Reassessment of the Devonian problematicum Protonympha as another post-Ediacaran vendobiont
Abstract
Protonympha is an enigmatic fossil represented by two species from the Middle Devonian (Protonympha transversa) and Late Devonian (Protonympha salicifolia) of New York. Although interpreted in the past as a polychaete worm or starfish arm, Protonympha is not found with marine fossils, but with fossil plants. This fossil plant community was a swamp woodland of Lepidosigillaria whitei, with ground cover of Haskinsia colophylla, fringing brackish to freshwater coastal lagoons of the Catskill Delta. Protonympha shares with Ediacaran Vendobionta a quilted body of unskeletonized biopolymer that is unusually resistant to burial compaction. In overall form, Protonympha is most like the Ediacaran genus Spriggina. Protonympha has branching and tapering tubular structures radiating from the bottom. These rhizine-like structures, thallus stratification and internal chambers revealed by petrographic thin sections suggest affinities with lichenized fungi. As for Cambrian Swartpuntia and Ordovician–Silurian Rutgersella, Protonympha may have been a post-Ediacaran vendobiont.
Keywords
‘Ediacaran Fossils’ is a term which lost much of its original meaning, once Ediacaran was ratified as the name of a new geological period (MacGabhann ). Fossils found in rocks of the Ediacaran Period include discoids like microbial colonies (Nemiana of Leonov ), flexuous carbonaceous compressions like sea weeds (Doushantouphyton of Xiao et al. ), calcareous stromatolites (Tungussia of Walter et al. ), other calcified cyanobacteria (Girvanella of Riding ), chitinous tubes like scyphozoans (Corumbella of Warren et al. ; Schiffbauer ), calcareous tubes like those of pogonophoran worms (Cloudina of Hua et al. ; Zhuravlev et al. ), burrows like those of worms or slime moulds (Lamonte of Meyer et al. ), and microfossils like mesomycetozoa (Tianzhushania of Huldtgren et al. ). Then, there are the most iconic and enigmatic of the Ediacaran fossils, the large quilted problematica called Vendobionta by Seilacher (, p. 607), with the following definition: ‘immobile foliate organisms of diverse geometries that were only a few millimetres thick, but reached several decimetres in size. A shared characteristic is the serial or fractal quilting of the flexible body wall, which stabilized shape, maximized external surface and compartmentalized the living content’. Vendobionta have no clear living relatives and are widely considered extinct since a basal Cambrian mass extinction (Laflamme et al. ). However, problematic fossils like classical Ediacaran vendobionts are also known in geologically younger rocks: Cambrian Swartpuntia of Jensen et al. and Hagadorn et al., Ordovician Rutgersella of Retallack, Silurian Rutgersella of Retallack and Devonian Protonympha of Conway Morris & Grazhdankin (, ). These plausible holdover fossils are of interest because they are sometimes better preserved than fossils in Ediacaran rocks, and are associated with a variety of other fossils as a guide to their habitats, which have become controversial for the Ediacaran (Retallack ; Tarhan et al. ).
This study concerns the last known vendobionts, Protonympha from the Middle and Late Devonian of New York state (Figs , ). When first described by Clarke, the three available specimens of Protonympha salicifolia were considered polychaete worms, along with other supposed polychaetes (Palaeochaeta devonica) from what is now regarded as lower Gardeau Formation (Sevon & Woodrow ). Ruedemann assigned a comparable fossil from the Marcellus Shale to another species: Protonympha marcellensis. Conway Morris & Grazhdankin regard Palaeochaeta devonica as a possible millipede, and ‘Protonympha’ marcellensis as a likely crustacean, but considered P. salicifolia a problematicum preserved in a manner comparable with vendobionts. Conway Morris & Grazhdankin added a second species with vendobiont-like preservation, Protonympha transversa, by emending the name of a fossil formerly considered to be the arm of a starfish, and assigned to Palaeaster (Hall ), Devonaster (Schuchert 1916) and Foliaster (Blake ). My own examination of these specimens, collection of additional material for petrographic study and observations on the sedimentology of their fossil localities confirms that these are two distinct species best placed within the same problematic genus, and also documents their occurrence with fossil land plants, freshwater fish and eurypterids, rather than marine fossils.
