Type species

Earltonella fredricksi LoDuca, n. gen. n. sp., by monotypy, from the Earlton Formation (Llandoverian) at the Dionne Concrete Products quarry near Earlton, Ontario.

Diagnosis

As for the type species by monotypy.

Occurrence

Silurian (Llandoverian, Telychian); Lake Timiskaming area, Ontario, Canada (Laurentia paleocontinent).

Etymology

In reference to the geographic origin of the material, near the town of Earlton, Ontario, Canada.

Remarks

Earltonella LoDuca, n. gen. shares key characteristics with the early Paleozoic noncalcified macroalga genera Buthograptus Hall, Reference Hall1861, and Menieria Wang et al., Reference Wang, Jin and Zhan2014. In particular, all three taxa are characterized by fronds consisting of a central axis bearing elongate pinnules. Buthograptus, which is known only from the Upper Ordovician Platteville Formation in southwestern Wisconsin and northern Illinois (LoDuca, Reference LoDuca2019; Kolata, Reference Kolata2021), includes the species B. laxus Hall, Reference Hall1861, B. gundersoni LoDuca, Reference LoDuca2019, and B. meyeri LoDuca, Reference LoDuca2019. Buthograptus was recently redescribed on the basis of new material and interpreted as a bryopsidalean alga with fronds similar to those of living Caulerpa (LoDuca, Reference LoDuca2019). The fronds of Buthograptus are characterized by the same basic architecture as those of Earltonella LoDuca, n. gen., these consisting of elongate pinnules in opposite to subopposite arrangement along the supporting central axis, with the vertical spacing between adjacent pinnules (0.2–0.4 mm for B. laxus; 0.7–1.8 mm for B. meyeri) overlapping with the range present within individual fronds of the new genus (0.3–0.8 mm). Earltonella LoDuca, n. gen. is distinguished from Buthograptus on the basis of pinnule morphology. Specifically, the pinnules of Buthograptus are comparatively robust (widths of 0.4–0.7 mm vs. 0.15 mm) and, crucially, these never show branching. Earltonella LoDuca, n. gen. also differs from Buthograptus in that no specimens attributed to the latter show a stolon (LoDuca, Reference LoDuca2019). The arrangement of multiple fronds in series on slabs that bear the holotypes of B. laxus and B. meyeri (see LoDuca, Reference LoDuca2019, fig. 1.1, 1.8) is, however, suggestive of such a structure. A further difference between the new genus and Buthograptus concerns the details of pinnule spacing within a given frond. Specifically, in the new genus, this decreases by a factor of nearly three from the base to the top of a frond, whereas in Buthograptus this spacing remains essentially uniform within a particular frond.

The monotypic genus Menieria was erected by Wang et al. (Reference Wang, Jin and Zhan2014) on the basis of M. minuta Wang et al., Reference Wang, Jin and Zhan2014, from the Silurian (Llandoverian, Aeronian) Gun River Formation of Anticosti Island. This taxon differs from Earltonella LoDuca, n. gen. in that the “central axis,” as described by Wang et al. (Reference Wang, Jin and Zhan2014, p. 361), is wider than the stolon of the new genus (1.0–1.6 vs. 0.7 mm) and bears fronds with highly irregular spacing. Differences are also evident with regard to the pinnules. Specifically, those of Menieria are larger in terms of both length (2.4–9.1 vs. 1.6–4.0 mm) and distal width (0.6–1.0 vs. 0.15 mm), with the latter difference being particularly pronounced, and show a different overall form, with a marked distal expansion, by roughly a factor of two. In addition, as with Buthograptus, the pinnules of Menieria never show branching. Menieria further differs from the new genus in the phyllotaxis of both the fronds and the pinnules. Although these give the appearance of having a distichous alternate arrangement, after careful observation, Wang et al. (Reference Wang, Jin and Zhan2014, p. 361) concluded that “lateral branches are attached to the central axis with a helical phyllotaxis” and determined the same arrangement for the pinnules. On the basis of the aforementioned differences, Earltonella LoDuca, n. gen. is regarded as distinct from Menieria. It must be noted, however, that the high degree of similarity in thallus morphology among Menieria, the new genus, and Buthograptus suggests that all three taxa share a very close evolutionary relationship.

