Editing Onychophora (section)

== Evolution ==

=== Insights on decay and fossilization ===
[[File:Onychophora Decay Timeline.png|left|thumb|400x400px|Feature decay timeline. Yellow means "pristine", orange means "decaying", red means "some or all lost"]]
Due to being soft-bodied, onychophorans need excellent conditions to [[Fossil|fossilize]]. However, even when this happens, their fossils can be subject to [[taphonomic bias]]. Experiments were done with modern velvet worms to analyze their decomposition in various [[Saline water|saline solutions]].<ref name=":5">{{Cite journal |last1=Murdock |first1=Duncan JE |last2=Gabbott |first2=Sarah E. |last3=Mayer |first3=Georg |last4=Purnell |first4=Mark A. |date=2014-11-29 |title=Decay of velvet worms (Onychophora), and bias in the fossil record of lobopodians |journal=BMC Evolutionary Biology |volume=14 |issue=1 |pages=222 |doi=10.1186/s12862-014-0222-z |doi-access=free |issn=1471-2148 |pmc=4266977 |pmid=25472836|bibcode=2014BMCEE..14..222M }}</ref> The study also investigated whether they experienced something called [[stemward slippage]]. In this phenomenon, animals are falsely categorized as more primitive due to the decay of certain features.<ref name=":5" />

The researchers found that different features [[Decomposition|decayed]] at significantly different rates.<ref name=":5" /> [[Salinity]] and time of [[Moulting|moult]] had little effect on decay, and the way things decomposed remained the same for different species (though it could happen at different speeds).<ref name=":5" /> Before any degradation, velvet worms flex into a S, U, or circular shape. Most flexing happens in the first 24 hours, but the process can continue for around two more days.<ref name=":5" /> In the early stages of decay, the [[epidermis]] and outer cuticle separate, causing a bloated appearance. The elongates while increasing in width by around 10–30%. The [[Limb (anatomy)|limbs]] do the same, increasing in length and width by around 10–25%.<ref name=":5" /> Around the same time, the internal organs begin to degrade. This eventually culminates in the [[Gastrointestinal tract|gut]] rupturing, destroying the other organs.<ref name=":5" /> In later stages of decay, the body cuticle shrinks close to its original size. A similar trend was found with the limbs, but it was just short of being statistically significant.<ref name=":5" /> It's around this time that many external features begin to deteriorate. These include the [[Dermal papillae (Onychophora)|dermal papillae]], leg rings, [[anus]], [[gonopore]], [[Antenna (zoology)|antenna]], slime papillae, and eventually [[Eye|eyes]]. Interestingly, the dermal papillae on the trunk disappear faster than those on the limbs.<ref name=":5" /> Even as decay progresses, the body is still recognizable. This stops once the outer cuticle finally ruptures. After that, the animal’s anatomy is extremely difficult to interpret. At this stage, the only identifiable features would be the [[Chitin|chitinous]] jaws and claws.<ref name=":5" />

Onychophorans are unlikely to experience any [[stemward slippage]] since their defining features (jaws, feet, and slime papillae) are generally decay-resistant.<ref name=":5" /> However, [[decomposition]] has a significant impact on fossil anatomy. For starters, the preserved body outline is probably somewhat inaccurate, as this gets bloated in the decay process. A similar thing happens with the [[Limb (anatomy)|limbs]], so this needs to be accounted for when analyzing locomotion and/or leg length.<ref name=":5" /> While fossilized onychophorans can appear to display [[Patterns in nature|patterning]], these aren't true to life. [[Biological pigment|pigment granules]] are one of the first things to degrade and can easily move around in the body.<ref name=":5" /> Another finding is that characters such as [[Internal organ|internal organs]] or the [[Pseudocoelom|body cavity]] are highly unlikely to fossilize. If these appear preserved in a fossil, they should be treated with skepticism, especially if they’re [[Mineralization (biology)|unmineralized]].<ref name=":5" /> When it comes to placement of the [[mouth]], even moderate decay makes it hard to tell if it's at the front or underside of the head.<ref name=":5" /> Additionally, [[Fossil|fossils]] that lack decay-resistant features probably lacked them in life. This is even more plausible if the fossil preserves decay-prone features, as their presence indicates a better level of preservation. For example, an onychophoran (or related animal) with [[Eye|eyes]] but no claws likely never had them.<ref name=":5" />

