The Life and Death of Jamoytius Kerwoodi White; a Silurian Jawless Nektonic Herbivore?

1. Introduction

Jamoytius kerwoodi White was a primitive, eel-like jawless fish that lived in the Llandovery epoch (444-433 Ma) of the Early Silurian period (White, 1946) (Figure 1). The fossil is preserved as rare carbonized films on bedding planes in one laminated siltstone horizon in the bank of the Logan water in the Lesmahagow inlier of Lanarkshire, SW Scotland (Ritchie, 1968a). It was once considered the most primitive known vertebrate (White, 1946), but with additional studies, its affinities are now debatable (Janvier, 1996, 2008; Sansom et al., 2010; Keating and Donaghue, 2016; Chevrinais et al., 2018; Miyashita et al., 2019, 2021). Because the interpretations of the even such exceptionally preserved soft-bodied fossils is difficult, observed features can be interpreted in different ways (Reeves and Sansom, 2023) (Figure 2). Various cladistic analyses of Jamoytius with other jawless vertebrates, using different character codings, give divergent results (Janvier 1981; Forey, 1984, 1995; Forey and Janvier 1993; Turner, 2004; Miyashiro et al., 2019; Reeves et al., 2023). Choice of the in-group taxa affects its placement (Donoghue et al., 2003; Gess et al. 2006; Reves et al., 2023). The position of Jamoytius on cladograms has consequently not stabilized, though it often appears as a sister taxon to the lampreys, the anaspids, or euphaneropids (Janvier, 1981; 2008; Ritchie, 1984; Janvier and Arsenault, 2007; Donoghue and Keating, 2014; Reeves et al., 2023) (Figure 3).

As a sister taxon to the lampreys, Jamoytius has been compared with parasitic lampreys which attack fish. But, only 18 of the 38 known species of lamprey, are carnivorous. Non-carnivorous lampreys are smaller (less than 40cm long) than the parasitic sea lampreys (35-120cm long) (Potter et al., 2015). The size and anatomy of Jamoytius is thus more compatible with non-carnivorous lampreys, though Priscomyzon riniensis, the oldest undoubted lamprey, from the Devonian (419-359 Ma) of South Africa, is also very small (Gess et al., 2006). The fossil evidence for early evolution of lampreys is scanty because only four undoubted Palaeozoic parasitic lamprey species have been recorded, the above Devonian Priscomyzon, and three from the Carboniferous (359-299 Ma). These lack, however, the specialised, heavily toothed discs with plate-like laminae present in modern lampreys, and it is possible that they were grazers, scraping algae off surfaces (Wu et al., 2023). Furthermore, their post-Paleozoic fossil record of lampreys is equally bad. Based on both morphological and molecular evidence, Brownstein and Near (2023) estimated that 90% of living lamprey clades originated only since the late Cretaceous. As reconstructed by Reeves and Sansom (2023), carnivorous lampreys evolved from non-carnivorous early Paleozoic forms and then radiated from the late Cretaceous times (~100Ma) and especially from Miocene times (~25 Ma) onwards into many both carnivorous and non-carnivorous forms.

As a sister taxon to the anaspids, Jamoytius resembles the genera, Loganiella, Birkenia, and, especially, Lasanius (Blom and Märss, 2010) (Figure 2B and Figure 4). Loganiella also occurs in the Jamoytius bed, while Birkenia and Lasanius occur in slightly younger fish beds in the Lesmahagow succession (Lovelock, 1998; Dineley, 1999).

As a sister taxon to the Upper Devonian, Euphanerops longaevus, (though many of the structures observed on the available fossils remain unexplained) (Janvier and Arsenault, 2007) (Figure 6 A, B), Jamoytius was originally classed with Euphanerops in the Jamoytiiformes (Tarlo 1967) (Figure 3C). Several other euphaneropids have now been recognized: one, Ciderius cooperi van der Brugghen from the fish beds above the Jamoytius bed at Lesmahagow (van der Brugghen, 2015). These are similar to Jamoytius, both in anatomy and possibly mode of life (Figure 6C).