Materials and methods
The lectotype of P. salicifolia could not be found in the State Museum of New York (NYSM 5170) when sought with the aid of Ed Landing, but Dmitry Grazhdankin provided excellent photographs for study. The holotype of P. transversa is still available in the American Museum of Natural History in New York (AM 39220 part and counterpart). Additional poorly preserved material of both species was collected for thin sectioning and measurement from the type localities near Naples and Summit in upstate New York, and curated in the Condon Collection of the Museum of Natural and Cultural History of the University of Oregon (UO F118355, OU F119432-434, UO F119443-5). New collections also allowed precise location of the fossils within newly measured geological sections (Figs , ). Also prepared were petrographic thin sections of the fossils and associated sediments, and these were scanned in a Nikon slide scanner under plane light. Associated plant fossils were also collected for the Condon Collection (Fig. ). Most specimens were measured using digital calipers, but incomplete specimens were measured by digital superposition and scaling of a complete outline of a type specimen on the fragment in the program Photoshop. Segments were sometimes effaced in the newly collected material, but were clear at the margins. These measurements are added to those of Conway Morris & Grazhdankin (, ). Four known specimens of P. transversa complete enough to measure have the following accession numbers, width in mm, length in mm and number of segments, respectively: AM F39220, 57, 20, 43: UO F119434, 39, 13, 28: UO F119443, 50, 16, 39: UO F119444, 31, 12, 29. Four known specimens of P. salicifolia have the following accession numbers, width in mm, length in mm and number of segments, respectively: NYSM 5168, 55, 14, 45: NYSM 5169, 58, 14, 50: NYSM 5170, 58, 15, 55: UO F118355, 45, 13, 42.
Geological setting of Protonympha transversa
The specimen label of the holotype (American Museum F39220) of P. transversa (Hall) Conway Morris & Grazhdankin has ‘Summit, Schoharie County, New York’, and indicates collection and publication by Hall, who offers no further details. Prosser made extensive collections of marine invertebrates in shales of this area (distinct from the sandy matrix of P. transversa), and found no additional specimens. Nor was Protonympha mentioned in exhaustive collections of marine fossils for palaeoecological study (Sutton & McGhee ).
The most likely locality of the holotype is flaggy sandstones (locality UO15582; Fig. B) on the northern margin of Summit village (N42.59251° W74.57941°), which yielded a variety of fossil plants, eurypterid fragments and Protonympha (Table , Figs A–G, D–F). This hillside littered with fossiliferous slabs is directly behind an historic marker sign: ‘Old Toll Gate on Richmondville-Charlotteville Plank Road 1856′. This rocky hillside was thus easily accessible when James Hall was collecting, and similar flagstones are in the masonry of the historic gate and house. The orange weathered, non-calcareous, flagstones here have plant compressions including the zosterophyll Serrulacaulis furcatus (Fig. F), like the specimen on the holotype slab (Fig. D, E). Among a variety of fossil plants (Fig. A–F), three possible specimens of P. transversa were found here, all poorly preserved. Shales above these flaggy sandstones yielded ostracods (locality UO15584), and a massive calcareous sandstone stratigraphically 3 m lower than the sandstones (OU15583) yielded broken fragments of marine brachiopods no larger than 1 cm (UOF119436-9), including Tropidoleptus carinatus (Conrad) Hall, Orthospirifer mesastrialis (Hall) Cooper & Dutro, Mucrospirifer mucronatus (Conrad) Tillman and Chonetes deflecta Hall, but no plant fragments.
| Taxon | Explanation | Localities (2 = UO15582, 5 = UO15585) | Source |
|---|---|---|---|
| Protonympha transversa (Hall) C.Morris & Grazd. | Foliose lichen | 2, 5 | Conway Morris & Grazhdankin |
| Hepaticites devonicus Hueber | Liverwort | 2 | Herein |
| Taeniocrada sp. indet. | Rhyniophyte | 2 | Herein |
| Serrulacaulis furcatus Hueber & Banks | Zosterophyll | 2 | Herein |
| Lepidosigillaria whitei Kräusel & Weyland | Lycopsid | 2, 5 | Grierson & Banks ; herein |
| Haskinsia colophylla (G.& B.) Grierson & Banks | Lycopsid | 2, 5 | Grierson & Banks ; herein |
| Svalbardia banksii Matten | Progymnosperm | 2 | Herein |
| Eddya sullivanensis Beck | Progymnosperm | 2 | Herein |
| Eurypterida gen. indet. | Eurypterid | 2 | Herein |
| Gyroptychius granulatus Newberry | Coelacanth-like fish | 5 | Herein |
Unlike the historic fossil locality on the outskirts of Summit (UO15582), a 20th century road cut 2.1 miles northeast of Summit (N42.60370° W74.5691°) is a fossil plant locality of Grierson & Banks, recollected here as UO15585 (Fig. , C), and yielding an assemblage of possible Protonympha, lycopsids and fish scales (Table : Fig. H). These sandstones are little modified by weathering, unlike the holotype of P. transversa, but do have sparse, fine root traces indicative of immature palaeosols, such as Psamments of Soil Survey Staff. The shales immediately overlying these plant-bearing sandstones include marine brachiopods (locality UO15586).
