Systematic paleontology

CHELICERATA Heymons, 1901

EURYPTERIDA Burmeister, 1843

EURYPTERINA Burmeister, 1843

DIPLOPERCULATA Lamsdell, Hoşgör and Selden, 2013

CARCINOSOMATOIDEA Størmer, 1934

MEGALOGRAPTIDAE Caster and Kjellesvig-Waering in Størmer, 1955

Pentecopterus gen. nov.

LSID: urn:lsid:zoobank.org:act:803A3DD9-AF58-4ADC-AA64-CEA1AA051124

Pentecopterus decorahensis sp. nov.

LSID: urn:lsid:zoobank.org:act:37E2232A-70D7-48DD-9CE6-90B5214B5A0E

Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 23 and 24

Fig. 1 Pentecopterus decorahensis, prosomal ventral plate. a SUI 139914, posterior lobe of lateral portion of ventral plate. b SUI 139978, lateral portion of ventral plate showing carapace locking mechanism (arrowed). c SUI 139936, anterior portion of ventral plate including rostrum, retained on shale. d SUI 139936, posterior portion of ventral plate shown in Fig. 1c including linguoid projection, removed from sediment. e SUI 1139921, linguoid projection. f SUI 139917 part, ventral plate. g SUI 139917 counterpart, lateral portion of ventral plate. h SUI 139916, lateral portion of large ventral plate. Scale bars = 10 mm Full size image

Fig. 2 Pentecopterus decorahensis, SUI 139979 - prosomal ventral plate, rostrum and proximal limb podomeres. a Rostrum and lateral portion of ventral plate with overlying proximal portions of prosomal appendages II–VI, direct light. b Transmitted light. c Interpretive drawing: red = ventral plate, green = appendage II, blue = appendage III, yellow = appendage IV, pink = appendage V, purple = appendage VI, and gray = ventral carapace cuticle, MR = marginal rim, II–VI = appendages II–VI. Scale bars = 10 mm Full size image

Fig. 3 Pentecopterus decorahensis, SUI 139941 (holotype) - rostrum and linguoid posterior projection underlain by left and right prosomal appendages II and III. a Specimen. b Interpretive drawing: red = ventral plate, green = appendage II, and blue = appendage III, II-2–II-4 = appendage II podomeres 2–4, III-2 = appendage III podomere 2. The coxa are angled anteriorly and covered by the ventral plate, which can be peeled back to reveal their position. Scale bar = 10 mm Full size image

Fig. 4 Pentecopterus decorahensis, cuticular structures on prosomal ventral plate. a SUI 140009, lateral portion of prosomal ventral plate showing row of scales (arrowed) along the inner margin. b SUI 140009, prosomal ventral plate with terrace lines and ventral prosomal integument bearing setae. c SUI 140011, setae covering ventral prosomal integument. Each figure is oriented with the specimen anterior to the left. Scale bars = 1 mm Full size image

Fig. 5 Pentecopterus decorahensis, juveniles. a, b SUI 139963, prosomal appendages III and IV, (a) counterpart, (b) part. c SUI 139965, prosomal appendages II–IV. Scale bars = 10 mm Full size image

Fig. 6 Pentecopterus decorahensis, cuticular structures on prosomal appendages. a SUI 139963, juvenile appendage showing denticulations of podomere margin and dorsal row of scales. b SUI 139951, coxa of appendage III with dense covering of conical scales. c SUI 139952, fourth podomere of appendage III showing raised scales with apical follicle. d SUI 139913, coxa of appendage V exhibiting dense covering of small scales. e SUI 139945, coxa of appendage VI with large scales. f SUI 139949, third to fifth podomeres of appendage VI with scattered small scales. Scale bars = 1 mm Full size image

Fig. 7 Pentecopterus decorahensis, chelicera. a, b SUI 139952, complete chelicera. a Part. b Counterpart. c SUI 139939, dorsal view of chelicera. d SUI 139934, free finger. e SUI 139935, complete chelicera. f SUI 139983, free and fixed fingers. F 1 = fixed finger, F 2 = free finger. Scale bars = 10 mm Full size image