=== Emergence from lobopodians ===
[[File:Antennacanthopodia.jpg|left|thumb|300x300px|''[[Antennacanthopodia]]'', a close relative of onychophorans]]
Certain [[Fossil|fossils]] from the [[Early Cambrian]] bear a striking resemblance to the velvet worms. These fossils, known collectively as [[lobopodian]]s, were an [[evolutionary grade]] that gave rise to [[Arthropod|arthropods]], [[Tardigrade|tardigrades]], onychophorans, and the extinct [[Radiodonta|radiodonts]].<ref name="Smith-OrtegaHz-2014">{{cite journal | last1 = Smith | first1 = M.R. | last2 = Ortega Hernández | first2 = J. | year = 2014 | title = ''Hallucigenia''<nowiki/>'s onychophoran-like claws and the case for Tactopoda | journal = Nature | volume = 514 | issue = 7522 | pages = 363–366 | doi = 10.1038/nature13576 | pmid = 25132546 | bibcode = 2014Natur.514..363S | s2cid = 205239797 | url = http://rdcu.be/bKoD }}</ref>

How different lobopodians are related varies from study to study. However, with the exception of a single paper,<ref>{{Cite journal |last=Aria |first=Cédric |last2=and Caron |first2=Jean-Bernard |date=2024-12-31 |title=Deep origin of articulation strategies in panarthropods: evidence from a new luolishaniid lobopodian (Panarthropoda) from the Tulip Beds, Burgess Shale |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2024.2356090 |journal=Journal of Systematic Palaeontology |volume=22 |issue=1 |pages=2356090 |doi=10.1080/14772019.2024.2356090 |issn=1477-2019}}</ref> ''[[Antennacanthopodia]]'' is the only animal to be confidently viewed as a close onychophoran relative.<ref name="Smith-OrtegaHz-2014" /><ref name=":8">{{Cite journal |last=Yang |first=Jie |last2=Ortega-Hernández |first2=Javier |last3=Gerber |first3=Sylvain |last4=Butterfield |first4=Nicholas J. |last5=Hou |first5=Jin-bo |last6=Lan |first6=Tian |last7=Zhang |first7=Xi-guang |date=2015-07-14 |title=A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora |url=https://www.pnas.org/doi/10.1073/pnas.1505596112 |journal=Proceedings of the National Academy of Sciences |volume=112 |issue=28 |pages=8678–8683 |doi=10.1073/pnas.1505596112 |pmc=4507230 |pmid=26124122}}</ref><ref>{{Cite journal |last1=Zhang |first1=Xi-Guang |last2=Smith |first2=Martin R. |last3=Yang |first3=Jie |last4=Hou |first4=Jin-Bo |date=2016-09-01 |title=Onychophoran-like musculature in a phosphatized Cambrian lobopodian |journal=Biology Letters |volume=12 |issue=9 |pages=20160492 |doi=10.1098/rsbl.2016.0492 |pmc=5046927 |pmid=27677816}}</ref><ref name=":13">{{Cite journal |last1=Murdock |first1=Duncan J. E. |last2=Gabbott |first2=Sarah E. |last3=Purnell |first3=Mark A. |date=2016-01-22 |title=The impact of taphonomic data on phylogenetic