"In the end we had to just speak that heresy out loud" -- Refuting Arachnid Monophyly

[goliathusdavid]

Just wanted to share this February paper for those who have not seen it. It has been on my radar since the week it first came out, but only really got to analyze it in an academic context this week. Talk about blowing up an entire taxonomic class...turns out horseshoe crabs are arachnids and that's just the beginning. The whole arachnid class as we know it kind of has to go out the window . And yes, the methods are impeccable, a whole other level of thoroughness.
Comprehensive Species Sampling and Sophisticated Algorithmic Approaches Refute the Monophyly of Arachnida - Journal of Molecular Biology and Evolution

Act of "Heresy" adds horseshoe crabs to arachnid family tree - NYTimes

 

[DaveM]

I have been following this story as well. An earlier paper from the same people (https://academic.oup.com/sysbio/article/68/6/896/5319972?login=false) was threatening what they have now done.
I really do not believe they will ever throw out the class Arachnida. They will just make the class monophyletic again by moving Xiphosura into Arachnida as a new sister clade of Ricinulei. This is a beautiful explanatory sentence, "Our results suggest that morphological convergence resulting from adaptations to life in terrestrial habitats has driven the historical perception of arachnid monophyly, paralleling the history of numerous other invertebrate terrestrial groups."
This is part of the greater movement toward DNA supplanting morphology as the top phylogenetic determinant in taxonomy. Any possible concession to historical antecedents will be made, and preserving Arachnida is one of the least problematic and most important concessions, barely even a concession, just reclassify horseshoe crabs and their ilk. Good. Soon we can have a new kind of creature to add to discussions on Arachnoboards!

Exciting! Now we can talk about book gills instead of book lungs. Oooo! Oooo! And since these guys need water circulation and room to swim, I want to be the first on AB ever to say it: Add LESS substrate!

 

[l4nsky]

Exciting! Now we can talk about book gills instead of book lungs. Oooo! Oooo! And since these guys need water circulation and room to swim, I want to be the first on AB ever to say it: Add LESS substrate!

And no one will be debating whether they need high humidity or not

 

[Wolfram1]

And no one will be debating whether they need high humidity or not

well, i would argue humidity is irrelevant , it needs moist substrate, so moist in fact it tops the capacity of said soil to absorb it.

 

[Patherophis]

Old news

Critical Appraisal of the Placement of Xiphosura (Chelicerata) with Account of Known Sources of Phylogenetic Error

Abstract. Horseshoe crabs (Xiphosura) are traditionally regarded as sister group to the clade of terrestrial chelicerates (Arachnida). This hypothesis has been

academic.oup.com

But it is nice to see more robust support for it

 

[goliathusdavid]

I have been following this story as well. An earlier paper from the same people (https://academic.oup.com/sysbio/article/68/6/896/5319972?login=false) was threatening what they have now done.
I really do not believe they will ever throw out the class Arachnida. They will just make the class monophyletic again by moving Xiphosura into Arachnida as a new sister clade of Ricinulei. This is a beautiful explanatory sentence, "Our results suggest that morphological convergence resulting from adaptations to life in terrestrial habitats has driven the historical perception of arachnid monophyly, paralleling the history of numerous other invertebrate terrestrial groups."
This is part of the greater movement toward DNA supplanting morphology as the top phylogenetic determinant in taxonomy. Any possible concession to historical antecedents will be made, and preserving Arachnida is one of the least problematic and most important concessions, barely even a concession, just reclassify horseshoe crabs and their ilk. Good. Soon we can have a new kind of creature to add to discussions on Arachnoboards!

Exciting! Now we can talk about book gills instead of book lungs. Oooo! Oooo! And since these guys need water circulation and room to swim, I want to be the first on AB ever to say it: Add LESS substrate!

A good take-- I think you are correct that we will never see a full throwout of the class Arachnida, but I would argue that this paper has an impact that will reach far more broadly than the simple inclusion of Xiphosura. While, as you say, the supplanting of morphology with genetic analysis is a mass movement, a few of the findings and assertions in the section "Morphology may be confounded by convergence in chelicerate phylogeny" as well as the integration of Xiphosura into the class are highly likely (in my opinion) to bring about a whole new lens for examining terrestrialization events. Putting an aquatic in the middle of a group composed of almost entirely terrestrials is certainly going to lead to some really fascinating questions that extend far beyond just arachnida, that is to say, more questions about the various times that a terrestrial group has said "land is overrated" and returned to water, a type of occurence around which there is a lot to be studied.

I should also say that the choice of words in my first post was poor. It is less that a "class has to go out the window" and much more "wow an evolutionary history we've accepted for a really long time for thousands of species was really pretty darn wrong ." Arachnida will be around for a long time-- I would just be prepared for it to shift A LOT!

And AMEN to the addition of a new forum for Xiphosura. We really should change the name to ArthroBoards...

 

[DaveM]

A good take-- I think you are correct that we will never see a full throwout of the class Arachnida, but I would argue that this paper has an impact that will reach far more broadly than the simple inclusion of Xiphosura. While, as you say, the supplanting of morphology with genetic analysis is a mass movement, a few of the findings and assertions in the section "Morphology may be confounded by convergence in chelicerate phylogeny" as well as the integration of Xiphosura into the class are highly likely (in my opinion) to bring about a whole new lens for examining terrestrialization events. Putting an aquatic in the middle of a group composed of almost entirely terrestrials is certainly going to lead to some really fascinating questions that extend far beyond just arachnida, that is to say, more questions about the various times that a terrestrial group has said "land is overrated" and returned to water, a type of occurence around which there is a lot to be studied.

I should also say that the choice of words in my first post was poor. It is less that a "class has to go out the window" and much more "wow an evolutionary history we've accepted for a really long time for thousands of species was really pretty darn wrong ." Arachnida will be around for a long time-- I would just be prepared for it to shift A LOT!

And AMEN to the addition of a new forum for Xiphosura. We really should change the name to ArthroBoards...

Agreed. You may be right that the other parts of the subphylum Chelicerata will see some more of a shaking up as well. I like Arachnoboards. Arthroboards would just be a step to far for me. You wouldn't want all of the crustaceans to become first-class citizens in this place, would you? All the other crabs? Just too crabby!

 

[goliathusdavid]

Agreed. You may be right that the other parts of the subphylum Chelicerata will see some more of a shaking up as well. I like Arachnoboards. Arthroboards would just be a step to far for me. You wouldn't want all of the crustaceans to become first-class citizens in this place, would you? All the other crabs? Just too crabby!

But... but... CARCINIZATION they all gonna be crabs anyway . Plot twist...

 

[DaveM]

But... but... CARCINIZATION they all gonna be crabs anyway . Plot twist...

Ah Ahah! Get out of my mind! I read that too. However, carcinization is a different story. That is convergent evolution, separate from phylogenetic changes, and hence will not change our understanding of what belongs as an arachnid.
At most, we can say that scorpions have crab-like pincers, which appear to be evolutionarily favored across disparate clades. Heck, I wish I had a pair of chelae for use when annoying people get up in my face or cut ahead of me in line!
Let carcinization progress freely, and may it visit my mammalian genus some day, but it will not change how we name living creatures, just make livid creatures out of people we don't like anyway

 

[goliathusdavid]

Completely agreed . Really no hard evidence of it occurring outside of Crustacea despite what the memes may asset... Still, gotta be doing something right if it keeps showing up every after mass extinction. Perhaps one day we shall have chelae and a flattened carapace

***I should say TRAGICALLY no hard evidence