All these localities are in the lower Moscow Formation, Hamilton Group (Rickard & Fisher ), of Middle Givetian age (Sevon & Woodrow ). The overlying Tully Limestone has ammonoids of the Pharciceras amplexum Zone (Kirchgasser ; Work et al. ) and conodonts of the Polygnathus varcus Zone (Davis ; Sparling ). The assemblage of plants found during this work is typical for the Givetian Svalbardia biozone (Banks et al. ; Edwards et al. ), better known from the laterally equivalent Manorkill Formation of the Catskill Front (Retallack & Huang ). These localities are also stratigraphically correlative with the Pumilio anoxic event in the ocean, which was a precisely dated greenhouse spike (Gradstein et al. ) represented by deep-calcic palaeosols on the Catskill Front (Fig. ).
At each locality, Protonympha was found with fossil plants in the same facies of wavy-bedded sandy flagstones, grey–green in colour, but weathering rust orange (Fig. ) like the holotype specimens (Fig. D–F). Marine fossils are found in associated shales (Prosser ) and also in a single massive calcareous sandstone (base of Fig. A), which is the thin eastern edge of the Portland Point Member of the Moscow Formation (McCave ). Fossils above the marine band are considered a brackish bay fauna from a lagoon ponded by a barrier bar, ultimately overlain by coastal plain facies of the Gilboa Formation (McCave ). These plant-bearing shales and sandstone are part of the Cooperstown Member of the Moscow Formation, interpreted as estuarine sandstone channels by Bridge & Willis. The outcrops examined do not show deep palaeochannels with trough cross-bedding, but low-angle heterolithic lateral accretion sets and shallow scour and fill with claystone breccia (Fig. A, B), of the kind interpreted as chutes of crevasse splays on levees of meandering streams elsewhere in the Catskill magnafacies (Gordon & Bridge ).
Fossils found with P. transversa were mainly plants, most commonly the spreading herbaceous lycopsid Haskinsia colophylla (Grierson & Banks) Grierson & Banks (Fig. A) and tree lycopsid, Lepidosigillaria whitei Kräusel & Weyland (Fig. B). Also found were a few fragmentary remains of the progymnosperm seedling Eddya sullivanensis Beck (Fig. C) and progymnosperm tree Svalbardia banksii Matten (Fig. D). Fragments of herbaceous plants include the liverwort Hepaticites devonicus Hueber (Fig. E) and the zosterophyll S. furcatus Hueber & Banks (Fig. F). A short fragment of the likely rhyniophyte, Taeniocrada sp. indet. (Fig. G), has characteristic strongly marked stele and compressed cortex, but lacks branching, and is wider than most recognized species of that genus (Taylor ). Animal remains found in association with P. transversa include scales of the freshwater fish Holoptychius granulosus Newberry from locality UO15585 (Fig. H) and indeterminate fragments of eurypterids (UOF119427) from locality UO15582.
Geological setting of Protonympha salicifolia
Clarke (, ) reported three specimens of P. salicifolia from ‘fine grained feldspathic argillaceous flagstones’ at two localities near Naples: ‘Italy Hill, 3 miles north of Naples’ and ‘Tannery Gully, Naples’. He also reported a specimen regarded by Conway Morris & Grazhdankin as a likely millipede (Palaeochaeta devonica) from correlative strata in Grimes Gully north of Naples (Fig. B), which is famous as the locality for the ‘Naples tree’, an unusually complete trunk of L. whitei (White ; Grierson & Banks ). Tannery Gully is now very overgrown, even in winter, but fossil plants (Table ) were found in flaggy sandstones 100 m east of where Tannery Gully joins Naples Creek at the foot of Hatch Hill (UO13428 of Fig. B, at N42.60875° W77.40277°). These sandstones include corms and rootlets of lycopsids (Fig. I, K), but no other pedogenic alteration, so are interpreted as immature clayey soils, comparable with Fluvents of Soil Survey Staff.