The mode of life of Jamoytius kerwoodi is thus unresolved; even its life orientation is still not certain (Sansom et al., 2010). In this paper I am not particularly concerned with its affinities, but with its mode of life as inferred from its anatomy (which bears, of course, on its affinities), adaptative morphology and palaeoenvironment based on the sedimentology of the enclosing strata and the life styles of it and its associated biota.

Figure 1. Jamoytius reconstructions: A) with ventral ‘lamprey’ mouth (wuht permission from Nobu Tamura); B) with terminal suspension/detritus feeding mouth (Lingham-Soliar, 2014, fig. 2.11.

Figure 1. Jamoytius reconstructions: A) with ventral ‘lamprey’ mouth (wuht permission from Nobu Tamura); B) with terminal suspension/detritus feeding mouth (Lingham-Soliar, 2014, fig. 2.11.

Figure 2. Jamoytius fossil and inferred features: A) with the conflicting interpretations of White (1946), in bold, and Ritchie (1960, 1963, 1968, 1984) in plain italics (modified from Sansom et al. 2010, Plate 1); B) body parts and topological interpretation of the holotype (NHM P11284a) (from Sansom et al., 2010, text fig. 5).

Figure 2. Jamoytius fossil and inferred features: A) with the conflicting interpretations of White (1946), in bold, and Ritchie (1960, 1963, 1968, 1984) in plain italics (modified from Sansom et al. 2010, Plate 1); B) body parts and topological interpretation of the holotype (NHM P11284a) (from Sansom et al., 2010, text fig. 5).

Figure 3. Examples of cladistic analyses showing three interpretations for Jamoytius (from Sansom et al., 2010).

Figure 3. Examples of cladistic analyses showing three interpretations for Jamoytius (from Sansom et al., 2010).

Figure 4. Living and oldest fossil (Devonian) lampreys. A) parasitic sea lamprey (Petromyzon marinus Linnaeus), 35-60 cm long; B) American brook lamprey (Lethenteron appendix DeKay) 15-25cm long (A&B courtesy of North Carolina Wildlife Resources Commission; C) Devonian parasitic fossil lamprey (Priscomyzon riniensis Gess et al., ~5 cm long (public domain).

Figure 4. Living and oldest fossil (Devonian) lampreys. A) parasitic sea lamprey (Petromyzon marinus Linnaeus), 35-60 cm long; B) American brook lamprey (Lethenteron appendix DeKay) 15-25cm long (A&B courtesy of North Carolina Wildlife Resources Commission; C) Devonian parasitic fossil lamprey (Priscomyzon riniensis Gess et al., ~5 cm long (public domain).

Figure 5. Agnathan reconstructions: A) Thelodont Loganellia scotica (permission of Opal Raptor); B) Anaspid Birkenia (permission of Highlander Fossils, www.highlanderfossils.com; C) Anaspid? Lasanius (permission of Rob Van Assen, Museon Omniversum, Den Haag).

Figure 5. Agnathan reconstructions: A) Thelodont Loganellia scotica (permission of Opal Raptor); B) Anaspid Birkenia (permission of Highlander Fossils, www.highlanderfossils.com; C) Anaspid? Lasanius (permission of Rob Van Assen, Museon Omniversum, Den Haag).

Figure 6. A) Euphanerops longaevus reconstruction (Upper Devonian, Canada) (from Phillipe Janvier in Parc Nationale de Miguasha 2003); B) Euphanerops longaevus as a swimming nektonic detritivore/ herbivore (with permission from Nobu Tamura); C) various euphaneropds; Ciuderius couperi, (Lower Silurian), Achanarella trewinii, Cornovichthys blaaeweni (Middle Devonian), Endiolepis aneri (Upper Devonian) (with permission from Nobu Tamura).

Figure 6. A) Euphanerops longaevus reconstruction (Upper Devonian, Canada) (from Phillipe Janvier in Parc Nationale de Miguasha 2003); B) Euphanerops longaevus as a swimming nektonic detritivore/ herbivore (with permission from Nobu Tamura); C) various euphaneropds; Ciuderius couperi, (Lower Silurian), Achanarella trewinii, Cornovichthys blaaeweni (Middle Devonian), Endiolepis aneri (Upper Devonian) (with permission from Nobu Tamura).

