User:RenaMoonn/Workspace 2

General body plan

Slime glands

Appendages

Nervous system

Nervous System

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Circulation

Respiration

Digestion and excretion

Distribution

In the modern day, velvet worms are found in the tropics and in the temperate zone of the Southern Hemisphere. Members of the family Peripatidae are found the tropical regions of South America, Central America, the Caribbean islands, Gabon, Northeast India, and Southeast Asia. Meanwhile, members of Peripatopsidae are found Chile, Australia, Southern Africa, New Guinea, and New Zealand.^[2] One deviation is the South African Peripatopsis capensis. This animal inadvertently introduced to Santa Cruz Island in the Galapagos and co-occurs with native species.^[3] When looking at velvet worms as a whole, the majority of are found in Australia and South America.^[2]

Extinct onychophorans have been discovered in parts of the Northern Hemisphere that are currently uninhabited by the phylum.^[4][5][6] While the onychophoran affinities of Succinipatopsis and Helenodora of are questioned,^[7][8] others like the Antennipatus are at a minimum close relatives of crown-group onychophora (Peripatidae and Peripatopsidae).^[8][9][10] This indicates that velvet worms were far more widespread in the past but subsequently died off for unknown reasons.^[11]

Habitat

Locomotion

Diet

Sociality

Growth and development

Reproduction

Internal phylogeny

Living velvet worms are divided into two families: Peripatidae and Peripatopsidae. These diverged around 274 million years ago during the Late Devonian and have since diversified.^[12] Within Peripatidae, the genera Eoperipatus (found in Southeast Asia) and Mesoperipatus (found in Gabon) were the most basal, while the rest of the group is found in tropical regions of the Americas.^[12] On the other hand, Peritpatopsidae can be divided into two main clades. One has members in Southern Africa and Chile, while the others live in Australasia.^[13][12]

Below is a genus-level cladogram of velvet worms. Note that this phylogeny doesn't analyze every species and lumps most peripatids into Neopatida, as many Peripatidae genera are paraphyletic.^[14] As of 2023, there are around 232 total living species, meaning this phylogeny should eventually be updated.^[15]

Crown Onychophora Family Tree

All of confirmed Onychophora family tree

If Succinipatopsis is a basal holdover without slime papillae

External phylogeny

Basal panarthropod

Tardigrades closest

Arthropods closest

Genomics

As of February 2025, velvet worms have had only two nuclear genomes sequenced.^[16][17] These are of Euperipatoides rowelli (a peripatopsid) and Epiperipatus broadwayi (a peripatid). The first one is highly fragmented, while the second is less so, but still needs improvement.^[18] Velvet worms seem to display genome gigantism, with the more complete assembly (E. broadwayi) having an size of 5.60 giga-base pairs.^[18] Around 70.92% of its genome are repeat sequences, something that contributes to the bulk of it's size. While less substantial, it also has very large introns, or parts of a gene that don't become proteins.^[18]

Insights on decay and fossilization

Due to being soft-bodied, onychophorans need excellent conditions to 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 solutions.^[19] 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.^[19]

The researchers found that different features decayed at significantly different rates.^[19] Salinity and time of moult had little effect on decay, and the way things decomposed remained the same for different species (though it could happen at different speeds).^[19] 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.^[19] 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 limbs do the same, increasing in length and width by around 10–25%.^[19] Around the same time, the internal organs begin to degrade. This eventually culminates in the gut rupturing, destroying the other organs.^[19] 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.^[19] It's around this time that many external features begin to deteriorate. These include the dermal papillae, leg rings, anus, gonopore, antenna, slime papillae, and eventually eyes. Interestingly, the dermal papillae on the trunk disappear faster than those on the limbs.^[19] 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 chitinous jaws and claws.^[19]

Onychophorans are unlikely to experience any stemward slippage since their defining features (jaws, feet, and slime papillae) are generally decay-resistant.^[19] 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 limbs, so this needs to be accounted for when analyzing locomotion and/or leg length.^[19] While fossilized onychophorans can appear to display patterning, these aren't true to life. pigment granules are one of the first things to degrade and can easily move around in the body.^[19] Another finding is that characters such as internal organs or the body cavity are highly unlikely to fossilize. If these appear preserved in a fossil, they should be treated with skepticism, especially if they’re unmineralized.^[19] 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.^[19] Additionally, 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 eyes but no claws likely never had them.^[19]

Emergence from Lobopods

Certain fossils from the Early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as lobopodians, were an evolutionary grade that gave rise to arthropods, tardigrades, onychophorans, and the extinct radiodonts.^[20]

How different lobopodians are related varies from study to study. However, with the exception of a single paper,^[21] Antennacanthopodia is the only animal to be confidently viewed as a close onychophoran relative.^{[20][22][23][24][25]} Antennacanthopodia lived during the Cambrian Stage 3 and possessed a variety of onychophoran traits: stubby lobopods, spinous foot pads, an annulated body, and small eyes behind the main antennae. An obvious difference from onychophorans was that the animal had not one, but two pairs of antennae.^[26] These secondary antennae were shorter than the first pair and thought to be homologous with slime papillae^[26] or jaws of velvet worms.^[27] When Onychophora first arose or moved onto land is currently unknown. However, it could have plausibly happened between the Ordovician and Silurian – approximately 490 to 430 million years ago – via the intertidal zone.^[28]

Velvet worms and their diversification

The earliest potential onychophoran, Helenodora, originates from the Mid Pennsylvanian Mazon Creek lagerstätte in the United States. Originally described in 1980,^[29] Helenodora was long regarded as the earliest known onychophoran.^{[30][31][20][22]} This changed in 2016 when a paper by Murdock et al. doubted its affinity. Based on the absence of many decay-resistant features (claws, jaws, slime papillae) and unclear ecology (Carbotubulus, a late-surviving aquatic lobopodian, was found in the same formation), the authors of this study placed Helenodora as a basal lobopodian most closely related to Paucipodia.^[32]

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Later studies contested some of these findings, (claws and jaws still there), but agreed on its uncertainty of being terrestrial or aquatic.

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The earliest definite onychophoran is the French Antennipatus from a Stephanian (Late Pennsylvanian^[33]) Lagerstätte in Montceau-les-Mines. This animal had a ventral (bottom facing) mouth, rings of dermal papillae on its trunk and limbs, and was at least somewhat terrestrial because of its slime papillae.^[34] However, due to the way it was preserved, it is unknown if Antennipatus belonged to the stem or crown group of the two living families of velvet worm (Peripatidae and Peripatopsidae).^[34][35][36]

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.^[37] The earliest known crown-group onychophoran, Cretoperipatus, is known from multiple specimens in Burmese amber.^[31][38][35] 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).^[35] Based on its morphology, Cretoperipatus was early-diverging member of Peripatidae, most closely related to the Asian genera Eoperipatus and Typhloperipatus.^[38]

Despite being preserved in amber, the affinities of the Cenozoic Tertiapatus and Succinipatopsis are surrounded in controversy. Some sources consider them to be onychophorans,^[30][38] while others dismiss this.^[34] Ultimately, these animals should be reanalyzed to better grasp their position on the tree of life.^[38]