Mazon Monday #280: Palaeocampa anthrax Redux

This is Mazon Monday post #280. What’s your favorite Mazon Creek fossil? Tell us at email:esconi.info@gmail.com.

_{Historical plate illustrating specimens and anatomy of Palaeocampa anthrax, from Scudder, 1884 Scudder, S. H. Two new and diverse types of Carboniferous myriapods. Boston Soc. Nat. Hist., Mem. 3, 283–297 (1884).}

The classification of Palaeocampa anthrax has long been controversial. The animal was first described as a catepillar in 1865 by Fielding Bradford Meek (1817-1876) and Amos Henry Worthen (1813-1888) in “Notice of some new types of organic remains from the Coal Measures of Illinois“. Joseph Even, “an amateur collector typical of the nineteenth-century gentleman naturalists” (see “Mazon Creek Fossil Beds“), found by found the holotype in the early 1860s. Unfortunately, that specimen was lost in a fire along with the type specimen of Amphibanus grandiceps between 1866 and 1868. In 1868, Samuel Hubbard Scudder classified P. anthrax as a worm in “The Fossil Insects of North America“. Scudder later reconsidered and classified it as a millipede in “The affinities of Palaeocampa Meek and Worthen as evidence of the wide diversity of type in the earliest known myriapods” in 1882 and “Two new and diverse types of Carboniferous myriapods” in 1884. Now, exciting new research published in the journal Nature, has concluded that Palaeocampa anthrax is actually the earliest known species of armored freshwater lobopodian.

The paper “Palaeocampa anthrax, an armored freshwater lobopodian with chemical defenses from the Carboniferous” was authored by Richard J Knecht, Christian R.A. McCall, Cheng-Chia Tsai, Richard A. Rabideau Childers and Nanfang Yu. Lobopods are members of the informal group Lobopodia (lobe-foot). They are panarthropods with stubby legs. They are soft-bodied and worm-like and are known from marine and freshwater environments. Their fossil relatives include Aysheaia and Hallucigenia from the Burgess Shale.

Palaeocampa anthrax, an armored freshwater lobopodian with chemical defenses from the Carboniferous

Abstract – Lobopodians are an evolutionary grade of panarthropods characterized by their vermiform bodies and paired, unjointed lobopodous legs. A paraphyletic group, their study is of particular significance in understanding the evolution of extant panarthropods. Found exclusively in marine deposits from the Paleozoic, the great majority of species come from Cambrian Konservat-Lagerstätten, with only a few representatives known from the Ordovician, Silurian, and Carboniferous. Here we redescribe Palaeocampa anthrax from the Carboniferous Mazon Creek (USA) and Montceau-les-Mines (France) Lagerstätten as a lobopodian. First published in 1865, nearly fifty years before the discovery of the Burgess Shale, Palaeocampa is historically the first discovered lobopod, and its presence at the slightly younger Montceau-les-Mines (Gzhelian), makes this the youngest known fossil ‘xenusiid’ lobopodian species. We present the case that Palaeocampa most likely inhabited a freshwater environment, contesting the view that Paleozoic lobopodians were exclusively marine. Palaeocampa bears biomineralized dorso-lateral and lateral sclerite sets with a unique architecture unseen in other lobopodian sclerites, which may have been capable of secreting defensive chemicals at their tips. Palaeocampa anthrax represents a major evolutionary step in lobopodians, both in environmental adaptations and in defensive abilities.

Emended Diagnosis– Lobopodian with 10 annulated lobopod pairs and corresponding biomineralized dorso-lateral and lateral sclerite sets. The lateral sclerites are composed of a bundle of radiating spines approximately equal in length to the width of the body, and the dorso-lateral sclerites are composed of a bundle of radiating spines approximately double the length of the width of the body. An additional set of long spines is situated on the posteriormost end radiating outwards. Each individual spine is straight, slightly tapering at the distal end, and externally has prominent longitudinal ridges separated by 2-3 smaller, less prominent ridges in between. Head bears a pair of stout, annulated antenniform appendages, a second pair of shorter and thinner appendages, and a smooth, slightly domed dorsal sclerite. Head and trunk covered in plicae with dermal papillae.

