Helenodora

Helenodora Temporal range: Pennsylvanian ~
Helenodora holotype FMNH PE 29049
Scientific classification
Kingdom:	Animalia
Stem group:	Onychophora
Genus:	†Helenodora Thompson and Jones, 1980
Species:	†H. inopinata
Binomial name
†Helenodora inopinata Thompson and Jones, 1980

Helenodora is an extinct basal onychophoran or lobopodian genus known from the Carboniferous Carbondale Formation of Illinois. The only known species described is H. inopinata. The ecology of this animal is not well known, but it is thought that it may have lived on land and/or underwater.

Discovery and naming

At the moment, Helenodora is known from ten specimens found in the Middle Pennsylvanian, Francis Creek Shale of Illinois. All were found in siderite concretions from Mazon Creek fossil beds, a lagerstätte that contains animals like the famous "Tully Monster".

The first specimens found (the holotype FMNH PE 29049 and paratype FMNH PE 29050) were described during the 80's and are composed of both a part and counterpart. These were deposited in the invertebrate fossil collection of the Field Museum of Natural History of Chicago, Illinois. Other specimens include FMNH PE 13966, 33380, 33822, 45049, and 49784 and FMNH ROM 47513, 45565, and 47978.

Helenodora was suggested by some authors to be a junior synonym of Ilyodes Scudder 1890, a fossil genus that includes two species and was originally described as a myriapod. A re-examination of the type material of Helenodora inopinata and the Ilyodes species found significant differences between them. Additionally, neither species of Ilyodes were adequately diagnosed, so they and the genus are all considered nomina dubia.

The genus name "Helenodora" is Greek for "Helen’s gift". This name honors Mrs. Helen Piecko of Chicago, Illinois for finding both the holotype and paratype. The species name "inopinata" is Latin and means "unexpected".

Description

Antennae

FMNH PE13966 specimen from Murdock et al. showing noticeable antennae

In its initial description, researchers were unsure about Helenodora's overall orientation. However, because one region tapered more than the other, contained a small dark patch (originally interpreted as jaws), and had appendages more reminiscent of antenna, it was tentatively labeled as the front. Later research confirmed this hypothesis, with multiple specimens possessing well-defined antennae on the more tapered region of their body. In newer specimens, the antenna are most notable in FMNH PE13966 (both the part and counterpart) but are also present in ROM 47978. In FMNH PE13966 where the antennae are complete, the appendages are 4-5 mm long and 1.2 mm wide at base.

Dubious jaws

Holotype specimen showing a dark hook-like patch originally viewed as a jaw

When first described, researchers noticed both the Helenodora holotype and paratype preserved a small dark patch at their front ends. These were assumed to be since they were found in roughly the same position as those in modern onychophorans. The patch in the holotype was noted to have some lobe-like relief, while the one in the paratype was smaller and pyritized.

Later research disputed this interpretation. In their reanalysis of the holotype, Murdock et al. found that despite what was originally claimed, the holotype patch was flat and showed no relief. They also found that it was chemically indistinguishable from other dark patches, only composed of siderite and some framboidal pyrite. Notably, this patch lacked any traces of a carbon film, something expected of preserved jaws.

A similar thing was found with the paratype’s jaw patch. Matching the original description, the paratype patch was composed of pyrite and had an indistinct shape. However, like the holotype patch, it was indistinguishable from other patches found on the fossil or outside its margins. Additionally, the paratype patch was found to exist between the second and third pairs of appendages. This is behind where the jaws are found in modern onychophorans. Combined with the lack of jaw patches in other specimens (including ones with a complete front), Murdock concluded Helenodora lacked jaws altogether.

Potential slime papillae

Helenodora specimens FMNH PE13966 (A and B), FMNH 33380 (C and D), FMNH PE33822 (E and F), FMNH PE45049 (g), and FMNH PE49784 (h)

2012 Carbotubulus study

• Had paired lateral extensions resembling retracted slime papillae (used Phallocephale for comparison)

2016 Murdock study

• Helenodora interpreted to lack jaws and slime papillae
• No consistent evidence that appendage pair two is differentiated from following appendages or that the animal had slime papillae
• From Carbotubulus description, ROM 47978 had round swelling on one side of head behind antennae which was interpreted as evidence of slime papillae
• However, specimens with a complete head (list specimens) lack this, making it more likely that the bulge was caused by the decay process

Trunk

Helenodora had a long trunk (body) with multiple annulations (rings of skin) on its cuticle. Around 9 of these were present on each segment, though they were sometimes obscured by the animal's legs. In complete or near complete specimens, the trunk ranged in length from 45 to 66 millimeters long and 6 to 13 millimeters wide. Additionally, it had a short rounded extension that continued past the last leg pair.

Helenodora was originally described as having dermal papillae, though later authors were more uncertain. When structures resembling them were preserved, they weren't always arranged in rows, and in certain specimens (FMNH PE13966), they could be found outside of annulations or even the body outline. Helenodora possessing dermal papillae wasn't fully rejected, but they were removed as a diagnostic feature of the animal.

Lobopods

Outdated artist's reconstruction of Helenodora inopinata.

Anatomical reconstruction by Murdock et al. based on newer fossil material

While originally thought to have more or less, Helenodora is now known to possess 20 pairs of legs known as lobopods. In complete specimens with a differentiated front and back, this number is certain. These lobopods are preserved as stubby triangles that taper off towards the end. They had a mean length of 1.7 millimeters, and a mean width at their base of 2.1 millimeters. Other than a single leg on the paratype (appendage 11), none of Helenodora’s lobopods preserve any part of the cuticle. However, even where it exists, the cuticle is poorly preserved. Because of this, whether the lobopods had any annulations or limb ornaments is unknown.