Noncalcified stoloniform thalli somewhat similar to that of Earltonella LoDuca, n. gen. are also characteristic of Parallelphyton Wu and Zhao in Wu et al., Reference Wu, Zhao, Tong and Yang2011, from the Cambrian Kaili Biota of South China and an unnamed form with a similar thallus morphology from the Neoproterozoic of South China (Wu et al., Reference Wu, Zhao, Tong and Yang2011; Ye at al., Reference Ye, Tong, Xiao, Zhu, An, Tian and Hu2015; Bykova et al., Reference Bykova, LoDuca, Ye, Marusin, Grazhdankin and Xiao2020). These, however, develop markedly simpler thalli than Earltonella LoDuca, n. gen., with the erect parts consisting only of unbranched cylindrical elements (analogous to central axes without pinnules). The nonbiomineralized Cambrian taxon Margaretia Walcott, Reference Walcott1931, which had been considered as a stoloniform Caulerpa-like green alga (Conway Morris and Robison, Reference Conway Morris and Robison1988), has since been reinterpreted as the tube of an enteropneust hemichordate (Nanglu et al., Reference Nanglu, Caron, Conway Morris and Cameron2016). The same relationship has been proposed by Fatka and Vodička (Reference Fatka and Vodička2022) for the similar form Krejciella Obrhel, Reference Obrhel1968b, from the Ordovician of the Prague Basin, which was previously interpreted as an alga by Havlíček et al. (Reference Havlíček, Vaněk and Fatka1993). These taxa differ markedly from Earltonella LoDuca, n. gen. in having much wider axes (4–20 mm vs. 0.7 mm) and by the complete lack of pinnate fronds.

With regard to calcareous macroalgae, there are no taxa, living or fossil, similar to Earltonella LoDuca, n. gen. This is not surprising, given that heavy calcification in marine siphonous green macroalgae appears to be restricted to forms with densely packed side branches, particularly those that develop a cortex (Borowitzka, Reference Borowitzka, Leadbeater and Riding1986). Such an architecture produces, in the microenvironment between the branches, conditions conducive to the precipitation of calcium carbonate (Pentecost, Reference Pentecost and Riding1991). Calcareous bryopsidalean algae with complex thalli of this sort are known from the Silurian and include Palaeoporella Stolley, Reference Stolley1893, Dimorphosiphon Høeg, Reference Høeg1927, Maslovina Obrhel, Reference Obrhel1968a, Paralitanaia Mamet and Préat, Reference Mamet and Préat1985, and Vitinellopsis Vachard, Bucur, and Munnecke, Reference Vachard, Bucur and Munnecke2022.

Holotype

ROMIP 66270.1 (Fig. 2.1) from the Earlton Formation (Llandoverian) in the Dionne Concrete Products quarry, Lake Timiskaming area, Ontario, Canada (47.744004°N, 79.818975°W).

Figure 2.

Earltonella fredricksi LoDuca, n. gen. n. sp., Earlton Formation (Llandoverian), Dionne Concrete Products quarry, Earlton, Ontario: (1) holotype, ROMIP 66270.1; arrows indicate detailed views in Figure 4.2, 4.3; (2) 66270.2; (3) 66270.3. Scale bars = 10 mm.

Diagnosis

Thallus noncalcified, comprising an elongate and creeping stolon (runner) bearing pinnate fronds at semi-regular intervals; stolon cylindrical; fronds consisting of a central axis (rachis) with delicate pinnules arrayed in opposite to subopposite fashion; pinnule form roughly cylindrical but slightly increasing in diameter distally, bifurcated at distal end, the resulting divisions being thin and hairlike; stolon, central axes, and pinnules without segmentation or serration.