Fig. 8 Pentecopterus decorahensis, prosomal appendage II. a SUI 139919, fragmentary appendage showing moveable spines. b SUI 140018, enlarged fixed spine with serration. c SUI 139975, coxa with moveable endite and podomeres 2–3. d SUI 139969, small coxa with moveable endite. e, f SUI 139920, podomeres 3–7 showing moveable and fixed spines. All spines are in situ except for the large fixed spine originating from the fourth podomere, which is detached and angled anteriorly to the appendage. e Part, (f) Counterpart. II-1–II-7 = coxa and appendage podomeres 1–7, EN = endite, FS = fixed spines, MS = moveable spines. Scale bars for a–c, e–f = 10 mm, for d = 1 mm Full size image

Fig. 9 Pentecopterus decorahensis, prosomal appendage III. a SUI 102857, complete appendage lacking armature. b SUI 139953, podomeres 2–8 showing enlargement of fixed lateral spines on more distal podomeres. c SUI 139951, podomeres 1–5. d SUI 139948, complete appendage, podomeres 1–5 preserved as imprints. e SUI 139973, sixth podomere showing moveable and fixed spines. f SUI 139990, podomeres 7–8 in ventral view. g SUI 140013, podomeres 6–8 in ventral view. h SUI 140007, podomeres 6–8 in lateral view. i SUI 139925, gnathobase of coxa. j SUI 139952, podomeres 1–5. k SUI 139922, large podomere 6 displaying fixed spine. l SUI 139944, podomeres 7–8 with enlarged fixed spine of podomere 6. m SUI 139929, podomeres 7–8 with proximal region of fixed spine of podomere 6. n, o SUI 139930, podomeres 2–6. n Part, o Counterpart. III-1–III-8 = podomeres 1–8, GB = gnathobase, FS = fixed spine, MS = moveable spine, PR = podomere rotation. Scale bars = 10 mm Full size image

Fig. 10 Pentecopterus decorahensis, prosomal appendage IV. a SUI 139937, fragmentary appendage showing moveable spines. b SUI 140000, podomeres 2–3 showing development of spines on the ventral distal podomere margin. c SUI 139927, termination of appendage. d SUI 139926, most complete example of appendage known, comprising podomeres 5–8, and lateral portion of carapace with marginal rim. e SUI 140012, podomeres 6–8. f SUI 139928, fourth podomere showing moveable spines. g SUI 139938, podomere displaying ventral distal extension and development of marginal spines. h SUI 139940, podomeres 6–8. i SUI 139959, coxa showing development of ancillary spines surrounding gnathobase. j SUI 139946, coxa and second podomere. IV-5–IV-8 = podomeres 5–8, CP = coxal projection, DS = distal swelling, GB = gnathobases, MR = marginal rim. Scale bars = 10 mm Full size image

Fig. 11 Pentecopterus decorahensis, prosomal appendage V. a SUI 140014, podomeres 4–7. b SUI 139986, podomeres 6–9. c SUI 139989, podomeres 3–4. d SUI 139942, podomeres 1–5. e SUI 139913, coxa and podomeres 6–9 showing distal denticulation of podomeres. f SUI 139970, complete appendage. g SUI 139924, coxa. h SUI 139985, podomeres 4–6. i SUI 139912, appendage termination. j SUI 139991, podomeres 4–5. k SUI 139996, podomeres 4–5. l SUI 139968, podomeres 6–9. V-1–V-9 = podomeres 1–9, DD = distal denticulations, FS = fixed spine. Scale bars = 10 mm Full size image

Fig. 12 Pentecopterus decorahensis, prosomal appendage V. a SUI 139998, podomeres 2–9 showing distal podomere serrations and distribution of setal follicles. b SUI 140016, podomeres 2–7. V-2–V-9 = podomeres 2–9, DD = distal denticulation, FO = follicles, FS = fixed spine. Scale bars = 10 mm Full size image

Fig. 13 Pentecopterus decorahensis, prosomal appendage VI. a SUI 139945, coxa. b SUI 139987, damaged coxa. c SUI 139994, coxa with gnathobase compressed into ‘neck’. d SUI 139997, podomeres 5–6 showing distal denticulations. e SUI 139993, lateral portion of sixth podomere showing attachment point for preceding podomere. f SUI 139961, isolated sixth podomere. g SUI 139967, isolated sixth podomere. AP = attachment point, DD = distal denticulation, GN = gnathobase, NE = ‘neck’, VP = ventral projection. Scale bars = 10 mm Full size image

