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Andúril Flame of the West

Hello all,


In my investigation of the fauna of the Devonian Mahantango Formation, I have become quite interested in the trilobite Dipleura dekayi. I have not yet had the opportunity to encounter this bug on the field, but it’s size and strange appearance have draw my interest. From the sources that I have read regarding the Mahantango and Hamilton Group formations in Pennsylvania and New York, I have noticed that trilobites with a very similar appearance have been referred to the separate genera Dipleura and Trimerus. In Fossil Collecting in the Mid-Atlantic States, Jasper Burns seems to treat Dipleura and Trimerus as synonyms. However, Wikipedia (I am fully aware this is not the most reliable source) has separate pages for Dipleura dekayi and Trimerus dekayi. I have also heard more references to Trimerus when the specimens are found in the New York area and I vaguely remember a posting on the forum where the question of Dipleura vs. Trimerus was addressed but which post that was has escaped me. 

Here are the main questions that I have regarding this topic: 


1.) Are Trimerus dekayi and Dipleura dekayi different valid trilobite species or are they synonyms? If they are synonyms, which would be the most proper to use?


2.) If Trimerus dekayi and Dipleura dekayi are two different valid species, how can they be differentiated? Does this depend on the region where the specimen was recovered?


3.) Are there other species of Trimerus present in the Mahantango Formation/Hamilton Group and how can they be identified and differentiated from Dipleura?

Thank you in advance to all who view and comment on this post. I am very eager to learn more about Mahantango fauna and hopefully it will not be long before I find my first Dipleura/Trimerus in the field. 


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Dipleura occurs in Devonian deposits (such as the Mahantango). Trimerus is restricted to the Silurian (such as in in the Middleport of NY). 


1) Dipleura and Trimerus are not different species, but genera.

2) As different genera, Dipleura have completely effaced pygidia (except when juveniles) whereas Trimerus has an obvious pygidial axis.

3) There are no Trimerus  in the Mahantango, so the question is moot. 


There is no longer a Trimerus dekayi, and the proper taxon would be Trimerus delphinocephalus for eastern North America.

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I recall that the Devonian species was originally called Trimerus dekayi.  Subsequently that species was recognized as being different enough from Trimerus to require a different genus, which was named Dipleura.  Both names refer to the same species, but the correct current name is Dipleura dekayi, Trimerus dekayi is an obsolete name.



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Jasper Burns did not treat Trimerus and Dipleura as synonyms. The designation of Trimerus (Dipleura) indicates that Dipleura was previously considered a subgenus of Trimerus. The concept of Dipleura as a subgenus of Trimerus was first introduced by Klaus Sdzuy in the 1959 Treatise on Invertebrate Paleontology. Subsequently, Tomczykowa 1975 elevated Dipleura to a generic rank based on differences in shape and lobation of the glabella and trilobation and segmentation of the pygidium.



Moore, R.C. (ed.) 1959
Treatise on Invertebrate Paleontology. Part O. Arthopoda 1. Trilobita.
Geological Society of America and University of Kansas Press: 560 pp.  PDF LINK


Tomczykowa, E. 1975.
The Trilobite Subfamily Homalonotinae from the Upper Silurian and Lower Devonian of Poland.
Acta Palaeontologica Polonica, 20(1):3-46  PDF LINK

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Andúril Flame of the West

I appreciate everyone’s responses. I figured that Dipleura dekayi was more likely to be the current name due to its use here on the forum. Thank you for the clarification @Kane. I had thought I saw a post where a species of Trimerus was offered as an identification for a Hamilton Group fossil but it was likely from one of the Silurian formations in the area. 


On 5/23/2023 at 11:34 AM, piranha said:

Jasper Burns did not treat Trimerus and Dipleura as synonyms. The designation of Trimerus (Dipleura) indicates that Dipleura was previously considered a subgenus of Trimerus.

Well, that is embarrassing… my mistake. I appreciate the clarification though and the articles that you linked to your response @piranha. It will make for some interesting reading!

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