Description

Thallus of holotype, ROMIP 66270.1, noncalcified, preserved as a carbonaceous compression on bedding plane surface, oval in gross outline, 110 mm in length, 48 mm in width, comprising an elongate and creeping central stolon (runner) bearing pinnate fronds typically 18–22 mm in length, the latter in the flattened specimen oriented at roughly right angles to the stolon in the central part of the thallus but at progressively smaller angles toward the inferred thallus end, with those at the extreme end running nearly parallel to stolon (Figs. 1, 2.1, 3). Stolon cylindrical with fairly smooth margins (glabrous), hollow, 0.7 mm wide, with a subtle zigzag form along its length, the inflections spaced at intervals of ~2 mm, with fronds extending from both sides in a roughly distichous alternating pattern (Figs. 2.1, 3, 4.1). Fronds consisting of a central axis (rachis) with pinnules arrayed in opposite to subopposite fashion (Fig. 3, 4.2. 4.3). Central axis cylindrical, with a width of 0.3 mm proximally, expanding to a width of 0.4–0.5 mm along most of its length, rounded at tip, margin smooth, lacking cross-walls or other evidence of segmentation (Fig. 4.2–4.4). Pinnules continuous with central axis, without evidence of cross-walls at junctions (Fig. 4.4), in lower and middle parts of frond meeting central axis at an angle of ~80°, this angle progressively decreasing along upper part of frond, reaching a minimum of 40–50° along uppermost part of frond (Fig. 4.2, 4.3); vertically adjacent pinnules separated by intervals of 0.8 mm in lower part of frond, this distance decreasing to 0.3 mm in uppermost part of frond. Primary pinnule element roughly cylindrical, without pronounced basal constriction, increasing slightly in diameter from 0.11 mm at base to 0.15 mm at distal end and with a length of 1.1–1.3 mm (Fig. 4.2–4.4), distal end bifurcated, secondary elements hairlike, with a width of 0.08 mm and having a length roughly comparable to that of primary element or slightly longer, diverging at an angle of 20–40° (Fig. 4.2–4.4); primary elements in lower and middle part of frond fairly straight along their length but with subtle upward curvature, those along uppermost part of frond show strong upward curvature and extend well above tip of central axis (Fig. 4.3); pinnule margins fairly smooth, without serrations (Fig. 4.2–4.4). ROMIP 66270.2, 66270.3, and 66271 generally similar to holotype, but overall thallus form for 66270.2 and 66270.3 less elongate (Figs. 1, 2.2, 2.3, 5.1–5.5). SEM images of fronds show ‘mudcracked’ texture on central axes (Fig. 6.1, 6.2, 6.6) and, more subtly, on pinnules (Fig. 6.4), fairly smooth margins for both central axes and pinnules (Fig. 6.1–6.4), a rounded form for central axis tip (Fig. 6.6), and an expanded region at or near the base of some, but not all, pinnules (Fig. 6.7, 6.8), this appearing in reflected light images as a distinctly darkened area (Fig. 6.5, 6.9).

Figure 3.

Detail of part of the holotype of Earltonella fredricksi LoDuca, n. gen. n. sp., Earlton Formation (Llandoverian), Dionne Concrete Products quarry, Earlton, Ontario, ROMIP 66270.1. Scale bar = 2 mm.

Figure 4.

Details of holotype of Earltonella fredricksi LoDuca, n. gen. n. sp., Earlton Formation (Llandoverian), Dionne Concrete Products quarry, Earlton, Ontario, ROMIP 66270.1: (1) stolon, ring-shaped cross-section at far right (arrow) indicates hollow structure; (2) fronds at right arrow in Figure 2.1, arrows mark well-preserved pinnules that show bifurcated tips; (3) frond at left arrow in Figure 2.1; (4) enlargement of area indicated by arrow in (3) showing pinnules in continuity with central axis. Scale bars are (1–3) 2 mm; (4) 0.5 mm.

Figure 5.