| Taxon | Explanation | Localities (7 = UO13427; 8 = UO13428) | Source |
|---|---|---|---|
| Rugalichnus matthewii Stimson et al. | Cyanobacterial wrinkles | 8 | Herein |
| Protonympha salicifolia Clarke | Foliose lichen | 7 | Conway Morris & Grazhdankin |
| Lepidosigillaria whitei Kräusel & Weyland | Lycopsid | 8 | Grierson & Banks ; herein |
| Hydnoceras sp. indet. | Hexactinellid sponge | 8 | Clarke |
| Orbiculoidea magnifica (Herrick) Schuchert | Inarticulate brachiopod | 8 | Clarke |
| Paropsonema cryptophya Clarke | Eldoniid echinoderm? | 8 | Clarke |
| Palaeophycus tubularis Hall | Worm burrow | 7 | Herein |
| Planolites beverleyensis (Billings) Alpert | Worm burrow | 7 | Herein |
| Palaeochaeta devonica Clarke | Millipede | 8 | Conway Morris & Grazhdankin |
An effaced specimen of P. salicifolia with marginal segmentation and weak midline (Fig. A) was found on the sole of a loose slab which also had trace fossils (Fig. L): Palaeophycus tubularis Hall and Planolites beverleyensis (Billings) Alpert and indeterminate plant fragments in a quarry above the road (UO13427 at N42.63462° W77.3680°) exposing the upper part of the section better seen 200 m north in Conklins Gully, 3 miles northeast of Naples, at the junction of Rainbow Hill Road and county road 245. Rainbow Hill Road leads east over the hill to the village of Italy, and the excellent exposures in Conklin's Gully (Fig. A) are the only outcrops that distance from Naples and on the correct stratigraphical level to be the ‘Italy Hill’ locality of Clarke (, ). This is the only fossiliferous level at this locality, and the deep gorge is the likely locality of the little weathered type specimens of P. salicifolia, which has on the same slab a short section of stem and three enations like the zosterophyll S. furcatus (Fig. A).
Clarke regarded this fossiliferous horizon as an informal unit of ‘Hatch sands’, but later (Clarke & Luther ) included this fauna in the ‘Grimes Sandstone’. In the terminology of Rickard & Fisher and Sutton & McGhee, this is the Gardeau Member of the Rhinestreet Shale, West Falls Group, corresponding to the former ‘Palmatolepis gigas conodont Zone’, since revised to Palmatolepis rhenana Zone (Klapper & Foster ). This level is between the mid-Frasnian ammonoid zones of Schindewolfoceras chemungensis and Playfordites tripartitus (House & Kirchgasser ; Becker & House ). This locality is also within the Frasnian–Famennian Archaeopteris plant biozone (Edwards et al. ; Retallack & Huang ). These localities are also correlative with the lower Rhinestreet anoxic event in the ocean, and a greenhouse spike is represented by deep-calcic palaeosols on the Catskill Front (Fig. ).
Fossils found with P. salicifolia by Clarke (, p. 1238) were ‘Paropsonema cryptophyum, Orbiculoidea magnifica, a linguloid of peculiar aspect, Hydnoceras, with fragments of Lepidodendron and other plant remains’. A list updating these records and adding specimens collected for this study is given in Table . Most of the plant fossils were L. whitei Kräusel & Weyland, including fragments of corm (Fig. I), corms with adventitious rootlets (Fig. K) and small twigs with leaf scars (UOF118360). Lack of articulate brachiopods or trilobites, and the presence of O. magnifica (Herrick) Schuchert and microbial wrinkle structures (Rugalichnus matthewii Stimson et al. ) can be taken as indications of brackish intertidal water, as inferred elsewhere in Devonian rocks (Bjerstedt ; Mata & Bottjer ). Paropsonema cryptophya (Clarke ) is a problematic unskeletonized discoid fossil, not examined during this study. This assemblage represents a brackish to freshwater interval between Chemung facies nearshore sands with shallow marine brachiopod communities of Orthospirifer-‘Camarotoechia’ (below) and Tylothyris-Douvillina (above) of Sutton & McGhee. Again, Protonympha is part of a plant community dominated by Lepidosigillaria, fringing brackish lagoonal waters.
One or two species or genera?
Additional material collected for this study (Fig. ) supports Conway Morris & Grazhdankin's (, ) division of a single genus Protonympha into two species, a narrow tapering one (P. salicifolia) and a wide lenticular one (P. transversa). Although the sample size may have been doubled, there are still only eight good specimens and two other fragments: Protonympha is a rare fossil. With that caution of low sample size and no clear juveniles, the two form species have different growth trajectories (Fig. ). From the few specimens known, growth of the two species may have been different: allometric elongation of the tip in P. salicifolia, but orthometric expansion of P. transversa.
The material collected for this study does not have the high relief and marked quilting of the types (Fig. ) and shows varying degree of effacement, decay and fragmentation. These are common taphonomic variations of fossil leaves (Ferguson ; Spicer ) and of vendobionts (Retallack ; Liu et al. ). Segmentation persisted longer at the margin than the middle, but the midline crease persisted even when quilting was obscure. Such taphonomic variation is an important criterion of biogenicity of dubiofossils (Hoffman ).