Remarks– This is a drastic revision to the previous diagnosis given by Pleijel et al. ²⁴ which defined Palaeocampa as a member of the polychaete order Amphinomida. The presence of lobopodous walking legs and the spine structure of Palaeocampa (unlike the structure of any known extinct or extant polychaete chaetae) invalidates the assignment of Palaeocampa to any annelid taxon. We are in agreement with Pleijel et al. ²⁴ that no marked difference exists between the geographically and temporally separate populations found in fossils from Mazon Creek and Montceau-les-Mines, and as such, we choose to keep Palaeocampa monospecific. An additional species of Palaeocampa, Palaeocampa(?) obscura, was doubtfully erected by Matthew³⁸; however, based on the sole specimen figured, this species from New Brunswick, Canada, is more likely a genuine arthropod or polychaete, and decidedly not Palaeocampa. Additionally, Pleijel et al.²⁴ following the suggestion of Pacaud et al.³³ synonymized the monospecific fossil polychaete genus Rhaphidiophorus with Palaeocampa based on the perceived assumption that both genera have externally, longitudinally striated chaete. However, SEM analysis of Rhaphidiophorus hystrix³⁹ chaete show no indication of external longitudinal striations nor do they have a similar internal structure (e.g., presence of septa) to that of Palaeocampa’s spines. Thompson³⁹ who erected Rhaphidiophorus, noted that the outer sheath of the chaete “had no preserved surface morphology” (p.192). Thompson went on to suggest that any fine lateral ribbing which may be observed was internal longitudinal canals that are noted in the chaetae of some polychaetes. Furthermore, body shape, variable number of segments, and the distinct presence of parapodia from which the chaete emerge distinguish Rhaphidiophorus (annelid) from Palaeocampa (lobopod). As such, we reject the synonymization of Rhaphidiophorus and Palaeocampa and maintain them as two distinct genera (see Supplementary Fig. 2).

Photos

Fig. 1: Neotype of Palaeocampa anthrax from Mazon Creek Lagerstätte.

a MCZ:IP:110208a, Overview of specimen. Faint red and blue grid lines from a wax pencil can still be seen across the specimen, made by the nineteenth century artist Katherine Pierson who illustrated pl. 26, Fig. 8 of²⁷. b Detail of trunk, showing bases of some spine tufts and the sclerotized dermal papillae reinforcing the dorsum. c Detail of head, showing dorsal sclerite with septarian fractures¹¹⁵ and part of a small, secondary frontal appendage. d Longitudinal view of spines showing external ridges and other spines with interior exposed revealing the septa dividing inner segments of the spine. e Spines with jagged serrations projecting upward from the modified basement papillae. f Bases of spines emerging from modified basement papillae. g Crenellated, jagged terminus of one of the spines. crh, chemical residue halo; dss, dorsal sclerite spines; hs, head sclerite; lb, lobopod limb; lss, lateral sclerite spines; sha, secondary frontal appendage. Scale bars represent 5 mm in (a), 2 mm in (b) and (c), and 100 μm in d–g.

Fig. 4: Yale Peabody Museum specimens of Palaeocampa anthrax from Mazon Creek Lagerstätte.

a YPM IP 042834, specimen in dorsal view. Details of the head, if preserved, are obscure. b YPM IP 042833, specimen in dorsal view. Head missing. c YPM IP 042826, specimen in dorsal view. Possibly represents a molt. d YPM IP 036198, specimen in dorsal view. Head missing. The counterpart of this specimen was long previously coated in lead. Largest available specimen. e YPM IP 042827, specimen in dorsal view. Annuli and dermal papillae are preserved across the trunk dorsum. f YPM IP 042828, specimen in dorsal view. Possibly represents a molt. The anterior end of the trunk dorsum is strongly delineated, with a squared-off profile dorsally. g YPM IP 042831, specimen in lateral view. Trunk is strongly flexed, possibly in a defensive reflex during burial to better expose the spine tufts and protect the soft underbody. Trunk lobopods are preserved down the length of the trunk, with roughly circular cross sections occasionally exposed. Details of head obscure. Images courtesy of the Yale Peabody Museum (CC0). Abbreviations as in Figs. 1, 3. Scale bars represent 5 mm.

Fig. 3: Palaeocampa anthrax from Mazon Creek Lagerstätte