Earltonella fredricksi LoDuca, n. gen. n. sp., Earlton Formation (Llandoverian), Dionne Concrete Products quarry, Earlton, Ontario, ROMIP 66271: (1) complete thallus; (2) detail of frond at upper right arrow in (1); (3) detail of frond at lower right arrow in (1); (4) detail of frond at left arrow in (1); (5) detail of frond at arrow in (3) showing pinnules with bifurcations. Images in (2–5) obtained using cross-polarized light. Scale bars are (1) 10 mm; (2, 3) 1 mm; (4, 5) 0.5 mm.

Figure 6.

Fronds of Earltonella fredricksi LoDuca, n. gen. n. sp. from the holotype slab, ROMIP 66270: (1) composite SEM-BSE image of area at left arrow in (5); (2) detail of area at arrow in (1) showing mudcracked texture on central axis; (3) SEM-BSE image of pinnule at lower right arrow in (5); (4) SEM-BSE image of pinnule at upper right arrow in (5); (5) reflected light image of distal part of frond, 66270.4; (6) SEM-BSE image of area at upper right arrow in (9); (7) SEM-BSE image of area at left arrow in (9); (8) SEM-BSE image of area at lower right arrow in (9); (9) reflected light image of distal parts of two fronds, 66270.5. Scale bars are (1, 3, 6–8) 0.2 mm; (2, 4) 0.1 mm; (5, 9) 2 mm.

Etymology

Named in honor of Dr. Walter Fredricks, Professor Emeritus of Biology at Marquette University, and his clan.

Remarks

This form is known only from the Dionne Concrete Products quarry and, with specimens known from only two slabs, is much less abundant at that locality than Thalassocystis striata. One of these slabs bears a single specimen (Fig. 5.1), the other appears to show several separate thalli (Figs. 1, 2.1–2.3). On the latter, however, the distribution of the material, together with the potential for concealment of some parts beneath matrix, leaves open the possibility that all or much of this material belongs to a single large thallus. Rhizoidal features were not observed among any of the material. It is possible, however, that this aspect of thallus morphology is also concealed by matrix.

SEM images show a distinctive ‘mudcracked’ surface texture on the central axes and, to a lesser extent, on the pinnules (Fig. 6.2, 6.4, 6.6). A similar surface texture was reported for Buthograptus laxus (LoDuca, Reference LoDuca2019, fig. 3.3). SEM images also clearly reveal the presence of an expanded region at or near the base of some, but not all, pinnules (Fig. 6.7, 6.8). This, too, has been reported for specimens of B. laxus (Whitfield, Reference Whitfield1894, pl. 1, fig. 3; LoDuca, Reference LoDuca2019, fig. 3.3), and it is evident in specimens of Buthograptus meyeri (LoDuca, Reference LoDuca2019, fig. 1.8). For all of these taxa, however, it is not evident on all pinnules, including those on a given frond, and it is entirely absent on Buthograptus gundersoni and Menieria minuta. The significance of this feature remains unclear.