2. Palaeoenvironments and paleoecology.

2.1. Sediments

The Jamoytius-bearing horizon is one of several eurypterid- and fish-bearing units in the Silurian (Llandoverian; 444-433 Ma) of the Lesmahagow inlier in the Midland Valley of Scotland (Ritchie, 1960,1968a; Clarkson and Harper, 2016)). It is exposed at Birk Knowes in a cliff next to the Logan Water (NS737346) (Figure 7A) near the top of the Patrick Burn Formation, an over 500-metre-thick section of alternating grey sandstones, siltstones, and mudstones (Jennings, 1961; Lovelock, 1998) (Figure 7B). The sediments of the Patrick Burn Formation change gradually upwards from deeper water interbedded mudstone/ turbidite sandstone facies into shallower water interbedded mudstone/cross-bedded and laminated sandstone facies. (Jennings, 1961; Lovelock, 1998) This is accompanied by changes in the taxonomy and ecology of the fossil biotas from marine to freshwater (Jennings, 1961; Ritchie,1963; Lovelock, 1998) (Figure 7B).

Throughout the Patrick Burn Formation, and in the Jamoytius bed itself, there is a complete absence of burrowing organisms and there are no tracks or trails on the bedding plane surfaces (Lovelock, 1998) The undisturbed nature of the sediments, together with abundant pyrite and organic matter, indicates anaerobic bottom conditions in very quiet water subject to periodic underflows (Ritchie, 1963); in keeping with the abundance of Ceratiocaris - most modern Phyllocarids prefer low energy conditions and are tolerant of low oxygen concentrations (Rolfe & Beckett, 1984). Geochemical evidence indicates a gradual salinity drop through the upper part of the Priesthill Group, which contains all the fish beds (Jennings, 1961).

The Jamoytius bed itself is about 10 metres thick and consists of alternating fine-grained thin sandstones, deposited by turbidity currents, and varved siltstones in which the varves consist of a lower siltstone sharply overlain by an organic-rich muddy layer: the latter enclose most of the fossils (Jennings, 1961; Ritchie, 1963) (Figure 8A, B). The bounding sandstones are low-angle tabular cross-bedded fine-grained mature muddy quartz- and felspathic-sandstones (Jennings, 1961), deposited by sand waves in shallow water by storms (Harms et al., 1975; Lovelock, 1998) (Figure 8C, D). The deeper water Jamoytius bed with its turbidite sandstones and laminated siltstone contrast markedly with the enclosing shallow water cross-bedded fine sandstones and testify to great fluctuations in water levels and climate at millennial scales typical of marginal marine and semi-arid lake basins such as those of Lake Cariaco in northern Venezuela and Lake Chad in the central Sahara (González et al., 2008; Armitage et al., 2015).

Both the Jamoytius bed, and higher fish-bearing beds at Lesmahagow, have alternations of fine silt-clay couplets (varves) which contain the biota, and olive-grey massive mudstones which are barren (Zolitschka, et al., 2015). In the Jamoytius horizon, itself, the alternations of laminated siltstones and graded quartz sandstones are almost identical to those of Cariaco basin in Venezuela (Athearn,1965; Hughen et al., 1996). Water circulation inside this partially isolated basin is restricted, which, combined with high annual primary productivity, causes it to be permanently anoxic at depth, where alternating light and dark coloured varved sediments correspond respectively to the winter-spring dry season and the summer-fall rainy season (Muller-Karger et al 2001).

The tectonic situation of the Midland basin Silurian inliers is also comparable with that of the Cariaco and related basins along the northern Venezuelan coast. Both are complex basins between ocean and continent, which evolved through time from marine to freshwater continental conditions due to shortening and inversion caused by major strike-slip faulting, occupying about he same ~ 100 Ma time period, Cretaceous to Recent for Venezuela, and Silurian through Devonian for the Midland Valley (Dewey and Strachan, 2003; James, 1990).