Both species of Protonympha have fundamental similarities of construction suggestive of relationship: bilaterally symmetrical body of hollow chambers formed by vertical partitions corresponding to surface quilting, extensions into surrounding matrix, wrinkle-fibre surface texture and high relief with different ends bent upwards and downwards, as noted by Conway Morris & Grazhdankin (, ).
Thin sections of newly collected Protonympha reveal discontinuous vertical as well as subhorizontal partitions of ferruginized organic matter (Fig. ), which may correspond with observed quilting of the fossils. An upper row of chambers as wide as visible ribs is intersected in vertical section, and these share a floor that is horizontal within the midline of the body. There is a suggestion of lower chambers as well, but that part of the fossil dissolves into matrix without clear resolution. The chamber cavities share a lateral wall and are not separately walled and articulated. They narrow outwards from the midline and bend like a knee (geniculation), but are not separately walled segments. The revealed surface of parts and counterparts (Fig. D–F) is the thick upper walls only, as can be seen where fractures and damage reveal the interior of chambers within the segments (Fig. C upper left).
Extensions into the matrix are especially clearly preserved near the end of the lectotype of P. salicifolia (Fig. C lower right), but are not bladed as described by Conway Morris & Grazhdankin. Close examination of specimens (Fig. ) and thin sections (Fig. ) shows that these extensions are tubes not much narrower than the segments, and in some cases, these extensions branch outwards (Fig. C). They branch and penetrate downwards to disrupt bedding planes of the matrix below in oriented specimens (Fig. A, B), but were not seen to disrupt overlying sediment (Fig. C). These structures are ptygmatically folded, presumably due to burial compaction of their matrix. Burial compaction for this stratigraphical level of the Catskill sedimentary succession has been determined by Retallack & Huang as 49–53% for silty palaeosols, but 80–82% of original thickness for fluvial sandstones more like the matrix of Protonympha. The tubes appear to be extensions of the quilting (Fig. C) rather than subsequent penetrating, parasitizing or decaying structures, because their ferruginized walls are comparable with those of chamber walls within the body (Fig. ).
The surface texture of Protonympha is like the fibres of a paper towel or papier maché, with the appearance of matted and flexuous filaments (Fig. C, F). This fabric is especially well illustrated by Blake and Conway Morris & Grazhdankin (, text-fig. 1). This fibrous to wrinkled lineation in many cases runs perpendicular to the body segments, but in some cases is moulded around geniculations along the segments. This wrinkle-fibre surface texture is found only on the fossils, and not on the surrounding matrix, so is unlikely to be a microbial film that covered the fossils before burial. Furthermore, associated fossil plants show no such surface texture, but rather cellular or woody texture (Fig. ). The types of P. salicifolia and P. transversa both show a brown weathered surface distinct from the black carbonized compressions of some fossil plants on the same slabs (Fig. A, D, E).
Finally, the fossils have high relief, comparable or greater than associated fossil plants, and like them, may have had some kind of biopolymer resistant to burial compaction. A carapace, shell or stereom was ruled out by Conway Morris & Grazhdankin (, ), because there is no evidence of deep articulation of skeletal elements, only geniculation of the surface. In thin section, the fossils are 3- to 4-mm thick, although partly supported by sediment within body chambers (Fig. ). This is thick for widths of only 12–20 mm, and thicker than known deeply buried fossil jellyfish and worms (Retallack ). Furthermore, Protonympha is broadly curled so that one end of the fossil is angled upwards from the bedding plane and the other down. In an oriented specimen of P. salicifolia (Fig. A), the narrow end arches up and the broad end arches down. The arrangement may be similar in P. transversa, in which the ends are similar and missing from what is interpreted as the upper counterpart (Fig. D). This curve may be a desiccation feature, but is also in part original, because the rhizine-like extensions are most common at one end, here considered basal.