Potential claws

Potential claws on the Helenodora paratype (FMNH PE 29050)

When first described by Thomson and Jones, the paratype (FMNH PE 29050) was interpreted to preserve seven pairs of claws on the tips of its lobopods. One of these was well-defined, while the rest less so. The claws were preserved as small, long dark patches composed of a thin film of carbon and framboidal pyrite. All were on the left row of the body and found on either side of the fossil concretion.

However, a 2016 redescription by Murdock et al. argued that Helenodora’s claws were a taphonomic artifact. In this study, claw-like patches were found throughout the paratype as well as in multiple newer specimens. These were found in areas inconsistent with claws, sometimes outside the fossil boundary. Based on this information, the claws on the paratype were likely a coincidence, causing Murdock to conclude that the animal lacked them in life. This was further supported by taphonomic evidence, as in modern velvet worms, claws are composed of sclerotized (hardened) chitin and are a highly decay-resistant feature.

Barely a year later, Helenodora’s lack of claws was contested in a study by Grimaldi et al. This paper redescribed Cretoperipatus, a velvet worm preserved in amber, based on three new specimens. The authors found that in one specimen, many of the claws had detached and were dispersed throughout the amber matrix. In modern velvet worms, the foot begins decaying soon after death. Because the claws are only connected by a fragile membrane of tissue, decay might cause them to detach. This can reach the point where the animal lacks feet and claws altogether. With this in mind, the small dark patches in Helenodora could be claws that separated before or during fossilization.

Classification

1980 study

• Was originally classified into the phylum Uniramia (a now obsolete group) and tentatively placed in Onychophora
• Excluded placement as a polychaete or tardigrade
• Can't be firmly placed within onychophora due to uncertainties about the head
• May have had a jaw and ventral mouth, but the dark patch that could be a jaw are poorly preserved
• Front appendages not well preserved enough to determine if these are antennae and oral papillae
• Reconstruction assumes helenodora was an onychophoran (first apendages of paratype assumed to be bases of antennae, second appendages are assumed to be slime papillae)
• Recognize that alternative interpretations are possible
• Mentioned that onychophora was a grade of animals
• May be a descendant of a Aysheaia, Onychophora, and Tardigrade clade
• Not sure if its in the onychophoran grade

2000 poinar study

• Placed in a variety of new taxa alongside the animals Tertiapatus and Succinipatopsis to distinguish them from living velvet worms
• Placed in Class Udeonychophora (terrestrial members with ventral mouth)
• Placed in order Ontonychophora, a group defined by their simple lobopods without  annulations of dermal papillae, variability of its members having claws, and lack of a specialized terminal foot
• Put in its own superfamily Helenodoroidea and family Helenodoridae
• Under this classification, simple lobopod persisted in Helenodora, Tertiapatus, and Succinipatopsis, but developed a foot portion in the lineage that gave rise to modern onychophorans

Onychophora Xenusia Paleozoic Lobopodians Udeonychophora Euonychophora Peripatidae Peripatopsidae Ontonychophora Tertiapatoidea Tertiapatidae Tertiapatus Succinipatopsidae Succinipatopsis Helenodoroidea Helenodoridae Helenodora

Outdated classification of Onychophora based on Poinar (2000)

2002 Cretoperipatus study

• Feet in claws not included in original description of Helenodora, but this may be an artifact of preservation

2016 Murdock study

• Helenodora generally regarded as a stem onychophoran more closely related to living onychophorans than cambrian lobopodians
• Placement is uncertain due to poor preservation of the head
• Huag et al described some new head details when looking at carbotubulus
• New helenodora fossils found allows for a full redescription
• Incorporated onychophoran decay into taphonomic character analysis
• Helenodora lacked jaws, claws, and slime papillae
• very basal lobopodian

Paleoenvironment

1980 study

• Lived alongside the tully monster
• Habitat of helenodora is unknown
• Mazon creek fossil beds had a mixed depositional environment (marine, freshwater, and terrestrial organisms)
• Pit 11 (where helenodora was found) contains mostly marine animals, though the occasional terrestrial animal can still be found (along with many land plants)
• Morphology of helenodra may show adaptations to terrestrial life, though this is only speculation
• Ex: shorter legs of helenodora and Peripatoides might be adaptations to terrestrial locomotion
• Ex: Reduced number of claws could reflect how terrestrial substrate is easier to grip than an aquatic one
• Ex: Slimmer body and legs could help better facilitate gas exchange

Gallery of fossils

Below are full-resolution images of various Helenodora fossils. For many of these, RGB values have been adjusted in certain parts of the fossil to better show anatomical details:

• 
  FMNH PE 29050 (paratype) part and counterpart
• 
  FMNH PE 29049 (holotype) part
• 
  FMNH PE 29049 (holotype) counterpart
• 
  FMNH PE 29050 (paratype) part
• 
  FMNH PE 29050 (paratype) counterpart
• 
  FMNH PE13966 part
• 
  FMNH PE13966 counterpart
• 
  FMNH 33380 part
• 
  FMNH 33380 counterpart
• 
  FMNH PE33822 part
• 
  FMNH PE33822 counterpart
• 
  FMNH PE45049
• 
  FMNH PE49784

External links

• Data related to Helenodora at Wikispecies