How many orders of trilobites are there?

[Top Trilo]

A seemingly simple question seems to not have a simple answer. How many orders of trilobites are there? Not everyone agrees on the number but is there a number that is generally accepted by the experts? I have seen anywhere from 9-14 different orders. @piranha has 14 in this thread, Sam Gon III has 11 on his site and I have seen many other sites with different numbers. Which is most up to date?

 

 

Thanks in advance for any feedback. I'm going to tag two resident trilobite experts ahead of time. @Kane @piranha

 

That was more than one question but they're all in the same order. Or are they? 

[jpc]

These things (taxonomy) seems to always be in flux.  Even the number of bird families is not a solid number according to ornithologists.  And that is without considering the fossil families.  The answer not only depends on who you ask, but when you asked it.  

[Tidgy's Dad]

The problem seems to be that some researchers are lumpers and some are splitters.

There is no exact definition of what constitutes an order compared to a suborder, superfamily etc. 

This old diagram is clearly too simplistic :

So people added sub, super and infra etc.  as prefixes but this is often still not sufficient so nowadays just "clade" followed by the name is often  used. 

And they change all the time. 

How many orders of trilobite exist depends on the researchers opinion on which differences are large enough to warrant a group being separated from another and labelled as an order, though it is clear that some orders are more closely related than others, hence such additions as the subclass Librostoma. 

It's all terribly confusing. 

Oh, and there are certainly orders that are used a "basket taxa" where genera are just dumped, such as the Ptychoparida and these will become divided in time. 

 

Edited May 25, 2022 by Tidgy's Dad

[Top Trilo]

17 minutes ago, jpc said:

These things (taxonomy) seems to always be in flux.  Even the number of bird families is not a solid number according to ornithologists.  And that is without considering the fossil families.  The answer not only depends on who you ask, but when you asked it.  

Thank you for your response JP. If extant families can't even get to an agreement I don't know how people can even get close to agreement when all the families being considered went extinct 250+ million years ago. That means not even a time machine would solve the problem.

 

10 minutes ago, Tidgy's Dad said:

The problem seems to be that some researchers are lumpers and some are splitters.

There is no exact definition of what constitutes an order compared to a suborder, superfamily etc. 

This old diagram is clearly too simplistic :

So people added sub, super and infra etc.  as prefixes but this is often still not sufficient so nowadays just "clade" followed by the name is often  used. 

And they change all the time. 

How many orders of trilobite exist depends on the researchers opinion on which differences are large enough to warrant a group being separated from another and labelled as an order, though it is clear that some orders are more closely related than others, hence such additions as the subclass Librostoma. 

It's all terribly confusing. 

Oh, and there are certainly orders hat are used a "basket taxa" where genera are just dumped, such as the Ptychoparida and these will become divided in time. 

 

Thanks for responding Adam, I guessed this was the case and with 20,000+ species of trilobite I'm sure classification changes regularly. Since there wasn't one day when a trilobite laid 11 eggs and they all belonged to separate orders, that means Instead of domain to species, there could be thousands in between. 

Since that's not going to happen I can see how taxonomy is rather difficult and confusing.

[Kane]

I believe the current consensus is that there are still the 14, with Trinucleida being the most recent addition after having been split out from Asaphida. There will, of course, still be heated arguments over whether Agnostida should be included, but that may be a minority view. 

[piranha]

There are 14 orders that are in use but there is no general consensus on a 'correct' combination. Adrain 2011 established the new orders Aulacopleurida and Olenida to the exclusion of Agnostida and Ptychopariida and includes a category of Order Uncertain comprising 58 families, many of which were previously grouped in the Ptychopariida.

 

Adrain, J.M. 2011.

Class Trilobita Walch, 1771.

In: Zhang Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-Level Classification and Survey of Taxonomic Richness. Zootaxa, 3148:104-109  PDF LINK

[Top Trilo]

1 hour ago, Kane said:

I believe the current consensus is that there are still the 14, with Trinucleida being the most recent addition after having been split out from Asaphida. There will, of course, still be heated arguments over whether Agnostida should be included, but that may be a minority view. 

 

57 minutes ago, piranha said:

There are 14 orders that are in use but there is no general consensus on a 'correct' combination. Adrain 2011 established the new orders Aulacopleurida and Olenida to the exclusion of Agnostida and Ptychopariida and includes a category of Order Uncertain comprising 58 families, many of which were previously grouped in the Ptychopariida.

 

Adrain, J.M. 2011.

Class Trilobita Walch, 1771.

In: Zhang Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-Level Classification and Survey of Taxonomic Richness. Zootaxa, 3148:104-109  PDF LINK

Thank you both for your insight, Is the idea of Agnotsida as an order of trilobite losing ground? Is it similar to the case of Pluto, not a planet? If so does it form its own class?

[Tidgy's Dad]

1 hour ago, Top Trilo said:

 Is the idea of Agnotsida as an order of trilobite losing ground? Is it similar to the case of Pluto, not a planet? If so does it form its own class?

I think it's still much debated. Some say they are trilobites, others suggest they're stem group trilobites or even a sister group to basal crustaceans.  

