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Ghost lineages!
Topic Started: Jun 21 2017, 06:59 AM (1,824 Views)
IIGSY
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A ghost lineage is a lineage of organisms that is infered to exist in the fossil record but has left no fossilized trace of its existence. Some examples include

Stem earthworms

Aquatic ancestors of arachnids

Stem hexapods

Basal bony fish that predate the lobe fin Ray fin split

Dozens of smaller metazoan phyla have no fossil record, including gastrotrichs and nemerteans

Early terrestrial velvet worms

But probably the biggest one of all, stem myriapods. Myriapods don't appear until the late silurian. Yet pancrustacea, the sister group to myriapods, have definitive cambrian fossils. This means there is a massive ghost lineage of myriapods and stem myriapods that extends from the cambrian to the middle silurian.

What might these groups have been like? Is there any notable ghost lineages you have to share?
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PortentosaMan
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I think the cambrian myriapods might have looked like some sort of pancrustacean worm or armored lobopod. The ordovician myriapods I think looked more like the myriapods we see today, but with a body made more for water (with gills and longer antennae).

The file under here is what I think an aquatic millipede would have looked like. (sorry for the bad quality)

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Edited by PortentosaMan, Jun 21 2017, 09:26 AM.
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HangingThief
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I don't think very primitive myriapods had ocelli arranged into the compound eye- like structure structure of modern millipedes though. Especially if we assume that centipedes and millipedes have a terrestrial common ancestor.
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For the longest time, the basal-iguanodontian family Rhabdodontidae had a 90 million year ghost lineage going from the Late Cretaceous to the Jurassic. However, Muttaburrasaurus was eventually reclassified to a position within the group, and it's about smack-dab in the middle. So that's still two gaps of 45 million years where they were doing something. Additionally, Muttaburrasaurus is from Australia, while the rest of the group are known from... Europe. There's more interesting stuff about the group, too, and they're one of my favorite groups of dinosaurs.
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In a similar vein, I find the ghost lineage for Pachycephalosauria especially vexing. As sisters to the ceratopsians, we know they had to have diverged from them at least 158 million years ago or so, and yet they only start definitively appearing in the fossil record less than 90 million years ago. That means we're missing over 80% of their evolutionary history in time. What were they doing all that time? We know that ceratopsians experimented and produced a number of clades before reaching Ceratopsidae. Did pachycephalosaurs do the same?

Pachycephalosaurids were weird to boot, too. Domed skulls aside, they have tiny arms, massively rotund rib cages and ossified tails, and they just show up in the fossil record with all these features already in place. This may not seem weird by itself, but consider that these had to have come from something that was along the lines of a Yinlong without an upper beak bone. Pachycephalosaur evolution, it is a mystery.
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IIGSY
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Especially mysterious are the stem lineages of land animals that are not vertebrates or arthropods. This includes nematodes, flatworms, nemerteans, and onychophores.

The majority of fossil nematodes are jurassic-onward in age, with some in the carboniferous. Some cambrian fossils are possible nematodes, but this remains dubious.

Archisymplectes and the infamus Amiskwia are both possible nemerteans, but their classification remains dubious. No confirmed nemertean has been found in the fossil record.

Platyhelminthese have basically no fossil record, with only a handful of specimens including...
-Permian tapeworms eggs found in shark coprolites
-Schistosoma eggs in Egyptian mummies
-Micropalaeosoma balticus, a free living flatworm found in Eocene baltic amber

Marine stem velvet worms are abundant in the early paleozoic, but their terrestrialization remains a mystery. Terrestrial stem onychophorans include Helenodora, Tertiapatus, and Succinipatopsis. Crown species are only known from amber Some of the amber specimens interestingly lack slime papillae, suggesting that the crown group may have been a recent development.

The nematode fossil record consists mainly of arthropod parasitic species trapped in amber along with their hosts.
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Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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Brock Slabson, Ph.D.
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For me it's a tossup between pterosaurs and bats, honestly. Both groups just show up in the fossil record, ready-made like store-bought cakes. The only thing with bats is that, IIRC, the earliest known genus might not have been able to echolocate. With pterosaurs we have the tantalizing, if poorly-preserved, Scleromochlus.

With both groups we can make educated guesses, but that's all.

