Sheltered Gregarious Behavior of Middle Ordovician Harpetid Trilobites

Abstract

The presence of six articulated exoskeletons of late holaspid specimens of the rare harpetid Eoharpes benignensis entombed under a pygidial shield of the large asaphid trilobite Nobiliasaphus repulsus from the Middle Ordovician Dobrotivá Formation of the Prague Basin, Czech Republic is interpreted as a unimodal monotaxic trilobite cluster. The sheltered preservation of the trilobites may be explained as; (1) hiding behavior associated with predation pressure; (2) storm disturbance; or (3) molting associated with feeding. It is herein suggested that these Middle Ordovician holaspid trilobites deliberately entered the restricted space under a large isolated asaphid trilobite pygidial shield to find a refuge and shared the space within restricted shelters with conspecifics. The completeness of all specimens of the rare taxon Eoharpes, combined with the presence of more than one individual in this restricted space, excludes the possibility of transportation by bottom currents. This exceptional find represents an example of ''frozen behavior'' and provides a new insight in the life strategy of Middle Ordovician benthic trilobites. Attack abatement, e.g., avoidance and dilution effects, is for the first time proposed as a possible explanation for this example of sheltered gregarious behavior in trilobites.

Recognising moulting behaviour in trilobites by examining morphology, development and preservation: Comment on Błażejowski et al. 2015

“…Gregarious behaviour in trilobites has been documented in many assemblages for protection , moulting , burrowing , feeding and reproduction . Unlike in modern arthropods, where behaviour can be observed directly, trilobite fossil aggregations must be considered in context to try to assess whether they are the result of gregarious behaviour or abiotic accumulations.…”

“…The early fossil record of moulting is also rich in examples of exuviae, particularly for Cambrian and Ordovician strongly biomineralised arthropods such as trilobites [5]. Several specimens from this time period show clusters of exuviae preserved in close proximity, indicating synchronous mass moulting events [13,14]. This rich fossil record of moulting has allowed for quantitative analyses investigating moulting behaviour trends, and their impact on trilobite evolution [5,15].…”

Seven 365-Million-Year-Old Trilobites Moulting within a Nautiloid Conch

“…They gradually lost their Cambrian predominance, and recessed since the middle Palaeozoic, when more prominent cryptic behaviour and survival strategy started to develop. In order to avoid the attacks of predators and the stresses of marine environments, they chose the conchs of other invertebrates as shelters 8 9 10 , or hid in the burrows of other creatures 3 11 , and some others may even have excavated open tunnels in the seafloor for self-protection 12 .…”

A Trilobite Cluster from the Silurian Rochester Shale of New York: Predation Patterns and Possible Defensive Behavior

“…There is considerable evidence to suggest that trilobites clustered in cryptic places on the seafloor (e.g., within burrows or reef cavities, under or inside empty shells and exoskeletons, etc.) to perform a variety of gregarious activities (especially molting) while avoiding predators or environmental perturbations (see Chatterton et al, 2003;Gutierrez-Marco et al, 2009;Fatka and Budil, 2014;Zong et al, 2016; references therein). However, the Rochester cluster was exposed on the sea¬ floor prior to burial, so perhaps the only line of defense from predation was a "safety in num¬ bers" approach.…”