4 The Indian Point Formation is a lithologically diverse unit consisting mainly of moderately to strongly calcareous mudstone, fine-grained sandstone, and wackestone, all of which commonly contain comminuted fossil debris. Minor lithotypes include coarse-grained sandstone, wackestone– packstone biostromes, limestone conglomerate, and polymictic conglomerate (Wilson et al. 2004). Fossils include stromatoporoids, brachiopods, trilobites, and crinoids; some sections contain abundant rugose corals. In the Flatlands area, including the study site, the dominant lithological association comprises greenish-grey, medium-to thick-bedded (up to roughly 1 m), locally fossiliferous, moderately to strongly calcareous mudstone with local thin beds of calcareous siltstone and fine-grained, parallel-laminated sandstone (Fig. 2). The fossils described herein were recovered in October 2011 from tumbled-out blocks of calcareous mudstone along a cut through the Indian Point Formation on the south side of Highway 134, opposite Island View Road, approximately 1 km east of Flatlands (Fig. 3). Additional fossils in the same blocks include scarce brachiopods, trilobites, and crinoid fragments. No trace fossils are present within the blocks.

5 Spores and acritarchs in the Indian Point Formation indicate a Pridolian to middle Lockhovian age (Wilson et al. 2004), as do brachiopod taxa described from the unit across the river from the study area, in Gaspé, Quebec (Bourque and Lachambre 1980). The Indian Point Formation is intruded by the Squaw Cap Felsite, which has yielded a middle Lochkovian U-Pb (zircon) age of 415.0 ± 0.5 Ma (Wilson et al. 2004), and provides an upper age limit for the unit.

6 The lithology, sedimentology, and paleontology of the Indian Point Formation indicate deposition in a near-shore marine-shelf environment (Wilson et al. 2004). Fully marine conditions are indicated by the presence of brachiopods, crinoids, corals, stromatoporoids, and trilobites, but palynological evidence indicates some input from terrestrial sources (Wilson et al. 2004), thereby pointing to a near-shore setting. The rarity of cross-laminated or ripple-laminated beds suggests deposition below fair-weather wave base. Nonetheless, water depth need not have exceeded a few metres because sea-floor topography related to reefs in the West Point Formation may have acted to baffle wave and current energy. Lack of trace fossils and preservation of organic material suggest low oxygen conditions near the sea floor. A complete lack of graded beds, sole markings, and convolute lamination argue against deposition as turbidite beds. Marine sedimentation was relatively short-lived, as a Lochkovian–Emsian regression began with local deposition of conglomerate and limestone (wackestone and packstone) at the top of the formation, and continued with subaerial volcanism of the conformably overlying Val d’Amour Formation (Dalhousie Group; Wilson et al. 2005).

7 Remarks: Twelve species have been assigned to Medusaegraptus, two with uncertainty. Seven of these, including one of the species assigned to the genus with uncertainty, are known only from the Ordovician of China (Mu et al. 1979; Lin 1984). In a review of species assigned to the genus from North America and Europe by LoDuca (1990), two were considered not to be medusaegraptids. Of the remaining three species, all of which are Silurian, Medusaegraptus mirabilis and Medusaegraptusgraminiformis are known exclusively from North America, whereas Medusaegraptus similis is known only from the Czech Republic.

8 Material: NBMG 11062, NBMG 16275, NBMG 16276, NBMG 16277, NBMG 16279, NBMG 16280. The specimens are reposited in the Palaeontology Collection of the New Brunswick Museum (NBMG).

9 Occurrence: All specimens are from tumbled-out blocks of the Pridolian to middle Lockhovian Indian Point Formation exposed along the south side of Highway 134 opposite Island View Road, Flatlands, New Brunswick (NTS Map 21 O/15; 47°59.49’ N; 66°51.47’W) (Fig. 1).

10 Description: Specimens are preserved as black carbonaceous compressions with little or no relief. The thallus for specimen 11062/1 is 38 mm long and 14 mm wide at its widest point and comprises an undivided main axis surrounded by thin, unbranched laterals (Fig. 4A, E). The basal part of the thallus is missing. The main axis is 22.5 mm long and 1.7 mm wide; it is largely concealed by overlying laterals. Laterals along the upper part of the main axis form a distinct capitulum. The capitulum-forming laterals are 14 mm long; each maintains a nearly uniform width, expanding very slightly distally to a maximum width of 0.3 mm. Laterals along the lower part of the main axis are shorter and thinner, on average being 7 mm long and 0.15 mm wide. All laterals have abrupt, bluntly rounded terminations. Laterals are arranged around the main axis in an irregular (aspondyl) fashion. Reproductive structures are not evident. Specimen 16275 appears to represent only the uppermost part of a thallus and consists of a cluster of elongate laterals up to 20 mm in length (Fig. 4F). Lateral width increases slightly distally; most are 0.55 mm wide near the base, increasing gradually to 0.7 mm at the distal tips. Specimens 16279 and 16280 are about half the size of 11062/1 but are otherwise similar in terms of thallus architecture (Fig. 4C, D). Specimens on slabs 16276 and 16277 consist of main axes, each nearly 3 mm wide, largely stripped of laterals (Fig. 4G, H). Because the matrix is highly friable, none of the specimens was examined using scanning electron microscopy.

11 Remarks: In having unbranched lateral appendages of near-constant width arranged irregularly around an undivided central axis, the New Brunswick material accords in all respects with the emended diagnosis of Medusaegraptus by LoDuca (1990). Measurements for species currently assigned to the genus most similar to the New Brunswick material are provided in Table 1. Among these, only one, Medusaegraptus mirabilis, has dimensions for lateral width that overlap with the New Brunswick material. All of the other species are characterized by thinner laterals. In addition, the largest of the nearly complete New Brunswick specimens (11062/1) accords with the type material of Medusaegraptus mirabilis in terms of main axis width and lateral length, lateral density along the main axis, and development of a distinct capitulum (Fig. 4A, B). For 16279 and 16280, it is possible that these specimens represent a species of Medusaegraptus apart from Medusaegraptus mirabilis owing to their relatively small dimensions. Such a determination, however, would require additional material and so these small specimens are herein regarded as early growth stages of Medusaegraptus mirabilis. Specimens on slabs 16276 and 16277 are regarded as main axes of Medusaegraptus mirabilis largely stripped of laterals, a condition known for thalli of this species from the type area (LoDuca, 1990). The width of these specimens is consistent with main axis width for Medusaegraptus mirabilis, but exceeds that for all other species assigned to the genus.

12 A key aspect of the anatomy of dasycladalean algae is the siphonous nature of their thalli (Berger and Kaever, 1992). Ruedemann (1925) described the appendages of Medusaegraptus mirabilis as hollow. Such a structure could not be confirmed in a reexamination of the type material by LoDuca (1990), but additional material collected by one of us (STL) from the type locality after publication of that study suggests that the laterals are in fact thin-walled, hollow tubes without internal cross walls and that the main axis has a similar construction. Siphonous structure is not evident in the New Brunswick material assigned to this taxon but, like most specimens of Medusaegraptus mirabilis from the type locality, this is regarded to be a consequence of compression.