It's frustrating. This is why my favorite evolutionary history is that of camels. I'm still bitter about North America losing its camels, honestly. #thisinterglacialruinedeverything
Edited by Brock Slabson, Ph.D., Jun 22 2017, 03:09 AM.
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IIGSY
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Gastrotrichs are non-existent in the fossil record, and there is only one fossil khinorhynch. Hell, if you think about, metazoans as a whole are a ghost lineage in a way.

Vertebrates, mollusks, arthropods, and brachiopods all have extremely detailed fossil records (though in the latter two, soft tissue preservation is nearly unheard of).

All other animal groups are a massive collection of ghost lineages with some known specimens in between.
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Punga: A terraformed world with no vertebrates
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ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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Inceptis
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I am the most miffed about entoprocts. I understand about the microscopic clades (still hoping for amber preserving more basal gnathiferan than dominican rotifer), but these guys are a decent size, up to seven millimeters long. And yet we only have Cotyledion and a few similar ones from the Cambrian, leaving more than a 500 million year gap. True, we're not sure about other phylums as well, but they have one of the most vague stories of Metazoa.

Nemerteans, though. Don't see how you made the connection to Amiskwia, considering how likely it's a chaetognath.

I'm wondering if the ancestors of myriapods looked similar to Euthycarcinoids but more segments, given the tree on their wiki page putting them right above them.
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IIGSY
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Inceptis
Jun 23 2017, 11:49 PM
I am the most miffed about entoprocts. I understand about the microscopic clades (still hoping for amber preserving more basal gnathiferan than dominican rotifer), but these guys are a decent size, up to seven millimeters long. And yet we only have Cotyledion and a few similar ones from the Cambrian, leaving more than a 500 million year gap. True, we're not sure about other phylums as well, but they have one of the most vague stories of Metazoa.

Nemerteans, though. Don't see how you made the connection to Amiskwia, considering how likely it's a chaetognath.

I'm wondering if the ancestors of myriapods looked similar to Euthycarcinoids but more segments, given the tree on their wiki page putting them right above them.
Well, the wiki page does state Amiskwia as being a possible nemertean. Not sure how accurate that is. Speaking of chaetognaths. While they are still extremely vague, they have a better fossil record than expected.
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Punga: A terraformed world with no vertebrates
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ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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kusanagi
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Stem or at least protomorphic hexapods like Leverhulmia are already known and someone else mentioned the euthycarcinoids are in the total group of Hexapoda + (Chilopoda + Progoneata). Amiskwia and Oesia are incertae sedis still but protoconodonts are total group chaetognnaths.

Most problematica are actually better known now: halwaxids and Odontogriphus are total group mollusks and Nectocaris is a crown mollusc; hyoliths are brachiozoan and "tommotians" ae a paraphyletic grade within the brachiozoa, which are in turn close to the newly recognised clade cambroernids. The hederellids seem to be similar to the phoronida and therefore also brachiozoan onfirming this group of animals was much more important in the past. Euconodonts are chordates but not crown craniates as is recovered when cladistic analyses do not allow them to be much anything else, dragging the otherwise stable vertebrate phylogeny into a polytomy. Archaocyathians and stromatoporoids were long ago discovered to be sponges and even the Ediacaran fronds are nowadays likely to be ctenophores. The vetulicolids are related to kinorhynchs in a cladistic analysis and importantly basal but definite vetulicolians (ie. banffozoans) do not have chordate-like gill slits undermining a chordate placement. The calcareous stylophorans definitely possess an echinoderm-like skeleton but they lack the water vascular system of the crown echinoderms and mitrates present a chordate-like dexothetism. Vetulocystis and Dianchicystis are two genera that resemble uncalcified stylophoran animals. Etacystis is an ambulacrarian and Tullimonstrum seems to have been a total group craniate nowadays even if other possibilities cannot be rejected out of hand. Sometimes problematica turn out not to be animals at all: a supposed Silurian deuterozoan Rutgersella has the histology of a fungus as well as rhizines and most non-triradial discoids in the Ediacaran biota turned out to be microbial colonies.
Edited by kusanagi, Jul 24 2017, 12:07 PM.
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IIGSY
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kusanagi
Jul 24 2017, 09:03 AM
Stem or at least protomorphic hexapods like Leverhulmia are already known and someone else mentioned the euthycarcinoids are in the total group of Hexapoda + (Chilopoda + Progoneata). Amiskwia and Oesia are incertae sedis still but protoconodonts are total group chaetognnaths.
1. Conodonts are vertebrates

