the slab of rock had been exposed. It was found that there were connections between the theca. It turns out that there actually are two types of connections and that they vary across  the colony. There are these ‘Hourglass’ and ‘Strap-like’ connections.
Graptolite colony with emphasis on network connecting theca

The image above emphasises the contours of these connections. They might be indicative that different type of individuals were inhabiting the colony and therefore that there might have been a division of labour within it. The thecae being all the same, it is thought to not represent sexual differences (dimorphism).

Could you imagine how many more discoveries are waiting to be made on fossils neatly put away on shelves or even exposed to the public?
Adrien for Sci&Fi, over and out.

Zalasiewicz, J.A. et al. (2013) Polymorphic organization in a planktonic graptoloid (Hemichordata: Pterobranchia) colony of Late Ordovician age. Geological magazine. (PDF)
Encyclopedia.com, Graptolithina.
Eurekalet. 2012. Prehistoric builders reveal trade secrets.

Visual credits:
Graptolite Specimen, Paul Witney, BGS, (c) NERC 2012
Rhabdopleura compacta, Dr. Atsuko Sato, University of Oxford.

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