Palaeosynecology of Protonympha
Protonympha salicifolia is preserved within coastal lagoon crevasse splay sandstone (Fig. C, D), and P. transversa preserved with a variety of plant fragments on an alluvial levee (Fig. A, B). They are not found with marine fossils, but with plants such as L. whitei (Fig. B), and may have been a part of the plant community dominated by that species. With a length at discovery of about 5 m, and diameter at breast height of 19 cm (White ), the ‘Naples tree’ of L. whitei would have had a height of 4.9 ± 0.9 m using the allometric all-tracheophyte algorithm of Niklas. This community was thus a low woodland. Corms and hollow rootlets of Lepidosigillaria, like those better known in Pennsylvanian lycopsids, are found near Naples (Fig. I, K; White ), close to their origin in a wetland surrounding a brackish to freshwater coastal lagoon. Haskinsia colophylla is a constant associate of both Lepidosigillaria and Protonympha at the Summit localities (UO15582, 15585) and was an herbaceous spreading understorey plant (Bonamo et al. ). The fossiliferous sandstones have sparse root traces, so that Haskinsia, Hepaticites, Serrulacaulis and corms of Lepidosigillaria may be in place of growth, as also is likely for variably preserved Protonympha with attached tubular penetrative structures (Figs ). Protonympha and the enigmatic discoid Paropsonema (Clarke ) were rare components of these lagoon-margin swamp woodlands.
Other plants found with Protonympha are known from associated palaeosols to form communities distinct from the Lepidosilligaria-Haskinsia community. Svalbardia banksi for example has been found in red Vertisol palaeosols of well-drained floodplains in the Manorkill Formation of the Catskill Front (Retallack & Huang ). The famous Gilboa trees are now regarded as cladoxyls such as Wattieza sp. cf. W. casasi (Stein et al. ), and found in drab pyritic Inceptisols with marine fossils comparable with modern mangrove soils (Driese et al. ). Svalbardia banksi was replaced in well-drained red palaeosols during the Frasnian by Archaeopteris macilenta, another progymnosperm tree (Retallack & Huang ) of the Oneonta Formation near Summit, laterally equivalent to the Genesee Group near Naples (Sevon & Woodrow ).
Ediacaran holdovers?
Protonympha is comparable in overall constructional morphology with Spriggina (Fig. ) and a variety of other quilted fossils that have been assigned to the Vendobionta of Seilacher. The hollow chambers behind the quilting are obvious in broken portions of the lectotype of P. salicifolia (Fig. B, C), and also seen in thin sections of Protonympha (Fig. ). These chambers are much shallower than body segments of insects, worms or echinoderms (Conway Morris & Grazhdankin, ). Chambered quilting is an apomorphy of Vendobionta considered as a clade, but the taxonomic rank of Vendobionta is more likely Class than Kingdom (Retallack ).
Protonympha also shows characteristic preservation of some Ediacaran Vendobionta, such as Dickinsonia and Spriggina, as concave hyporeliefs, or raised impressions on the soles of sandstone beds (Retallack, ). This form of Flinders-style preservation (of Narbonne ) is also known in other post-Ediacaran vendobionts such as Cambrian Swartpuntia (Jensen et al. ) and Ordovician Rutgersella (Retallack ). Other specimens of Protonympha (Fig. D–E) show parts and counterparts, one concave and one convex, as if only the thick upper surface was fractured during rock splitting: the Nama-style preservation of Narbonne known in a few Ediacaran Dickinsonia (Gehling ; Tarhan et al. ).
Newly collected specimens of Protonympha are similar to the Ediacaran vendobionts Dickinsonia and Ernietta in thin section (Elliott et al. ; Ivantsov et al. ; Retallack ). In all three taxa, ferruginized organic partitions form a thick upper wall, with orthogonal vertical partitions enclosing sediment. All show a poorly preserved middle seam with suggestion of additional chambers below. The wispy lower extensions of Dickinsonia and inner extensions of Erniettia (Retallack, ) are not as clearly tubular nor as large as in Protonympha (Fig. ). Furthermore, Ernietta has the thick wall down, upside down compared with Dickinsonia (Retallack, ). Silurian Rutgersella also shows two storeys of chambers around a central seam, thick upper wall and wispy extensions below, but these are pyritized, not ferruginized (Retallack ). Thus microconstructional details link Protonympha and Rutgersella with Ediacaran Vendobionta.
Whether palaeoenvironments of Devonian Protonympha and Ediacaran Vendobionta were comparable depends on interpretation. South Australian Ediacaran fossils most similar in preservational style to Protonympha were at first considered coastal lagoonal, intertidal and shallow marine (Jenkins et al. ), perhaps thrown up on the shoreface (Glaessner ). Subsequent discovery of palaeosols beneath vendobiont fossils in growth position supplied evidence that they lived on Ediacaran coastal floodplains (Retallack ) and intertidal flats (Retallack ). This palaeosol interpretation for the Ediacara Member of South Australia is based on 12 independent lines of evidence (Retallack ), to which now can be added early diagenetic silica cements with Ge/Si ratios (Tarhan et al. ) characteristic of soils (Retallack ). The Silurian vendobiont Rutgersella similarly colonized estuarine intertidal to supratidal mudflats (Retallack ). Comparable coastal lagoon-margin levees and swamp woodlands are envisaged here for Protonympha (Fig. ). On the other hand, the identification of Ediacaran palaeosols has been challenged (Tarhan et al., ), and Ediacaran vendobionts reconstructed as entirely subtidal (Gehling & Droser ) to abyssal marine (Narbonne ; Narbonne et al. ). Neither of these marine interpretations are likely for Protonympha in plant assemblages of the Devonian Catskill Delta of New York (Sutton & McGhee ; Bridge & Willis ).