The thallus of Earltonella fredricksi LoDuca, n. sp., in broad terms, resembles those produced by the living bryopsidalean alga Caulerpa. In particular, as with the well-known extant taxa C. sertularioides Howe, Reference Howe1905 (Fig. 7) and C. taxifolia Agardh, Reference Agardh1817, the thallus consists of an elongate creeping cylindrical stolon that bears a series of pinnate, complanate fronds. In addition, as with all bryopsidalean algae, the new species appears to have had a siphonous thallus organization. For the stolon, this is indicated by its hollow nature, as conveyed by the ring-like form of this feature in cross-section view (Figs. 3, 4.1), and for the entire thallus this is indicated by a lack of evident cross walls or segmentation. Trabeculae (i.e., irregular, strut-like cell wall ingrowths), which are ubiquitous for living Caulerpaceae and traverse the cell wall lumen to provide structural support, have not been observed in the new species, but it is doubtful that such internal structures would be evident in compressed fossil specimens. In terms of pinnule morphology, the new species shows pinnules with bifurcated tips, and pinnules of this nature are known for Caulerpa, including C. taxifolia. Key morphometric aspects also fall within the range known for Caulerpa, with width values for the stolon, central axes, and pinnules, as well as frond lengths, being comparable to those displayed by smaller species within the genus, albeit only about one-half to one-third those observed in the aforementioned C. taxifolia and C. sertularioides. In addition, the fairly wide pinnule spacing is comparable to that known for C. ashmeadii Harvey, Reference Harvey1858. No species of Caulerpa, however, shows pinnules with an overall shape that precisely matches that of the new species, particularly with regard to the relatively long secondary elements. Earltonella fredricksi LoDuca, n. sp. also appears to differ from all known Caulerpa species in the manner in which the fronds are arranged along the stolon. In Caulerpa, these are arranged in a single row along the top. In the new species, by contrast, the fronds seem to emerge from the sides of the stolon, in an alternating pattern. Such an arrangement for the new species must, however, be regarded as equivocal, owing to the flattened condition of the fossil thalli. In this regard, if the fronds were arranged largely upright in a single row along the top, but were tilted relative to perpendicular in an alternating pattern down the length of the stolon, a rapid influx of mud from above could have produced the pattern of frond distribution shown by the specimens. The orientation of some of the associated specimens of Thalassocystis striata is consistent with burial dynamics of this sort (e.g., Fig. 8.2). Moreover, flattening in this manner could have distorted the stolon into the zigzag form evident in the specimens (Figs. 3, 5.1). Some herbarium specimens of Caulerpa would appear to be comparable in these regards (Fig. 7).

Figure 7.

Herbarium specimen of Caulerpa sertularioides Howe, Reference Howe1905, University of Michigan Herbarium, 682353. Scale bar = 20 mm.

Figure 8.

Thalassocystis striata Taggart and Parker, Reference Taggart and Parker1976, Earlton Formation (Llandoverian), Dionne Concrete Products quarry, Earlton, Ontario: (1) two thalli preserved in lateral view, ROMIP 66272.1 (top) and 66272.2 (bottom); (2) thallus preserved in overhead view, ROMIP 66273A; (3) counterpart of specimen in (2), ROM 66273B. Scale bars = 10 mm.

The gross morphology and carbonaceous composition of Earltonella fredricksi LoDuca, n. sp. also invite comparisons with hydroid colonies, particularly those produced by extant aglaopheniids (feather hydroids), and with the pinnule-bearing arms of crinoids. With regard to a hydroid affinity, the hydrocladia of living hydroids can resemble the general form of the pinnules in the new species, including the development of distal bifurcation (e.g., Henry and Kenchington, Reference Henry and Kenchington2004, fig. 1), and fossils of early Paleozoic age with a form somewhat resembling the pinnate fronds of the new species have been described as hydroids. Among these are the Ordovician taxon Webbyites Kraft, Kraft, and Prokop, Reference Kraft, Kraft and Prokop2001, and Plumalina Hall, Reference Hall1858, the latter known mainly from the Devonian but also reported from the Silurian (Wenlockian) of New York (Muscente and Allmon, Reference Muscente and Allmon2013). Nonetheless, a hydroid affinity for the new species is regarded as unlikely for several reasons. First, specimens of the new species show no evidence of hydrotheca, including in SEM images. Specifically, the pinnule margins appear fairly smooth and continuous, and their surfaces lack sculpture or patterning that might point to the presence of such structures (Fig. 6.3, 6.4). Second, the hydrocladia of Plumalina, although lacking preserved hydrotheca or polyps, show in some cases distinctive box-like modules with a central pore or groove, which have been interpreted as polyp bases (Sass and Rock, Reference Sass and Rock1975; Muscente and Allmon, Reference Muscente and Allmon2013). The pinnules of the new species, however, do not show features along these lines. Finally, the strongly upcurved nature of the distal (uppermost) pinnules and the lack of internodes on the central axes would seem to be inconsistent with a hydroid affinity (LoDuca, Reference LoDuca2019; Song et al., Reference Song, Ruthensteiner, Lyu, Liu, Wang and Han2021). With regard to a crinoid affinity, decalcified examples of pinnule-bearing crinoid arms preserved as carbonaceous material that occur in direct association with a rich, noncalcified macroalgal flora in the Llandoverian (Aeronian) of Estonia (Tinn et al., Reference Tinn, Meidla, Ainsaar and Pani2009; Ausich et al., Reference Ausich, Wilson and Tinn2020) have a gross morphology somewhat similar to the individual fronds of the new species. The pinnules of crinoids, however, are not known to show distal bifurcation. In addition, the new species lacks key crinoid features (e.g., a calyx or a stem constructed of columnals).