[Top Trilo]

40 minutes ago, Tidgy's Dad said:

I think it's still much debated. Some say they are trilobites, others suggest they're stem group trilobites or even a sister group to basal crustaceans.  

Thanks Adam, do you consider your Agnostid fossils part of your trilobite collection?

[Tidgy's Dad]

1 hour ago, Top Trilo said:

Thanks Adam, do you consider your Agnostid fossils part of your trilobite collection?

Yes.

But I really don't know what I'm talking about. 

It's just easier, frankly, and saves me from making new labels and id cards. 

I've got enough to deal with concerning my brachiopods. 

[Top Trilo]

1 minute ago, Tidgy's Dad said:

I've got enough to deal with concerning my brachiopods. 

Is Brachiopod taxonomy just as controversial? Unfortunately I think most non-fossil people think they're just shells.

[Top Trilo]

I guess, regarding taxonomy, it doesn't matter what non-fossil people think of them. Its mostly done by the specialists anyway. Still though is there that much disagreement if a Spiriferid and an Atrypid are closely related or not. That's about the extent of my brachiopod knowledge. 

I learn more everyday though thanks to your brachiopod updates in your collection threads.

[Tidgy's Dad]

1 hour ago, Top Trilo said:

Is Brachiopod taxonomy just as controversial? Unfortunately I think most non-fossil people think they're just shells.

To be fair, since the old Spiriferida was split onto Atrypida, Athyridida, Spiriferida and Spiriferinida, plus the isolation of the Thecideida and the separation of 'the Inarticulata" into Linguliformes and Craniiformes, (subphylums)   I don't think we worry too much about orders. But below that there is an awful lot of debate about where a particular family or  genus belongs. 

[piranha]

Here is the most recent info on the "agnostid problem":

 

 

The weight of evidence currently available supports an artiopodan affinity for agnostinids and suggests that they form a clade with polymeroid trilobites. More precise resolution of their relationship with polymeroid trilobites will be facilitated by a better understanding of the polarity of character evolution of the artiopodan cephalon and the phylogeny of the earliest trilobite lineages. Whether agnostinids are regarded as an early diverging trilobite ingroup or as a distinct sister clade to polymeroid trilobites, they testify to the high degree of morphological differentiation achieved among the Cambrian members of this group. Among artiopodans, trilobites are notably characterized by a release from canalization in their trunk development, especially among basally diverging lineages [73]. Agnostinids conform to this pattern with their highly reduced trunk [74], but additionally point to the possibility that the segmental composition of the cephalon was labile in the early evolutionary history of the trilobite-agnostinid clade—a trait potentially inherited from a deeper artiopodan ancestry.

In combination with the material from the Orsten Lagerstätte, our study contributes towards the most completely known ontogenetic sequence of any artiopodan. The similarity between adult and juvenile agnostinids implies that their distinctive morphology is not simply attributable to ontogenetic changes or a differentiated larval ecology. Instead, the morphological differentiation of agnostinids from polymeroid trilobites is likely a product of both the plesiomorphically variable head tagma and adaptation to a specialized mode of life. Agnostinids, as small nektobenthic detritus feeders and opportunists, present an example of a clade of artiopodans occupying a niche in the Early Palaeozoic which was subsequently overtaken by mandibulates. Taken together, these findings hint that considerable morphological and ecological diversity among artiopodans may still await discovery, particularly with continued study of Burgess Shale-type material.

 

Moysiuk, J., Caron, J.B. 2019

Burgess Shale Fossils Shed Light on the Agnostid Problem.

Royal Society of London, Proceedings, Series B 281(20182314):1-9  PDF LINK

 

 

The results of the phylogenetic analysis conducted by Moysiuk & Caron (2019) show that Agnostina (Agnostus + Peronopsis) is sister to the polymerid trilobites, which is mainly supported by shared features such as the calcified tergites, style of tergal articulation, and a dorsally segmented cephalon (including the occipital lobe). In summary, although the combined soft-tissue and biomineralized morphology of agnostines may now suggest an affinity closer to artiopodans (even as sister to polymerid trilobites; Moysiuk & Caron, 2019) than to pancrustaceans (Walossek & Müller, 1990; Bergström & Hou, 2005), further analyses are needed to test whether they are trilobites sensu stricto.

 

Paterson, J.R. 2020

The Trouble with Trilobites: Classification, Phylogeny and the Cryptogenesis Problem.

Geological Magazine, 157(1):35-46  PDF LINK

[Top Trilo]

3 minutes ago, Tidgy's Dad said:

the old Spiriferida was split onto Atrypida, Athyridida, Spiriferida and Spiriferinida, plus the isolation of the Thecideida and the separation of 'the Inarticulata" into Linguliformes and Craniiformes,

Say that five times fast! I don't think I could say it once. 

