Migrating Devonian Trilobites

[piranha]

excerpts and figures from:

 

Błażejowski, B., Brett, C.E., Kin, A., Radwański, A., & Gruszczyński, M. (2016)

Ancient animal migration: a case study of eyeless, dimorphic Devonian trilobites from Poland.

Palaeontology, 59:743-751

 

 

ABSTRACT

We report evidence of one of the oldest known animal migratory episodes in the form of queues of the eyeless trilobite Trimerocephalus chopini Kin & Błażejowski, from the Late Devonian (Famennian) of central Poland. In addition, there is evidence for two morphs in this population, one with nine segments and the other with ten. We infer that these queues represent mass migratory chains coordinated by chemotaxis, comparable to those observed in modern crustaceans such as spiny lobsters, and further suggest that the two forms, which occur in an approximately 1:1 ratio, may be dimorphs. These ancient arthropods may have migrated periodically to shallow marine areas for mass mating and spawning. The sudden death of the trilobites in the queues may have been caused by excess carbon dioxide and hydrogen sulphide introduced into the bottom water by distal storm disturbance of anoxic sediments. This study demonstrates the potential for further research on the evolution and ecology of aggregative behaviour in marine arthropods.

 

CONCLUSIONS

Results of our research demonstrate that repetitive migratory behaviour had already evolved among marine animals by at least the mid-Palaeozoic Era. We infer that the breeding strategy for the blind trilobite Trimerocephalus chopini Kin & Błażejowski 2013 included mass migration to spawning-grounds; the mass migrations in queues were likely coordinated by primary chemosensory phenomenon, as in some modern crustaceans. An important argument related to periodic migration of T. chopini to the breeding areas is the co-occurrence of numerous enrolled juveniles, which occur in each queue horizon; these abundant juveniles could record ‘trilobite nurseries’. Moreover, individuals of T. chopini forming migratory queues do not show a fixed number of thoracic segments. A plausible explanation for the 9- and 10-segmented variants, considering their nearly 1:1 ratio, is that they represent sexual dimorphs in a single species; these two morphotypes each show considerable and overlapping size ranges, although the 10-segmented morph is generally larger.

 

As to causes of sudden death of the described blind trilobites, we suggest poisoning by hydrogen sulphide, which is very diffusive and highly toxic, and/or hypercapnia, owing to carbon dioxide released from sediment, replacing oxygen in the bottom waters. Regardless, the similar state of in situ preservation of these blind trilobites suggests almost simultaneous death of all of the individuals (both enrolled juveniles and sexually mature adults arranged in the queues). Abrupt burial of T. chopini populations followed very shortly after the mass mortalities, probably from fallout of flocculated muds stirred up by storms (Type 1 assemblages of Brett et al. 2012). This must have been a recurrent phenomenon in this area during the Late Devonian because at least three occasions of storm burial overlapped with reproductive periods of T. chopini to produce similar queues at three distinct horizons.

 

Finally, we conclude that chemotaxis may have functioned as a long-term macroevolutionary process controlling reproductive behaviour of arthropods, the largest known phylum of animals on Earth.

 

 

[Fossildude19]

Wow, Scott.

That is rather fascinating.

Thanks for posting this.

Regards,

[Fossildude19]

Another one with a shot of the trilos - LINK

Regards,

[Raggedy Man]

Amazing! Thanks Piranha. I had Tim remove that rubbish from earlier. Lol.

[Shamalama]

This is cool, sorry I missed it the first time around.