Carbotubulus

Carbotubulus Temporal range:
Scientific classification
Kingdom:	Animalia
Clade:	Panarthropoda
Phylum:	†Lobopodia
Clade:	†Hallucishaniids
Family:	†Hallucigeniidae
Genus:	†Carbotubulus Haug et al., 2012
Species:	†C. waloszeki
Binomial name
†Carbotubulus waloszeki Haug et al., 2012

Carbotubulus is a genus of lobopodian known from the Carboniferous Carbondale Formation of the Mazon Creek area from the state of Illinois, in the United States. A monotypic genus, it contains one species: Carbotubulus waloszeki, and is known from a single fossil. Overall, the animal had a very tubular body. Its head, trunk, and 10 pairs of limbs (9 of them being legs) all resembled long thin tubes. With an age of about 300 million years, it is the first long-legged lobopodian discovered to persist following the Cambrian explosion.

Discovery and naming

Carbotubulus is known from a single specimen, the holotype ROM 47514, and is composed of a part and counterpart. This was found in the Francis Creek Shale of Illinois's Carbondale Formation. More specifically, Carbotubulus was recovered from Pit 11 of the Mazon Creek fossil beds. The animal was discovered and described by Joachim T. Haug, Georg Mayer, Carolin Haug, and Derek E.G. Briggs in 2012. The holotype is currently being held in the Invertebrate Paleontology Collection at the Royal Ontario Museum of Toronto, Canada.

The animal's genus name is derived from two words. The first, "carbo", refers to it living in Carboniferous. The second, "tubulus", is Latin for "small pipe" and is a reference to its pipe-like legs. The species name "waloszeki" honors Dieter Waloszek for his work on arthropod evolution.

Description

Carbotubulus was preserved in 3D, with the holotype being nearly 7 times longer than wide (35 millimeters in length vs 4.8 millimeters in width). Additionally, the body surface was generally smooth. It lacked any annulations (rings) on its skin other than a few longitudinal wrinkles.

The head made up a significant part of Carbotubulus's body, around 1/3rd of the animal's total length. On the sides of the head were two pairs dark circular spots, one slightly fainter than the other. The paper never suggested what these might be. However, the authors dismissed them being an artifact due to their symmetrical arrangement. Behind the dots were a pair of small appendages. These were situated in a ventro-lateral position (on the bottom of the head towards the sides), being separate from and significantly smaller than the animal's legs.

Carbotubulus had 9 pairs of long, tube-like legs known as lobopods. These came in different lengths and sizes, with the largest (pair #8) being 11 millimeters long and 2 millimeters in diameter (more than 5 times longer than wide). The diameter of legs 7 and 9 are somewhat wider, though the authors considered this an artifact of preservation. Compared to the body's diameter, the legs are more than twice as long. It should also be mentioned that in the fossil, the ends of pairs #8 and #10 were not incorporated in the fossil concretion.

Classification

When Carbotubulus was first described, its systematic position was not clear and was loosely assigned to the group Panarthropoda (or in the original description, Arthropoda sensu lato). Discovery of the Cambrian lobopod Ovatiovermis cribratus from the Burgess Shale in 2017 led to a reanalysis of lobopodian classification, and Carbotubulus was confidently labled a panarthropod, specifically assigned the family Hallucigeniidae along with Cardiodictyon and the various species of Hallucigenia.

This classification was controversial, especially after the discovery of the second post-Cambrian (Silurian) long-legged lobopodian, Thanahita distos, from the Herefordshire Lagerstätte at the England–Wales border in UK in 2018. The new interpretation suggest that Carbotubulus and Cardiodictyon may lie outside the hallucigeniid family. A later study from 2024 disagreed with this, placing Carbotubulus along with Thanahita and Microdictyon in Hallucigeniidae. A year later, another paper describing Palaeocampa anthrax found Carbotubulus to be in a polytomy with a variety of traditional Hallucigeniids (though Cardiodictyon and Hallucigenia hongmeia were outside the group). A phylogeny of this can be found below.

	†Diania
	†Lenisambulatrix †Paucipodia †Thanahita †Eoconchariidae †Microdictyon †Fusuchoncharium †Quadratapora †Xenusion †Onychodictyon †Strictocorniculum †Rhombocorniculum †Orstenotubulus †Carbotubulus †Hallucigenia fortis †Hallucigenia sparsa †Hallucigenia hongmeia †Cardiodictyon Tardigrada (tardigrades) †Luolishaniida †Ovatiovermis †Facivermis †Luolishania †Collinsovermidae †Acincocricus †Collinsovermis †Collinsium †Entothyreos †Aysheaiidae †Aysheaia †Hadranax †Palaeocampa Antennopoda †Antennacanthopodia Onychophora (velvet worms) †Siberiidae †Siberion †Megadictyon †Jianshanopodia †Pambdelurion †Kerygmachelidae †Opabiniidae †Radiodonta (e.g. Anomalocaris) Deuteropoda (modern arthropods)

Evolutionary implications

The majority of the long-legged lobopodians such as Hallucigenia, Paucipodia and Orstenotubulus are known only during the Cambrian explosion. Living around 300 million years ago (well after the Cambrian came to a close), Carbotubulus supports the fact that long-legged Cambrian-type worms survived for over 200 million years after extinction events during the Middle Cambrian. These occurred between 510 and 502 million years ago, and by them many groups animals ultimately disappeared. There are other lineages that persisted for longer than originally thought, for example Mimetaster hexagonalis (a marrellomorph) and Schinderhannes bartelsi (a radiodont) from the Early Devonian. Still, the survival of Carbotubulus is much more extreme.

Paleoenvironment

Carbotubulus shared its environment with another lobopodian, the controversial Helenodora inopinata, and differed from it in many respects. While Carbotubulus had a long tube-like head and pair of tiny appendages on its underside, Helenodora resembled a velvet worm, having a pair of pre-cerebral forward-facing antenna and appendages resembling slime papillae (though later studies disagree on this). Carbotubulus had 9 pairs of legs while Helenodora had 20. These were long and tube-like in Carbotubulus, and stubby in Helenodora. Carbotubulus was mostly smooth, while Helenodora preserved annulations like those of a modern velvet worm.