resolution: Helenodora inopinata (Carboniferous, Mazon Creek Lagerstätte) and the onychophoran stem lineage |url=https://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-016-0582-7 |journal=BMC Evolutionary Biology |volume=16 |issue=1 |pages=19 |bibcode=2016BMCEE..16...19M |doi=10.1186/s12862-016-0582-7 |doi-broken-date=17 March 2025 |issn=1471-2148 |pmc=4722706 |pmid=26801389 |doi-access=free}}</ref><ref name=":04">{{Cite journal |last1=Caron |first1=Jean-Bernard |last2=Aria |first2=Cédric |date=2017-01-31 |title=Cambrian suspension-feeding lobopodians and the early radiation of panarthropods |journal=BMC Evolutionary Biology |volume=17 |issue=1 |pages=29 |bibcode=2017BMCEE..17...29C |doi=10.1186/s12862-016-0858-y |issn=1471-2148 |pmc=5282736 |pmid=28137244 |doi-access=free}}</ref> ''Antennacanthopodia'' lived during the [[Cambrian Stage 3]] and possessed a variety of onychophoran traits: stubby lobopods, spinous foot pads, an annulated body, and small [[Eye|eyes]] behind the main [[Antenna (zoology)|antennae]]. An obvious difference from onychophorans was that the animal had not one, but two pairs of antennae.<ref name="Ou">{{cite journal |author1=Qiang Ou |author2=Jianni Liu |author3=Degan Shu |author4=Jian Han |author5=Zhifei Zhang |author6=Xiaoqiao Wan |author7=Qianping Lei |year=2011 |title=A rare onychophoran-like lobopodian from the Lower Cambrian Chengjiang lagerstätte, southwestern China, and its phylogenetic implications |journal=[[Journal of Paleontology]] |volume=85 |issue=3 |pages=587–594 |bibcode=2011JPal...85..587O |doi=10.1666/09-147R2.1 |s2cid=53056128}}</ref> These secondary antennae were shorter than the first pair and thought to be homologous with either the slime papillae<ref name="Ou" /> or jaws of velvet worms.<ref name=":22">{{Cite journal |last1=Ortega-Hernández |first1=Javier |last2=Janssen |first2=Ralf |last3=Budd |first3=Graham E. |date=2017-05-01 |title=Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective |url=https://www.sciencedirect.com/science/article/abs/pii/S1467803916301669 |journal=Arthropod Structure & Development |series=Evolution of Segmentation |volume=46 |issue=3 |pages=354–379 |bibcode=2017ArtSD..46..354O |doi=10.1016/j.asd.2016.10.011 |issn=1467-8039 |pmid=27989966}}</ref> When Onychophora first arose or moved onto land is currently unknown. However, it could have plausibly happened between the [[Ordovician]] and [[Silurian]] – approximately {{Ma|Ordovician|Late Silurian|round=-1}} – via the [[intertidal zone]].<ref name="Monge1995" />

=== Velvet worms and their diversification ===
[[File:Helenodora.jpg|thumb|Reconstruction of the Carboniferous possible onychophoran ''[[Helenodora]]''|350x350px]]