Figure 7. A) Birk Knowes exposure of Jamoytius beds. B) Section of Priesthill Group with fossil associations, ranges and inferred salinity (modified from Lovelock, 1998).

Figure 7. A) Birk Knowes exposure of Jamoytius beds. B) Section of Priesthill Group with fossil associations, ranges and inferred salinity (modified from Lovelock, 1998).

Figure 8. Sediments, A) View of part of Jamoytius bed showing varved siltstones alternating with graded fine-grained sandstones; B) photomicrograph of Jamoytius bed varved siltstone showing lower silt overlain by organic-rich silt cap; C) view of outcrop of fine-grained quartz-sandstones overlying Jamoytius bed; D) Photomicrograph of fine-grained sandstone from C, dominated by subangular quartz grains with rare plagioclase felspar, in carbonate cement.

Figure 8. Sediments, A) View of part of Jamoytius bed showing varved siltstones alternating with graded fine-grained sandstones; B) photomicrograph of Jamoytius bed varved siltstone showing lower silt overlain by organic-rich silt cap; C) view of outcrop of fine-grained quartz-sandstones overlying Jamoytius bed; D) Photomicrograph of fine-grained sandstone from C, dominated by subangular quartz grains with rare plagioclase felspar, in carbonate cement.

3. Discussion

The Jamoytius reconstructions with a terminal mouth suggest a filter-feeder or a detritus-feeder, analogous to larval lampreys (Denison, 1961) (Figure 1 C, E) and possibly to the Loganelliform thelodonts with which it is associated: the latter are interpreted as pelagic slow swimmers in open water (Ferrón and Botella, 2017). Larval lampreys can feed on highly concentrated food suspensions so thick that they border on organic deposits (Mallat, 1984, 1985). Jamoytius, however, lacks any obvious adaptations to suspension feeding (Lammons, 2009), and the more likely anterior ventral position of the mouth indicates particulate feeding or grazing (Oleh, 2018).

A bottom detritus feeding life style proposed by Parrington (1958) is unlikely given the anoxic bottom inferred from sedimentology, and the hypocercal tail which would give lift to a fish whose morphology also suggests an active lifestyle (Kermack, 1943). Jamoytius, in fact, like other euphaneropids, resembles elongate arrow-like bony fish, like pike (Esox spp.) and barracuda (Sphyraena spp.), with posterior dorsal and anal fins, which assist the tail in bursts of rapid acceleration, but are inefficient at steady swimming (Fletcher et al., 2014). Ritchie (1968) considered that its highly developed metamerism (a linear series of body segments fundamentally similar in structure) and large eyes of Jamoytius indicated a very active, fast-swimming vertebrate.

Sedimentological and palaeoecological characteristics of the associated biota indicate that Jamoytius lived in a brackish water environment in which the bottom waters and sediments were anoxic, and inhospitable to benthos and a predominantly planktonic and nektonic biota lived only in the overlying oxic waters. A benthonic mode of life for any of the Jamoytius association organisms is unlikely. Jamoytius and its likely euphaneropid sisters, despite their possibly autapomorphic, elongated branchial basket, could be plausible stem lampreys (Janvier, 2008). The inferred herbivorous life style of Jamoytius may not be too far apart from that of living secondarily non-parasitic lampreys. Petromyzids may initially have evolved as microphagous filterers or herbivorous organisms and only later developed ectoparasitic modes of life (Strahan, 1963; Mallatt, 1984, 2023).

4. Conclusions

Jamoytius is associated with a low-diversity dominantly nektonic detritus and herbivorous fauna living over an anoxic bottom, at the transition from a marine-influenced, probably brackish-water, deep-water basin to a shallower-water, less saline and likely freshwater basin. Jamoytius was likely either a free-living surface feeding detritivore/herbivore. The paleoenvironment in which it lived compares well with that of living non-parasitic lampreys. Though its immediate descendants seem to be parasitic, this ectoparasitic mode of life may have evolved from ancestral microphagous filterers or herbivores.