Monday 30 December 2013

A mass death of Starfish in the Late Cretaceous of Morocco.

Starfish (Asteroidea) are benthic marine Echinoderms found across the globe. They have a simple bodyplan, with a central disk that has the mouth on the underside, surrounded by (usually) five arms, each with a double row of tube feet underneath. The upper surface of the Starfish is covered by an articulated armoured exoskeleton made up of thousands of small calcite plates. The animal moves by means of a water vascular system, which enables it to inflate and deflate the tube feet as required. The earliest Starfish appear in the fossil record in the Ordovician, however they are not common fossils, as the skeleton is made up of a large number of very small components, and tends to disarticulate rapidly after death.

In a paper published in the journal Palaeontology on 7 May 2013, Any Gale of the School of Earth and Environmental Sciences at the University of Portsmouth and Loïc Villier of the Laboratoire de Géologie des Systèmes et des Réservoirs Carbonatés at the Université de Provence describe a site at Bakrit in the Middle Atlas of Morocco where a very large number of Starfish belonging to a single (new) species are preserved as in a limestone concretion apparently representing a mass mortality event.

The Starfish are given the specific Cretasterias reticulatus, where 'Cretasterias' means 'Cretaceous Starfish' and 'reticulatus' refers to their reticulated skeleton. There are thousands of individuals present in the slabs recovered from the site, averaging 60-140 mm across, and essentially similar in morphology to modern Starfish.

Specimens of the Starfish Cretasterias reticulatus on a slab from the Late Cretaceous of Bakrit in the Middle Atlas. Gale & Villier (2013).

The Starfish in the slab appear to have died in a single mass mortality event, probably when they were swept into a marine gully during a storm and buried rapidly. Such mortality events are not unusual in modern Starfish, many species of which will undergo spectacular population explosions when suitable conditions arise, followed by mass deaths when conditions change again.

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