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
In its initial description, Thomson and Jones 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.
Potential jaws
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. 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 Back in 1980, Thomson and Jones tentatively described Helenodora as having "oral papilla" (slime papilla), though they were not completely sure of this due to poor preservation in both the holotype and paratype. Using Phallocephale for comparison, a 2012 study found the Helenodora specimen ROM 47978 had paired lateral extensions resembling retracted slime papillae. Though, like the study before them, these were labeled as putative. Things changed in 2016 when Murdock et al. interpreted Helenodora to lack slime papilla all together. In the specimens analyzed, they found no consistent evidence that the 2nd appendage was differentiated from ones behind it, or that the animal had signs of slime papilla. As the bulge in ROM 47978 wasn't present in other specimens with a complete head, it was likely a product of decay.
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
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
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