A terrestrial habitat for the new species can be excluded by its direct association with marine taxa, including trilobites and brachiopods, and by its general lack of disarticulation and fragmentation, indicating that the material was buried in situ or very nearly so. This, then, serves to effectively eliminate the possibility that it represents an early non-vascular land plant, such as a liverwort.

In consideration of the foregoing, together with co-occurrence with the noncalcified bryopsidalean alga Thalassocystis striata and broad similarity to the noncalcified Late Ordovician–Early Silurian macroalga taxa Buthograptus and Menieria, it is concluded that Earltonella fredricksi LoDuca, n. sp. is best regarded as a bryopsidalean alga with a stoloniform thallus (sensu LoDuca et al., Reference LoDuca, Bykova, Wu, Xiao and Zhao2017) roughly comparable to that of frondose forms of Caulerpa, such as C. taxifolia, although it must be noted that an animal affinity cannot be entirely excluded on the basis of the available data. Given the uncertainties regarding precise arrangement of the fronds along the stolon, and in consideration of the considerable age of the material, the new species is not assigned to Caulerpa. In this regard, molecular studies cast doubt on a close phylogenetic affinity between the new species and Caulerpa. According to the chronogram provided in Draisma et al. (Reference Draisma, Prud'homme van Reine, Sauvage, Belton, Gurgel, Lim and Phang2014), Caulerpaceae did not diverge from the sister-clade Pseudochlorodesmis until sometime after the Devonian, between the late Carboniferous and Late Triassic. Molecular clock analyses do, however, readily accommodate a bryopsidalean affinity for Earltonella fredricksi LoDuca, n. sp. (Verbruggen et al., Reference Verbruggen, Ashworth, LoDuca, Vlaeminck, Cocquyt, Sauvage, Zechman, Littler, Littler, Leliaert and De Clerk2009; Draisma et al., Reference Draisma, Prud'homme van Reine, Sauvage, Belton, Gurgel, Lim and Phang2014; Del Cortona et al., Reference Del Cortona, Jackson, Bucchini, Van Bel and D'hondt2020) and, as noted above, both calcified and noncalcified bryopsidalean algae with diverse and complex forms have been described from Ordovician and Silurian strata.

Considered as a bryopsidalean alga and in view of the molecular evidence, it is conceivable that Earltonella fredricksi LoDuca, n. gen. n. sp. belongs to an extinct lineage within this group, together with Buthograptus and Menieria and, perhaps, Parallelphyton. A similar scenario was raised in regards to the Silurian Codium-like taxa Thalassocystis and Inocladus (LoDuca et al., Reference LoDuca, Swinehart, LeRoy, Tetreault and Steckenfinger2021). The possibility of such a scenario, in general, is supported by well-documented examples of convergent evolution of complex thallus form among extant Bryopsidales, including between Codium and Pseudocodium and even within Caulerpa itself in terms of frond morphology where, as noted by Draisma et al. (Reference Draisma, Prud'homme van Reine, Sauvage, Belton, Gurgel, Lim and Phang2014, p. 1029), “Vesiculate, terete, and flattened ramuli [pinnules] all evolved multiple times” (see also Famà et al., Reference Famà, Wysor, Kooistra and Zuccarello2002).