4 minutes ago, piranha said:

Here is the most recent info on the "agnostid problem":

 

 

The weight of evidence currently available supports an artiopodan affinity for agnostinids and suggests that they form a clade with polymeroid trilobites. More precise
resolution of their relationship with polymeroid trilobites will be facilitated by a better understanding of the polarity of character evolution of the artiopodan cephalon and the phylogeny of the earliest trilobite lineages. Whether agnostinids are regarded as an early diverging trilobite ingroup or as a distinct sister clade to polymeroid trilobites, they testify to the high degree of morphological differentiation achieved among the Cambrian members of this group. Among artiopodans, trilobites are notably characterized by a release from canalization in their trunk development, especially among basally diverging lineages [73]. Agnostinids conform to this pattern with their highly reduced trunk [74], but additionally point to the possibility that the segmental composition of the cephalon was labile in the early evolutionary history of the trilobite-agnostinid clade—a trait potentially inherited from a deeper artiopodan ancestry.

In combination with the material from the Orsten Lagerstätte, our study contributes towards the most completely known ontogenetic sequence of any artiopodan. The similarity between adult and juvenile agnostinids implies that their distinctive morphology is not simply attributable to ontogenetic changes or a differentiated larval ecology. Instead, the morphological differentiation of agnostinids from polymeroid trilobites is likely a product of both the plesiomorphically variable head tagma and adaptation to a specialized mode of life. Agnostinids, as small nektobenthic detritus feeders and opportunists, present an example of a clade of artiopodans occupying a niche in the Early Palaeozoic which was subsequently overtaken by mandibulates. Taken together, these findings hint that considerable morphological and ecological diversity among artiopodans may still await discovery, particularly with continued study of Burgess Shale-type material.

 

Moysiuk, J., Caron, J.B. 2019

Burgess Shale Fossils Shed Light on the Agnostid Problem.

Royal Society of London, Proceedings, Series B 281(20182314):1-9  PDF LINK

 

The results of the phylogenetic analysis conducted by Moysiuk & Caron (2019) show that Agnostina (Agnostus + Peronopsis) is sister to the polymerid trilobites, which is mainly supported by shared features such as the calcified tergites, style of tergal articulation, and a dorsally segmented cephalon (including the occipital lobe). In summary, although the combined soft-tissue and biomineralized morphology of agnostines may now suggest an affinity closer to artiopodans (even as sister to polymerid trilobites; Moysiuk & Caron, 2019) than to pancrustaceans (Walossek & Müller, 1990; Bergström & Hou, 2005), further analyses are needed to test whether they are trilobites sensu stricto.

 

Paterson, J.R. 2020

The Trouble with Trilobites: Classification, Phylogeny and the Cryptogenesis Problem.

Geological Magazine, 157(1):35-46  PDF LINK

Thank you for this. It has everything I was wondering.

[Tidgy's Dad]

1 hour ago, Top Trilo said:

I guess, regarding taxonomy, it doesn't matter what non-fossil people think of them. Its mostly done by the specialists anyway. Still though is there that much disagreement if a Spiriferid and an Atrypid are closely related or not. That's about the extent of my brachiopod knowledge. 

I learn more everyday though thanks to your brachiopod updates in your collection threads.

We now know that the atrypids and spiriferids developed spiralia independently. The atrypids were first Atrypid spiralia bend outwards, rather than inwards in the spiriferids and atrypids usually have a very short hinge line while spiriferids are famously strophic for quite a distance. Atrypids are always rounded in outline whist spiriferids are rarely so. 

[Top Trilo]

2 hours ago, piranha said:

There are 14 orders that are in use but there is no general consensus on a 'correct' combination. Adrain 2011 established the new orders Aulacopleurida and Olenida to the exclusion of Agnostida and Ptychopariida and includes a category of Order Uncertain comprising 58 families, many of which were previously grouped in the Ptychopariida.

 

Adrain, J.M. 2011.

Class Trilobita Walch, 1771.

In: Zhang Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-Level Classification and Survey of Taxonomic Richness. Zootaxa, 3148:104-109  PDF LINK

While Adrian 2011 doesn't have Agnostida it does have Eodiscida. Does that mean some Agnostids are considered trilobites while others aren't, according to this publication?

[Top Trilo]

Is Trinucleida generally accepted as an order?

[piranha]

7 minutes ago, Top Trilo said:

Is Trinucleida generally accepted as an order?

 

Trinucleida was first introduced as a suborder in 1915. With the recent paper by Bignon et al. 2020 it will certainly get adopted by other workers in the future. Speaking of Sam Gon, his most recent chart iteration of trilobite orders includes the Trinucleida. This scheme also includes the Olenida with the "Pytchopariida" tentatively placed in quotation marks. 

 

 

Bignon, A., Waisfeld, B.G., Vaccari, N.E., Chatterton, B.D.E. 2020

Reassessment of the Order Trinucleida (Trilobita).

Journal of Systematic Palaeontology, 18(13):1061-1077

[piranha]

1 hour ago, Top Trilo said:

While Adrian 2011 doesn't have Agnostida it does have Eodiscida. Does that mean some Agnostids are considered trilobites while others aren't, according to this publication?

 

Adrain 2011 removed the Agnostida entirely. However, in the chart above (Gon 2020), agnostids and eodiscids are lumped together in the Agnostida.

 

As I mentioned previously, there is no general consensus....