Squirmarius testai: A new species of Cyclostome from the Carboniferous Mazon Creek Lagerstӓtte.

Vermiform (Worm-like) fossils assigned to the species Nemavermes mackeei have been described from the 308.6–308.4 million year old Mazon Creek Lagerstӓtte of Illinois, and the 330.3–323.4 million year old Bear Gulch Limestone in Montana. These are between 40 and 140 mm in length, and between 3 and 16 mm in width, and have been interpreted as free-living marine Nematodes. 

In a paper published in the Journal of Paleontology on 1 December 2023, Victoria McCoy of the Department of Geosciences at the University of Wisconsin-Milwaukee, and the School of Geography, Geology, and the Environment at the University of Leicester, Jack Wittry of the Field Museum of Natural History, Hamed Sadabadi of the Department of Materials Science and Engineering at the University of Wisconsin-Milwaukee, and Paul Mayer, also of the Field Museum of Natural History, re-examine specimens assigned to Nemavermes mackeei, and conclude that the taxon is not valid, and that the specimens assigned to this species belong to a variety of different Vermiform Animals, including a new species of Cyclostome (the group of Jawless Fish which includes modern Hagfish and Lampreys).

Examination of the holotype of Nemavermes mackeei, FMNH PE 21551, shows that it is a proboscis of the well-known (but hard to interpret) Mazon Creek fossil Tullimonstrum gregarium. In taxonomy, a specimen is deemed to belong to a species if it can be shown to belong to the same species as the holotype. Since the holotype of Nemavermes mackeei, FMNH PE 21551, clearly belongs to the same species as the holotype of Tullimonstrum gregarium, PE 10504, which was named first, then Nemavermes mackeei must be considered to be a junior synonym of Tullimonstrum gregarium, rather than a valid species.

Comparison of the holotype of Nemavermes mackeei to the proboscis and buccal apparatus of Tullimonstrum gregarium. (1) Holotype of Nemavermes mackeei, FMNH PE 21551; (2) line drawing of (1), showing in particular the projected shape of the bifurcate portion of the buccal apparatus; (3) Tullimonstrum gregarium proboscis with buccal apparatus, FMNH PE 39890, showing sharp bend in the proboscis, and the differential preservation between the proboscis and buccal apparatus; (4) Tullimonstrum gregarium proboscis with buccal apparatus, FMNH PE 39375, showing two sharp bends in the proboscis; (5, 6)Tullimonstrum gregarium buccal apparati, FMNH PE 28739 (5) and FMNH PE 31063 (6), showing the distinctive shape of the buccal apparatus and the preservational differences between the buccal apparatus and the rest of the proboscis. Scale bars are 10 mm. McCoy et al. (2023).

Many specimens assigned to Nemavermes mackeei, however, are clearly not specimens of Tullimonstrum gregarium, but of a vermiform Animal, which McCoy et al. interpret as a new species of Cyclostome, and name Squirmarius testai, where 'Squirmarius' means 'one who squirms' and 'testai' honours fossil hunter Tom Testa, who collected many of the specimens assigned to the new species, and who referred to these fossils as 'squirms'.

Specimens of Squirmarius testai: (1) holotype, FMNH PF 17809; (2) FMNH PF 17810; (3) FMNH PF 17812; (4) FMNH PF 17811; (5) LF 2101; (6) LF 5664, which was collected by David Douglass and donated by the David and Sandra Douglass collection to the Lauer Foundation for Paleontology, Science, and Education in Wheaton Illinois for this study. Scale bars are 10 mm. McCoy et al. (2023).

Squirmarius testai is a vermiform Animal, frequently showing soft tissue preservation, with a tapering posterior end (tail) and a blunter anterior end (head). None of the specimens preserve any fins, including specimens where the areas of a Cyclostome where fins would be expected are well preserved. The stem Hagfish, Gilpichthys greenei, also from the Mazon Creek Lagerstӓtte, is also interpretted as beign finless, so this is not an implausible state for a Cyclostome from this era. However, in some specimens of the Lamprey, Mayomyzon pieckoensis, and the Hagfish, Myxinikela siroka, fins are not preserved, though other specimens are known to have fins, suggesting that preservation can vary in Mazon Creek Cyclostomes.

Detailed morphology of Squirmarius testai holotype, FMNH PF 17809: (1) whole body with boxes indicating regions that correspond to (2) box A, (3) box B, and (4) box C, and dots indicating the spots that correspond to (5) dot D and (6) dot E; ; (2) head, box A in (1), showing the blunt point at the anterior end of the body, with two eyes preserved as dark ovals with a white dot in the right eye that might represent a lens; (3) gut near the middle of the body, box B in (1), showing the flat kaolinite preservation; (4) tail, box C in (1), showing the narrow point at the posterior end of the body, the gut with dark preservation ending before the posterior point of the tail (white arrow), and the lack of a tail fin; (5) SEM image in SE mode of the leftmost eye, dot D in (1), showing ovoid melanosomes along with siderite crystals with moulds of ovoid melanosomes; (6) SEM image in BSE mode of the gut with dark preservation in the tail, dot E in (1), with siderite crystals without melanosome moulds, and abundant pyrite microcrystals and occasional pyrite framboids. Scale bars are 10 mm (1); 2 mm (2)–(4); 10 μm (6); 5 μm (5). McCoy et al. (2023).

Several specimens assigned to Squirmarius testai have dark segmented structures which McCoy et al. interpret as myomeres (blocks of skeletal muscle tissue arranged in sequence). Also present in many is a lighter structure running through the centre of the body, interpreted as a gut, and some unidentified darker regions, which may be internal organs.

Detailed morphology of Squirmarius testai, FMNH PF 17812: (1) whole body with boxes indicating the regions that correspond to (2) box A, (3) box B, (4) box C, and (5) box D;  (2) head or anteriormost preserved part of the body, box A in (1), showing the mottled dark coating and two irregular, differently sized dark patches that are unlikely to be eyes; (3) triangular or C-shaped organ, box B in (1), directly anterior to gut; note also the dark mottled covering; (4) body region, box C in (1), showing dark mottled covering and 3D gut; (5) tail, box D in (1), showing the dark mottled covering, the gut ending before the tip of the tail (white arrow), and nothing that clearly represents a tail fin. Scale bars are 10 mm (1); 2 mm (2)–(5). McCoy et al. (2023).

Many specimens have eyes preserved as dark brown-black ovals on both the part and counterpart, with some having central infills of kaolinite, which could represent lenses. Under the scanning electron microscope, it is possible to see ovoid microbodies and moulds of microbodies, which are the size and shape of melanosomes. Melanosomes have been documented in a range of fossil lagerstӓtte, including other Chordates at Mazon Creek, and therefore are not implausible structures to find preserved in Squirmarius testai. Importantly, very few groups of Animals have melanostomes within their eyes; outside of the Chordates, these structures are found only in the eyes of Box Jellyfish and some Flatworms, neither of which resemble Squirmarius testai in form, supporting the idea that this Animal is a Chordate.

None of these features is consistent with Squirmarius testai being a Nematode, including its size, which would be remarkably large in a free-living Nematode (some modern parasitic Nematodes get much larger, but free-living forms tend to be microscopic), supporting McCoy et al.'s belief that Squirmarius testai is a Cyclostome, and probably a stem Hagfish.

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