Considered in broader terms, the appearance of complex Caulerpa-like thalli within Bryopsidales during the Late Ordovician–Early Silurian is part of a major revolution in macroalgal thallus morphology during this time documented by LoDuca et al., Reference LoDuca, Bykova, Wu, Xiao and Zhao2017. As suggested by that study, the Ordovician macroalgal revolution may signal a defensive response to intensified grazing pressure by meso-herbivores that arose as an outcome of the Great Ordovician Biodiversification Event (GOBE). Specifically, ‘complicating’ a simple siphonous thallus, consisting only of an unbranched cylindrical siphon, by developing dense outgrowths of lateral appendages (secondary elements) yields benefits by providing (1) a physical barrier (‘palisade’) between grazers and critical central parts of the thallus, and (2) a measure of anatomical compartmentalization, which in turn allows the cell walls of the secondary elements to be relatively thin for enhanced assimilation efficiency without incurring marked costs in survivorship (LoDuca and Behringer, Reference LoDuca and Behringer2009). Numerous variations on this theme originated among siphonous green macroalgae during the GOBE, of which complex Caulerpa-like thalli are but one example, along with a proliferation of heavily calcified thalli (LoDuca et al., Reference LoDuca, Bykova, Wu, Xiao and Zhao2017).

Type species

Thalassocystis striata Taggart and Parker, Reference Taggart and Parker1976, by monotypy.

Holotype

FMNH 43982 from the Schoolcraft Formation (Llandoverian) in an exposure along the Lake Michigan shoreline two miles east of Manistique, Michigan, U.S.A (Taggart and Parker, Reference Taggart and Parker1976, fig. 1).

Description

Thallus of ROMIP 66272.1 noncalcified, unsegmented, extending 46 mm above holdfast, comprising dichotomously branched axes; branching highly irregular (anisotomous); individual axes with a length of 22–27 mm, these in some cases narrow, with a maximum width of 3 mm, but in other cases broad and obovate with a maximum width of 7 mm; proximal parts of axes with a width of 1–3 mm; axis terminations obtuse; angle between axes at bifurcation typically between 30–40°; medullary tubes with a width of 0.16 mm, poorly preserved; cortical tubes not evident; holdfast small relative to thallus, 7 mm wide, 11 mm long, consisting of a loose array of thin filaments, each having a width of 0.3–0.4 mm (Fig. 8.1). Thalli of ROMIP 66272.2 and 66273 generally similar to 66272.1, but 66272.2 somewhat smaller, with a height of 32 mm (Fig. 8.1), and 66273 preserved in overhead as opposed to lateral view (Fig. 8.2, 8.3). ROMIP 66274 (Fig. 9.1) shows only relatively narrow axes, some with a papillate surface texture, others with well-preserved medullary and cortical tubes (Fig. 9.2); some axes show a strong degree of 3D preservation (Fig. 9.3). ROMIP 66272.3 shows in lower part of thallus well-preserved cortical tubes with a cylindrical form, a length of 0.5–1.0 mm, and a width of 0.1 mm, these arrayed along the supporting medullary tube in a single row and oriented to it at an angle of ~90°, with spacing between adjacent tubes of less than one tube width (Fig. 9.4, 9.5). ROMIP 66272.3 and 66275 show well-preserved holdfasts (Fig. 9.4, 9.6, 9.7). ROMIP 662726 appears to show a greater amount of decay relative to the other specimens (Fig. 9.8). Thalli from Wabi Point (Figs. 10.1–10.7, 11) generally similar to aforementioned Dionne quarry thalli in terms of branching details, surface texture, and internal features, and with a comparable range of thallus sizes and axis forms. Some show a pronounced degree of 3D preservation (Fig. 10.4).

Figure 9.

Thalassocystis striata Taggart and Parker, Reference Taggart and Parker1976, Earlton Formation (Llandoverian), Dionne Concrete Products quarry, Earlton, Ontario: (1) complete thallus, ROMIP 66274; (2) detail of area at lower arrow in (1) showing medullary and cortical tubes; (3) detail of area at upper arrow in (1) showing axis with strong degree of 3D preservation; (4) thallus with well-preserved holdfast, ROMIP 66272.3; (5) detail of area at arrow in (4) showing medullary and cortical tubes; (6) thallus with sparse branching, ROMIP 66275; (7) detail of thallus in (6) showing holdfast; (8) complete thallus, ROMIP 66276. Scale bars are (1, 4, 6, 8) 10 mm; (2, 5) 1 mm; (3, 7) 2 mm.