The earliest potential onychophoran, ''[[Helenodora]]'', originates from the [[Middle Pennsylvanian|Mid Pennsylvanian]] [[Mazon Creek fossil beds|Mazon Creek lagerstätte]] in the [[United States]]. Originally described in 1980,<ref>{{Cite journal |last=Thompson |first=Ida |last2=Jones |first2=Douglas S. |date=1980 |title=A Possible Onychophoran from the Middle Pennsylvanian Mazon Creek Beds of Northern Illinois |url=https://www.jstor.org/stable/1304204 |journal=Journal of Paleontology |volume=54 |issue=3 |pages=588–596 |issn=0022-3360}}</ref> Helenodora was long regarded as the earliest known onychophoran.<ref name="Jr2000">{{cite journal |last=Poinar |first=George |date=Winter 2000 |title=Fossil onychophorans from Dominican and Baltic amber: ''Tertiapatus dominicanus'' n.g., n.sp. (Tertiapatidae n. fam.) and ''Succinipatopsis balticus'' n.g., n.sp. (Succinipatopsidae n. fam.) with a Proposed Classification of the Subphylum Onychophora |journal=Invertebrate Biology |volume=119 |issue=1 |pages=104–109 |bibcode=2000InvBi.119..104P |doi=10.1111/j.1744-7410.2000.tb00178.x |jstor=3227105 |doi-access=free}}</ref>'''<ref name="G02" />'''<ref name="Smith-OrtegaHz-2014" /><ref name=":8" /> This changed in 2016 when a paper by Murdock et al. doubted its affinity. Based on the absence of many [[Decomposition|decay]]-resistant features ([[Claw|claws]], jaws, slime papillae) and unclear [[ecology]] (''[[Carbotubulus]]'', a late-surviving [[Aquatic animal|aquatic]] [[Lobopodia|lobopodian]], was found in the same [[Geological formation|formation]]), the authors of this study placed ''Helenodora'' as a basal lobopodian most closely related to ''[[Paucipodia]]''.<ref name=":07">{{cite journal |last1=Murdock |first1=D. J. E. |last2=Gabbott |first2=S. E. |last3=Purnell |first3=M. A. |date=2016 |title=The impact of taphonomic data on phylogenetic resolution: ''Helenodora inopinata'' (Carboniferous, Mazon Creek Lagerstätte) and the onychophoran stem lineage |journal=BMC Evolutionary Biology |volume=16 |issue=19 |pages=19 |bibcode=2016BMCEE..16...19M |doi=10.1186/s12862-016-0582-7 |pmc=4722706 |pmid=26801389 |doi-access=free}}</ref> 

The earliest definite onychophoran is the [[France|French]] ''[[Antennipatus]]'' from a [[Stephanian (stage)|Stephanian]] ([[Late Pennsylvanian]]<ref name=":06">{{Cite journal |last=Knight |first=John A. |last2=Cleal |first2=Christopher J. |last3=Álvarez-Vázquez |first3=Carmen |date=2023-06-14 |title=The challenge of relating the Kasimovian to west European chronostratigraphy: a critical review of the Cantabrian and Barruelian substages of the Stephanian Stage |url=https://www.lyellcollection.org/doi/10.1144/SP535-2022-189 |journal=Geological Society, London, Special Publications |language=en |volume=535 |issue=1 |pages=31–71 |doi=10.1144/SP535-2022-189 |issn=0305-8719 |doi-access=free}}</ref>) [[Lagerstätte]] in [[Montceau-les-Mines]].<ref name="GarwoodEdgecombe2016" /> This animal had a [[ventral]] (bottom facing) [[mouth]], rings of [[Dermal papillae (Onychophora)|dermal papillae]] on its trunk and [[Appendage|limbs]], and was at least somewhat [[Terrestrial animal|terrestrial]] because of its slime papillae.<ref name="GarwoodEdgecombe2016" /> However, due to the way it was preserved, it is unknown if ''Antennipatus'' belonged to the [[Stem-group|stem]] or [[crown group]] of the two living [[Family (biology)|families]] of velvet worm ([[Peripatidae]] and [[Peripatopsidae]]).<ref name="GarwoodEdgecombe2016">{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |last3=Charbonnier |first3=Sylvain |last4=Chabard |first4=Dominique |last5=Sotty |first5=Daniel |last6=Giribet |first6=Gonzalo |year=2016 |title=Carboniferous Onychophora from Montceau-les-Mines, France, and onychophoran terrestrialization |journal=Invertebrate Biology |volume=135 |issue=3 |pages=179–190 |issn=1077-8306 |pmid=27708504 |pmc=5042098 |doi=10.1111/ivb.12130 |doi-access=free }}</ref><ref name=":033">{{Cite journal |last=Giribet |first=Gonzalo |last2=Buckman-Young |first2=Rebecca S. |last3=Costa |first3=Cristiano Sampaio |last4=Baker |first4=Caitlin M. |last5=Benavides |first5=Ligia R. |last6=Branstetter |first6=Michael G. |last7=Daniels |first7=Savel R. |last8=Pinto-da-Rocha |first8=Ricardo |date=2018 |title=The ‘Peripatos’ in Eurogondwana? – Lack of evidence that south-east Asian onychophorans walked through Europe |url=https://bioone.org/journals/Invertebrate-Systematics/volume-32/issue-4/IS18007/The-iPeripatos-i-in-Eurogondwana--Lack-of-evidence-that/10.1071/IS18007.full |journal=Invertebrate Systematics |volume=32 |issue=4 |pages=840–863 |doi=10.1071/IS18007}}</ref><ref name=":7">{{Cite journal |last=Baker |first=Caitlin M |last2=Buckman-Young |first2=Rebecca S |last3=Costa |first3=Cristiano S |last4=Giribet |first4=Gonzalo |date=2021-12-01 |title=Phylogenomic Analysis of Velvet Worms (Onychophora) Uncovers an Evolutionary Radiation in the Neotropics |url=https://academic.oup.com/mbe/article/38/12/5391/6357048?login=true |journal=Molecular Biology and Evolution |volume=38 |issue=12 |pages=5391–5404 |doi=10.1093/molbev/msab251 |issn=1537-1719|pmc=8662635 }}</ref>