2. Uniramia is outdated, pancrustacea is true

3. Euthycarcinoids are stem mandibulates if memory serves right
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Punga: A terraformed world with no vertebrates
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ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

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Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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kusanagi
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Jul 24 2017, 10:44 AM
kusanagi
Jul 24 2017, 09:03 AM
Stem or at least protomorphic hexapods like Leverhulmia are already known and someone else mentioned the euthycarcinoids are in the total group of Hexapoda + (Chilopoda + Progoneata). Amiskwia and Oesia are incertae sedis still but protoconodonts are total group chaetognnaths.
1. Conodonts are vertebrates

2. Uniramia is outdated, pancrustacea is true

3. Euthycarcinoids are stem mandibulates if memory serves right
1. When conodonts are included in cladistic analyses that find them to be vertebrates the phylogeny of the vertebrata is destroyed, and the supposed homologies shared between euconodonts and vertebrates (and even specifically gnathostomes!) are dubious or outright wrong (Turner et al 2010). For example the euconodont degree of encephalisation is too primitive for them to be craniates and their fin rays resemble those of cephalochordates more than those of vertebrates. Much as paleontologists have a history of misidentifying or exaggerating primates and even artiodactyls such as Hesperopithecus as human ancestors so too the similarity of various early Palaeozoic animals to vertebrates or chordates is frequently overstated: euconodonts are at least chordates but Yunnanozoon, Pikaia and vetulicolidans? Quite probably they are not. Yunnanozoon anatomy is consistent with a placement in or near the ambulocrarians, and Pikaia is overall closest to the Emu Shale Myoscolex which Dzik interprets as a swimming annelid possessing rows of rod-like chaetae although Conway Moris still holds out that P. is a divergent stem chordate though he does not compare M. to P. in detail and he stresses fossils of P. do not definitely preserve a notochord. Janvier and Turner et al express doubt elsewhere that P. is a chordate and it is at the very least outside the crown chordates nowadays so it is interesting the way notions like Pikaia as the "first fish" or "our earliest ancestor" have taken root in the sci nerd consciousness - despite the problems with a chordate interpretation and the discovery of less problematic and even earlier vertebrate "ancestors" in the Cambrian. The Burgess Shale yields the total group craniate Metaspriggina and similar craniates are known from the earlier Maotianshan Shales along with a genus closer to the predicted chordate LCA, Cathaymyrus. Actually its a bit astonishing how many people have attempted to recognise soft bodied problematica as chordates or at least chordate ancestors and cousins.

http://www.bioone.org/doi/abs/10.5252/g2010n4a1

2. When fiossil taxa are added a monophyletic crustacea results, and the limited morphological data connecting incects to crustaceans is itself in conflict with molecular data by suggesting a position of hexapods close to the "higher" malacostracans etc instead of branchiopods. Cladistic analyses do still recover Atelocerata/Tracheata/Uniramia though not neccessarily the monophyly of clades myriapoda or hexapoda. I should like to see analysis of the crown arthropods rooted in canadaspidids and Fuxianhuia that includes euthycarcinoids, stem hexapods such as Leverhulmia and the late problematicum Camptophyllia to get a better picture of arthropod colonisation of the land.

?Megacheira + ((Pycogonida + Euchelicerata) + ((Trilobitomorpha + (Aglaspidida + Xenopoda)) + ((Burgessia + (Marrellomorpha + Crustacea)) + ((Ellipura + (Diplura + Insecta)) + Euthycarcinoidea + (Chilopoda + Progoneata))))

What did you mean by aquatic stem arachnids? Early fossil scorpiones like Proscorpius and therefore the arachnid LCA were thught to have been aquatic and OTOH I remember reading the earliest acari were found in marine deposits. But Proscorpius is now reinterpreted as a terrestrial arthropod anyway so the LCA of scorpions and mainline arachnids was presumably terrestrial. It looked like early scorpions and therefore the stemward eurypterids.

https://peerj.com/articles/641/
Edited by kusanagi, Jul 24 2017, 01:32 PM.
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IIGSY
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If euconodonts are chordates but not vertebrates, then how do they have hard, teeth like parts?