Finally, there are particular Ediacaran vendobionts similar to Protonympha, such as Spriggina (Fig. A), and comparable genera Cyanorus (Ivantsov ), Praecambridium (Glaessner & Wade ) and Marywadea (Glaessner ). These genera also share an elongate outline, segments with distinct geniculations forming lengthwise pleats and a wrinkle-fibre microfabric (Fig. B, C). These Ediacaran fossils all differ from Protonympha in having a semicircular end opposite a tapering end, although this is unclear in the holotype of Spriggina floundersi (Fig. B). Ediacaran Sprigginidae also differ in having distal segments oriented in one direction only, and lacking an S-shaped vertical undulation of the body. The large rounded end of Spriggina has been variously interpreted as a head (Birket-Smith ; McMenamin ) or a holdfast (Seilacher ), and thus Sprigginidae (Glaessner ) has been included within both sessile Rangeomorpha (Pflug ) and motile Bilateriomorpha (Erwin et al. ).
Comparable living organisms
Past research and general observations on Protonympha already discussed leave the following living organisms worth consideration as possible modern analogues for Protonympha: polychaete worms, starfish, elongate arthropods, algae, plant gametophyte, unlichenized fungi and lichens.
Polychaete worm?
Clarke considered P. salicifolia a polychaete worm, and similar affinities were also proposed for comparable Ediacaran Spriggina (Glaessner ) and Marywadea (Glaessner ). However, the segments of Protonympha and these other taxa are not scale-like structures (elytra), nor are the extensions into the matrix-like stiff bristles (chaetae) of polychaetes (Conway Morris & Grazhdankin ). Fossil polychaetes are soft-bodied and compressed to a film, even when partly permineralized within siderite nodules (Fitzhugh et al. ), unlike the high relief preservation of Protonympha in sandstone.
Starfish?
Hall, Schuchert and Blake all considered P. transversa an arm of a starfish. Conway Morris & Grazhdankin noted that Protonympha has a flexuous pellicle with wrinkle-fibre microfabric rather than smooth to pitted stereom, and also geniculations and divots rather than articulations and pores between skeletal segments. These surface observations are confirmed by thin sections (Fig. ) showing organic-chambered construction, rather than individual skeletal ossicles. Association with plants (Fig. ; Clarke ) also makes interpretation as an obligate marine organism unlikely.
Elongate arthropod?
Protonympha is comparable with the Ediacaran Spriggina for which Birket-Smith prepared a highly imaginative restoration of arthropod-like limbs and segmented head. Glaessner & Wade also considered Praecambridium, another member of the extinct Sprigginidae, an elongate arthropod. Millipede would also be a possibility for Protonympha, given association with plants, and associated Devonochaeta may have been a millipede (Conway Morris & Grazhdankin ). Both marine and non-marine arthropods are unlikely for Protonympha because of the lack of a mineralized exoskeleton, no articulations between segments, nor any legs nor pleopods (Conway Morris & Grazhdankin, ). The pellicle of Protonympha has a wrinkle-fibre fabric, that is geniculated rather than segmented, and extensions into the matrix below are sinuous and branching, unlike arthropod limbs (Fig. ).
Alga?
Could Protonympha have been a seaweed, as envisaged by Ford for the Ediacaran vendobiont Charnia? A few living sea weeds such as Padina pavonica (Phaeophyta), Codium effusum (Chlorophyta) and Delessaria serrulata (Rhodophyta) have superficially quilted thalli (Bold & Wynne ). With cellulose cell walls and surficial quilts, fossil sea weeds do not maintain the high relief (Xiao et al. ) seen in Protonympha (Fig. ), nor internal partitions (Fig. ).
Plant gametophyte?
Occurrence of Protonympha with diverse plant fossils at Summit and plant debris at Naples (Fig. ) raises the possibility that it might be the gametophyte stage of a pteridophyte, a small haploid plant with multiple sex organs above and rhizoids below. Known Devonian gametophytes are not only cordate like many modern gametophytes, but stellate and trumpet shaped (Remy et al. ), unlike Protonympha. Gametophyte fossils are either thin compressions or permineralizations with little woody tissue (Kerp et al. ) and would not have created such compaction-resistant fossils as Protonympha.
