Antennipatus


Antennipatus montceauensis is an extinct species of onychophoran, a group colloquially known as velvet worms, from the Montceau-les-Mines lagerstätte of what is now France. The animal is the sole member of its genus and notably the oldest confirmed onychophoran fossil. While known of since the 1980s, Antennipatus was described almost 4 decades later in 2016. Currently, the animal has three described specimens with varying levels of preservation. All of are 305 million years old and date to the Stephanian stage of the Late Carboniferous.
Anatomically, Antennipatus resembles modern velvet worms. The animal had rings of dermal papillae along most of its body, with those on the antennae alternating between wide and narrow bands. On its underside, Antennipatus had a putative ventral mouth that preserved what were likely lip papillae. The animal had stubby legs, and on one specimen, definite slime papillae.
Antennipatus was originally thought to be a member of the genus Helenodora. However, this quickly dismissed after its description. Based on its anatomy, Antennipatus was at least somewhat terrestrial, but due to the way it was preserved, is difficult to place phylogenetically. Antennipatus could either be basal to the velvet worm crown group or located somewhere inside it.

History of discovery and naming

While undescribed at the time, Antennipatus was known of as far back as 1981. A 1982 paper mentioned the animal, saying it was a definite onychophoran due to its well-preserved antennae. The same paper considered it virtually identical to Helenodora from the Mazon Creek [fossil beds]. Later studies from 1988 and 2014 displayed some of the undescribed fossils and documented additional specimens. In 2016, Antennipatus was officially described by Russel J. Garwood, Gregory D. Edgecombe, Sylvain Charbonnier, Dominique Chabard, Daniel Sotty, and Gonzalo Giribet.
Currently, there are three known fossils of Antennipatus: the holotype and two paratypes. All fossils can be divided into a part and counterpart and were found in the Montceau-les-Mines lagerstätte of Central France. All fossils are 305 million years old and date to the Late Stephanian, a stage equivalent to the Kasimovian-Gzhelian boundary of the late Pennsylvanian.
Both the holotype and paratypes were deposited in the Jacques de La Comble Natural History Museum of Autun, but belong to the National [Museum of Natural History, France|National Museum of Natural History] of Paris.
Antennipatus gets its genus name from its well-preserved, onychophoran-like antennae. The species name "montceauensis" was given due to originating in Montceau-les-Mines.

Description

Head

Antennipatus had slender, onychophoran-like antennae. In the holotype these preserved more than 40 annuli in alternating wide and narrow bands. This pattern is also seen living velvet worms, for example, Epiperipatus isthmicola. As of now, not a single specimen of Antennipatus preserves eyes. This could be an artifact of preservation, though alternatively, it could have lacked eyes all together.
Two specimens of Antennipatus preserved what is likely a ventral mouth. This was preserved as a hole that is longer than it is wide, being 3.2 × 1.6 millimeters in the holotype and 1.8 × 1.2 millimeters in MNHN SOT003122. The hole was also surrounded by a circular protrusion, likely the remains of lip papillae. Since neither specimens preserve jaws, the mouth was either closed or filled with sediment when being fossilized.
Behind the antennae and to the sides of the mouth were a pair of slime papillae. These are clearly defined in MNHN SOT003121b, might exist in MNHN SOT006706. The appendages were interpreted as slime papillae based on their anterolateral location, lack of annulation compared to other limbs, and due to being noticeably shorter than the proceeding legs.

Trunk and lobopods

No fossil of Antennipatus preserves the hind part of its anatomy, meaning how many legs it had, the position of its anus, and the position of its genital pad are unknown. Luckily, a few trunk segments managed to be preserved, giving some insight on their structure. Antennipatus's trunk had a minimum of 5 segments, with 8 plicae per segment. The plicae are preserved as rows of papillae separated by horizontal ridges. Like in modern onychophorans, each segment of the trunk possessed a pair of legs known as lobopods. These preserved annulations, having eight to ten per leg. However, the lobopods lacked claws or spinous pads. The trunk also lacked tracheal openings, though this is probably a result of being small and unlikely to preserve outside of amber.

Classification

Before its description, Antennipatus was considered virtually identical to Helenodora and possibly a member of the same genus. However, after being described, both were found to differ in a few key ways. On its trunk, Helenodora has nine plicae per segment while Antennipatus has eight. Antennipatus was also significantly larger, though this may be caused by the specimens being of different ontogenetic ages.
Antennipatus is regarded as the earliest-known onychophoran. However, it is difficult to place phylogenetically due to lacking several informative traits. The jaw blade and genital opening are either hidden or completely absent, preventing even a family-level assignment. Based on a time-calibrated phylogeny, Antennipatus predates the diversification of Peripatopsidae by around 100 million years and is within the error bar of early Peripatidae diversification.
Since it lived in the Northern Hemisphere and had a large body size, Antennipatus could have affinities with peripatids. Yet, as mentioned before, this can not be clearly demonstrated. The animal differs from living Peripatidae in the number of plicae on its trunk. Compared to Antennipatus's 8, most peripatids have 12 but up to 24 plicae per segment. Velvet worms are estimated to have first appeared in the Late Devonian. Combined with the animal's terrestrial lifestyle and placement in time, Antennipatus could easily be a crown-group onychophoran. Nonetheless, later studies treat this with caution. A 2018 paper by Gonzalo et al. viewed Antennipatus a true onychophoran of uncertain placement. It could either be in the crown group or outside of this in the stem group. A 2021 paper by Baker et al. agreed with this, but conservatively treated it as a stem-group representative for the purposes of their study.

Paleobiology

Terrestriality

Based on a variety of evidence, Antennipatus was a terrestrial or at least somewhat terrestrial animal. The best of these is at least one fossil preserves unequivocal slime papillae. Since these appendages can be drawn into the body, their absence fails to indicate that other specimens lacked them. For instance, in Cretoperipatus, the slime papillae were barely distinguishable from the surrounding body, even in one of its best-preserved fossils. These appendages imply terrestriality due to the way they oscillate to shoot slime, something that would not work in an aquatic environment. Other terrestrial features are absent due to being incredibly small. Beyond its anatomy, another pointer towards a terrestrial existence was Antennipatus's depositional environment.

Paleoenvironment

Antennipatus originated in siderite nodules from Montceau-les-Mines lagerstätte. This lagerstätte had a significant terrestrial influence, preserving numerous plants and animals that were purely terrestrial. Some of these include stem-spiders like Idmonarachne, giant Myriapods like Arthropleura, and the tree-sized horsetail Calamites. The ecosystem also supported a variety of aquatic life, including Syncarid crustaceans, lungfish, Alanops, as well as the amphibious Temnospondyls and Nectrideans. Montecau was likely a freshwater depositional environment, having no structural, sedimentological, or paleogeographic evidence of marine influence. However, this has come under scrutiny. The locality preserves a few marine-associated organisms and was not sampled in the isotopic analysis of nearby localities. Because of this, it was probably closer to the Paleotethys Ocean than originally thought.