Based on [[molecular dating]], crown-group onychophorans diverged around 376 million years ago in the [[Late Devonian]]. This estimation happens regardless if ''[[Antennipatus]]'' is used to constrain the divergence date, or if no calibration is done at all.<ref name=":7" /> The earliest known [[Crown group|crown-group]] onychophoran, ''[[Cretoperipatus]]'', is known from multiple specimens in [[Burmese amber]].'''<ref name="G02">{{cite journal |last1=Grimaldi |first1=David A. |last2=Engel |first2=Michael S. |last3=Nascimbene |first3=Paul C. |date=March 2002 |title=Fossiliferous Cretaceous Amber from Myanmar (Burma): Its Rediscovery, Biotic Diversity, and Paleontological Significance |url=https://www.biodiversitylibrary.org/bibliography/178519 |journal=American Museum Novitates |issue=3361 |pages=1–71 |doi=10.1206/0003-0082(2002)361<0001:FCAFMB>2.0.CO;2 |s2cid=53645124 |hdl=2246/2914}}</ref><ref name="Oliveira">{{cite journal |last1=Oliveira |first1=I. S. |last2=Bai |first2=M |last3=Jahn |first3=H |last4=Gross |first4=V |last5=Martin |first5=C |last6=Hammel |first6=J. U. |last7=Zhang |first7=W |last8=Mayer |first8=G |date=2016 |title=Earliest Onychophoran in Amber Reveals Gondwanan Migration Patterns |journal=Current Biology |volume=26 |issue=19 |pages=2594–2601 |bibcode=2016CBio...26.2594O |doi=10.1016/j.cub.2016.07.023 |pmid=27693140 |doi-access=free}}</ref>'''<ref name=":033" /> All are from the same general location and date to a maximum age of around 98.79 million years ago (the earliest [[Cenomanian]] of the [[Late Cretaceous]]).<ref name=":033" /> Based on its [[Morphology (biology)|morphology]], ''Cretoperipatus'' was early-diverging member of [[Peripatidae]], most closely related to the [[Asia|Asian]] genera ''[[Eoperipatus]]'' and ''[[Typhloperipatus]]''.'''<ref name="Oliveira" />''' 

Despite being preserved in [[amber]], the affinities of the [[Cenozoic]] ''[[Tertiapatus]]'' and ''[[Succinipatopsis]]'' are surrounded in controversy. Some sources consider them to be onychophorans,<ref name="Jr2000" />'''<ref name="Oliveira" />''' while others dismiss this.<ref name="GarwoodEdgecombe2016" /> Ultimately, these animals should be reanalyzed to better grasp their position on the [[Tree of life (biology)|tree of life]].'''<ref name="Oliveira" />'''