And, aren't hexapods widely agreed to be a sister to xenocarida (remipedia plus cephalocarida)?

And, craniata is redundant as hagfish and lampreys are sister taxa in cyclostomata. But are early forms like haikouichthys in the crown group?
Edited by IIGSY, Jul 24 2017, 01:20 PM.
Projects
Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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kusanagi
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Jul 24 2017, 01:15 PM
If euconodonts are chordates but not vertebrates, then how do they have hard, teeth like parts?

And, aren't hexapods widely agreed to be a sister to xenocarida (remipedia plus cephalocarida)?

And, craniata is redundant as hagfish and lampreys are sister taxa in cyclostomata. But are early forms like haikouichthys in the crown group?
Hard teeth-like parts are not unique t ognathostomes and conodont structures are not particularly teet like, nor limited to euconodonta which many people now talk about as though they were the only conodontophores ever. Traditional conodonta was merely a form taxon for fragmentary fossil evidence found in isolation - and such part-organismal data is poor grounds for establishing the morphology or relationships of the whole organism. Protoconodonts are chaetognaths and even the mollusc Odondogriphus seems to have been a westergaardodinid conodontophore. Though euconodonts are at least chordates their "teeth" do not have pulp cavities or enamel. There is no reason to interpret any of the conodont structures as homologous with teeth.

Cyclostomata is an old idea based on developmental data that was rfuted as the progressive loss of cranial elements forsced the reorganisation of skull development in stem gnathostomes. Were cyclostomata monophyletic then even the crownward stem gnathostomes like cephalaspidomorphs would have to be moved out of total group gnathostomes. This is obviously untrue as gnathostomes nest within the cephalaspidomorphs. Hagfish do at least have the genes for vertebrae and both the hagfish and lampreys have the genes neccessary to secrete enamel, but the present state of knowledge is still that hagfish diverged first. As a total group myxinoidea seems to include Palaeospondylus otherwise neither myxinoids nor hyperoartians are definitely pre-Carboniferous. Euphanerops seems to be a stem hyperoartian from the late Devonian and Jamoytius is probably the most stemward of total group gnathostomates which include all of the armoured and scaled agnathans. People like Jarvik talked about a monophyletic cyclostome clade when people still thought caudates might be lungfishes but a lot moved on since then, and it actually is a bit disturbing that many people still treat the molecular evidence as having precedence on this whilst still citing the developmental differences between cyclostome and gnathostome vertebrates as though the evolution of the latter from the former had never been demonstrated by developmental evidence.

https://zoologicalletters.biomedcentral.com/articles/10.1186/s40851-016-0057-0

Talking about Jarvik the polypteriformes have a long ghost lineage: cladistians like Bawitius in the Cretaceous were nonetheless modern enough bichirs. Concensus is that they are at least crownward of Cheirolepis among the actinopterygian fishes but their origins and stem group are a mystery. As late as the 1980s Bjerring cited the development of the bichir skull to refute an actinopterygian affinity and as far as I know he still holds that view. Very famously the polypteriformes possesss such a very rhipidistian-type lung that even from the perspective that the actinopterygian swim bladder is a degenerated lung the similarity of the polypteriform and rhipidistian lung is daunting. Only soft tissue preservation and missing links can clear up questions raised by polypteriforms as late appearing but extremely basal actinopterygians - or as Bjerring suggests, something else. Why do they have paired ventral spines like acanthodians? All round they seem to makes sense in the Palaeozoic but only appear in the Cretaceous strata.

And no, the relationships between the major clades of arthropods are largely not agreed on, I thought you would have known that. Some people now put myriapods as sisters of chelicerates and the monophyly of traditional, holophyletic crustacea, myriapoda and hexapoda has all been questioned even. and that is not even starting on the Palaeozoic fossils. Chelicerata is always regarded monophyletic with the exclusion of pycogonids sometimes, and that might be the only certainty about the living arthropod classes. Morphological evidence at the very least divides crown arthropods into Mandibulata and Chelicerata and thats about as definite as it gets.
Edited by kusanagi, Jul 24 2017, 02:50 PM.
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