Helenodora

Helenodora Temporal range: Pennsylvanian ~311–307  Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Helenodora holotype FMNH PE 29049
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Stem group:	Onychophora
Genus:	† Helenodora Thompson and Jones, 1980
Species:	†H. inopinata
Binomial name
†Helenodora inopinata Thompson and Jones, 1980 [1]

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. ^{[2] [3]} All were found in siderite concretions from Mazon Creek fossil beds, a lagerstätte that contains animals like the famous "Tully Monster". ^{[2] [4]}

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. ^[2] Other specimens include FMNH PE 13966, 33380, 33822, 45049, and 49784 and FMNH ROM 47513, 45565, and 47978. ^[3]

Helenodora was suggested by some authors to be a junior synonym of IlyodesScudder 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. ^[3]

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". ^[2]

Description

Dubious jaws

Outdated artist's reconstruction of Helenodora inopinata.

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

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 jaws 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. ^[2]

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. ^[3]

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. ^[3]

Lobopods

While originally thought to have more or less, ^[2] 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. ^[3] These lobopods are preserved as stubby triangles that taper off towards the end. ^[2] They had a mean length of 1.7 millimeters, and a mean width at their base of 2.1 millimeters. ^[3] 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. ^{[2] [3]}

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. ^[2] The claws were preserved as small, long dark patches composed of a thin film of carbon and framboidal pyrite. ^[3] All were on the left row of the body and found on either side of the fossil concretion. ^[2]

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. ^[3] 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. ^{[5] [3]}

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. ^[6] In modern velvet worms, the foot begins decaying soon after death. ^{[5] [6]} 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. ^[6]

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: ^[6]

References

1. Thompson, I.; Jones, D. S. (May 1980). "A Possible Onychophoran from the Middle Pennsylvanian Mazon Creek Beds of Northern Illinois" (PDF). Journal of Paleontology. 54 (3): 588–596. Bibcode:1974JPal...48..524M. JSTOR   1304204.
2. Thompson, Ida; Jones, Douglas S. (1980). "A Possible Onychophoran from the Middle Pennsylvanian Mazon Creek Beds of Northern Illinois". Journal of Paleontology. 54 (3): 588–596. ISSN   0022-3360. JSTOR   1304204.
3. Murdock, D. J. E.; Gabbott, S. E.; Purnell, M. A. (2016). "The impact of taphonomic data on phylogenetic resolution: Helenodora inopinata (Carboniferous, Mazon Creek Lagerstätte) and the onychophoran stem lineage". BMC Evolutionary Biology. 16 (19): 19. Bibcode:2016BMCEE..16...19M. doi: 10.1186/s12862-016-0582-7 (inactive 1 July 2025). PMC   4722706 . PMID   26801389.
4. Clements, Thomas; Purnell, Mark; Gabbott, Sarah (2018-10-04). "The Mazon Creek Lagerstätte: a diverse late Paleozoic ecosystem entombed within siderite concretions". Journal of the Geological Society. 176 (1): 1–11. doi:10.1144/jgs2018-088. hdl: 2381/44761 . ISSN   0016-7649.
5. Murdock, Duncan JE; Gabbott, Sarah E.; Mayer, Georg; Purnell, Mark A. (2014-11-29). "Decay of velvet worms (Onychophora), and bias in the fossil record of lobopodians". BMC Evolutionary Biology. 14 (1): 222. Bibcode:2014BMCEE..14..222M. doi: 10.1186/s12862-014-0222-z . ISSN   1471-2148. PMC   4266977 . PMID   25472836.
6. Oliveira, I. S.; Bai, M; Jahn, H; Gross, V; Martin, C; Hammel, J. U.; Zhang, W; Mayer, G (2016). "Earliest Onychophoran in Amber Reveals Gondwanan Migration Patterns". Current Biology. 26 (19): 2594–2601. Bibcode:2016CBio...26.2594O. doi: 10.1016/j.cub.2016.07.023 . PMID   27693140.

External links

• Data related to Helenodora at Wikispecies