Figure 10.

Thalassocystis striata Taggart and Parker, Reference Taggart and Parker1976, Earlton Formation (Llandoverian), Wabi (Dawson) Point, Ontario: (1) nearly complete thallus, ROMIP 66277; (2) nearly complete thallus, MPEP 1501.2a; (3) nearly complete thallus, MPEP 1501.9a; (4) nearly complete thallus showing a single dichotomy and a strong degree of 3D preservation, MPEP 1501.20; (5) an axis showing medullary tubes, MPEP 1501.14a; (6) nearly complete thallus, MPEP 1501.30b; (7) an axis showing medullary tubes, MPEP 1501.16a (images in 2–7 courtesy of Mario Cournoyer). Scale bars are (1–4, 6, 7) 10 mm; (5) 2.5 mm.

Remarks

This species is locally abundant at both of the Timiskaming localities, with dozens of thalli having been collected from each. In all key morphological respects, the Timiskaming material is entirely consistent with Michigan material, as described by LoDuca et al. (Reference LoDuca, Swinehart, LeRoy, Tetreault and Steckenfinger2021). This includes (1) thallus branching pattern; (2) axis shape, size, and surface texture; (3) details of the medullary and cortical tubes, including shape, size, and arrangement; and (4) holdfast configuration and size. In addition, all key variations among Michigan specimens are fully represented among the Timiskaming material. In this regard, as with all of the Michigan localities, some of the individual thalli from both of the Timiskaming localities show a mix of narrow axes and wider, distinctly bulbous axes (compare Figs. 8.1, 10.2 with LoDuca et al., Reference LoDuca, Swinehart, LeRoy, Tetreault and Steckenfinger2021, fig. 15.8), whereas others comprise only narrow axes (compare Figs. 9.1, 10.3 with LoDuca et al., Reference LoDuca, Swinehart, LeRoy, Tetreault and Steckenfinger2021, fig. 15.1). In addition, thalli with limited branching, in some cases comprising just a single dichotomy, are known from all occurrences (compare Fig. 9.6 with LoDuca et al., Reference LoDuca, Swinehart, LeRoy, Tetreault and Steckenfinger2021, fig. 16.2, 16.6).

Taphonomic details are also comparable between the Timiskaming and Michigan material. In particular, specimens in both areas usually show the thallus in lateral view, although overhead views are known (compare Fig. 8.2 with LoDuca et al., Reference LoDuca, Swinehart, LeRoy, Tetreault and Steckenfinger2021, fig. 16.10), and they tend to be fairly flat, although examples with a strong degree of 3D preservation are occasionally encountered (Figs. 9.3). The latter characteristic is particularly well developed among thalli from the Michigan type locality as well as the Wabi Point locality (Fig. 10.4). Specimens from the Michigan type locality are also similar to those from the Wabi Point locality in that detached axes (Figs. 10.4, 10.5, 11) are not uncommon, whereas specimens from the other localities tend to be complete or nearly so. Finally, as with the Michigan material, some of the Timiskaming specimens show only surficial details of the axes, including in some cases a distinctive papillate texture (Figs. 8.2, 8.3, 9.1, 9.6, 10.6), whereas others show internal details pertaining to the siphons (Figs. 9.2, 9.4, 9.5, 10.3, 10.5, 10.7). With regard to preservation of internal features, the Timiskaming specimens are more similar to those from the Manistique area than they are to those from the Carmeuse quarry near Gulliver, Michigan, in that the best specimens from the latter show these features in greater detail compared to specimens from all of the other localities.

Figure 11.

Thalassocystis striata Taggart and Parker, Reference Taggart and Parker1976, Earlton Formation (Llandoverian), Wabi (Dawson) Point, Ontario. Large slab bearing complete and partial thalli, MPEP 1501.32 (image courtesy of Mario Cournoyer). Scale bar = 20 mm.