Adder's tongue fern spike?
Another possibility is suggested by general similarity of Protonympha with the sporangial spike of adder's tongue fern, Ophioglossum (Chrysler ; Sen ), and less similar to moonwort, Botrychium (Wagner & Wagner, ). Ophioglossum and Botrychium (Ophioglossales) are generally considered eusporangiate ferns, but sometimes as living progymnosperms (Kato ). Crown group Ophioglossales are unknown as fossils earlier than Palaeocene (Rothwell & Stockey ), but progymnosperms are known from the Middle Devonian of New York (Fig. C, D: Retallack & Huang ). The sporangia of Ophioglossales are simple chambers (Sen ; Wagner & Wagner, ) rather than geniculate tubes as in Protonympha. Ophioglossales have no secondary xylem (Rothwell & Karrfalt ) that could explain observed compaction resistance of Protonympha in unpermineralized sediments. Finally, no leaves, stalks nor tubers have been found attached to Protonympha, but these are firmly attached to known fossil Ophioglossales (Rothwell & Stockey ).
Unlichenized fungus?
Quilted and bladed Ediacaran fossils have been compared with ascomycotan fruiting bodies of morels, such as Morchella angusticeps (Retallack ). With chitin cell walls (McFarlane et al. ), morels could be preserved in sandstones with relief comparable with known occurrences of Protonympha, as demonstrated by deeply buried trilobites in sandstone (Retallack ). The irregular extensions could be rhizines from near the base of the ascocarp (Retallack ). Fossil morels are unknown, but Ascomycota are known by Early Devonian (Taylor et al. ). The very regular quilting of Protonympha is unlike the more disorganized compartments of morels, and thin sections revealed no outer palisade-like layer comparable with an hymenium, nor a large internal hollow (Fig. ).
Lichen?
Lichens are a symbiosis between alga or cyanobacteria and fungus. The fungus has chitin walls and forms a stratified, chambered thallus, resistant to burial compaction, comparable with vendobionts in thin section (Retallack ). Lichens are classified according to the fungal component, and both basidiomycotan and ascomycotan lichens are known by the Early Devonian (Honegger et al.,b), but not before then (Berbee & Taylor ; Taylor et al. ). Chitinous spores and saccules with highly differentiated walls and attached hyphae are diverse and abundant in rocks as old as Ediacaran, and may have been extinct glomeromycotan lichens (Retallack ). Lichens are commonly attached to rock or wood, but rhizine-like penetration of sandstone by Protonympha (Figs C, A, B) invites comparison with the variety of lichens that live in soils (McCune & Rosentreter ). The principal objection to Spriggina and Protonympha as lichens is their high degree of symmetry and regularity of segments, so that first impressions are that they were something more complex and cellularly differentiated, like a polychaete worm (Clarke ) or starfish (Hall ).
Conclusions
None of these modern comparisons is ideal for Protonympha, which remains a problematicum. Nevertheless, Protonympha may have been the last of a lineage, like comparable Ediacaran Sprigginidae (Retallack ). Other comparable fossils include Ordovician and Silurian Rutgersella (Retallack, ), which shares a quilted construction and no clear head nor tail. Glomeromycotan lichens remains a possibility for Protonympha, but demonstrated glomeromycotan lichens such as Prototaxites (Retallack & Landing ; Retallack, ), had branching trunks and unquilted laminae, unlike Protonympha. These were all poorly understood fossils of tidal flats and alluvial lowlands (Retallack,c).
By Devonian time, Protonympha lived in swamp woodlands of lycopsids, including trees of L. whitei and ground cover of H. colophylla (Fig. ). Protonympha was preserved in sandstone levees of coastal streams and in sandstone of brackish to freshwater coastal lagoons of the Devonian Catskill Delta, in facies quite distinct from upland forest vegetation, coastal mangrove-like vegetation and benthic marine communities (Sutton & McGhee ; Retallack & Huang ). Rhizine-like structures from the base of Protonympha attached it to soil. The thallus was internally stratified with a complex system of chambers, like that demonstrated for Rutgersella (Retallack ) and Dickinsonia (Retallack 2016a).
Acknowledgements
Dima Grazhdankin, Ed Landing, Simon Conway Morris and Bruce Runnegar offered useful discussion. Selina Robson and Dima Grazhdankin provided photographs of types of the two Protonympha species. One field trip was partially funded by the Soil-4-Climate Group of Tufts University and another by University of Oregon Faculty Research Account.
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Received: 27 March 2017
Accepted: 8 September 2017
Published online: 8 December 2017
Issue date: 1 July 2018
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