Fig. 14 Pentecopterus decorahensis, prosomal appendage VI. a SUI 139918, seventh podomere showing serrations and anterodistal projection. b SUI 139999, podomeres 7–9 showing serrations on proximal region of seventh podomere. c SUI 139984, almost complete articulated paddle comprising podomeres 1–7. d SUI 139933, podomeres 7–9. e SUI 139960, paddle showing articulations between podomeres 5–7 and the distribution of setal follicles. f SUI 139992, podomeres 8–9. g SUI 139988, seventh podomere. h SUI 139995, podomeres 6–7 showing articulation and overlap. i SUI 139958, distal portion of sixth podomere shown in both anterior and posterior aspect displaying the continuation of serrations behind the lateral projection. j SUI 139981, incomplete seventh podomere and distal portion of eighth showing the insertion of podomere 9. k SUI 139966, distal portion of sixth podomere in both anterior and posterior aspect. l SUI 139949, podomeres 1–3. m SUI 139977, seventh podomere. n SUI 139964, podomeres 8–9. o SUI 140001, small paddle showing podomeres 6–7 and the positioning of podomere 7a posterior to the anterodistal projection. VI-1–VI-9 = podomeres 1–9, FO = follicles. Scale bars = 10 mm Full size image

Fig. 15 Pentecopterus decorahensis, mesosomal tergites. a, b SUI 139931. a Counterpart showing imprint of central scale rows in the sediment. b Part, articulated tergites probably representing segments 4–5. c SUI 139943, tergites 2–4. d, e SUI 139947. d Counterpart, fragment of cuticle removed from second tergite. e Part, tergite 1–4. f SUI 140018 part, two tergites in series. g SUI 139950, tergite. h SUI 139932, sixth tergite missing right lateral margin. i SUI 140015, fragment of exceptionally large tergite. T1–T5 = tergites 1–5, SR = scale row. Scale bars = 10 mm Full size image

Fig. 16 Limulus polyphemus and Pentecopterus decorahensis, cuticular features of tergites. a Limulus polyphemus, cracking of the cuticular surface. b–q Pentecopterus decorahensis, mesosomal tergites. b SUI 140020, portion of tergite showing row of enlarged scales and cracking of the cuticle. c SUI 140044, sparse scale ornamentation and cracking of the surface. d SUI 140039, weathered portion of cuticle showing advanced stages of cracking. e SUI 140046, regular scale ornament. f SUI 139954, lateral portions of articulated tergites. g SUI 140038, anterior portion of tergite showing the articulating facet devoid of ornamentation. h SUI 140027, tergite showing row of enlarged scales and posterior doublure. i SUI 140042, smooth anterior articulating facet and central row of enlarged guttalate scales. j SUI 140043, scale ornament. k SUI 140057, lateral portion of tergite showing ancillary row of scales along margin. l SUI 140045, anterior articulating facet of tergite. m SUI 140026, widely spaced follicles. n SUI 140028, uniform scale ornament. o SUI 140021, showing faint cracking of the cuticle. p SUI 140022, merging of scales towards posterior of tergite and fragment of posterior doublure. q SUI 140025, broadening and merging of rounded scales towards rear of tergite. AF = articulating facet, CC = cuticular cracking, FO = follicles, PD = posterior doublure, SC = scales, SR = scale row. Scale bars = 10 mm Full size image

Fig. 17 Pentecopterus decorahensis, genital appendage and operculum. a SUI 140003, genital appendage missing left lobe. b SUI 140008, incomplete genital operculum with left ala and deltoid plate. DP = deltoid plate, FO = follicles, SC = scales. Scale bars = 10 mm Full size image

Fig. 18 Pentecopterus decorahensis, metasomal tergites. a SUI 139955, tergites 9–12. b SUI 139976, tergites 8–9 showing dentate posterior margin. c SUI 139971, fragment displaying enlarged guttalate scale rows and posterior doublure with serrations. d SUI 140004, ventral cuticle with smooth articulating facet. e SUI 139974, cuticle showing central scale rows and posterior doublure. f SUI 140002, posterior margin of tergite with serrations. T9–T12 = tergites 9–12, AF = articulating facet, PD = posterior doublure, PD = posterior doublure, SR = scale row. Scale bars = 10 mm Full size image

Fig. 19 Pentecopterus decorahensis telson. a SUI 139956, almost complete, position of scales shown in Fig. 19c arrowed. b SUI 139957, showing partial exfoliation. c SUI 139956, detail of cuticular ornamentation. Scale bars for a–b = 10 mm, c = 2 mm Full size image

Etymology

The genus is named for the penteconter (Greek πεντηκόντορος), an early form of ancient Greek galley and one of the first true warships, which the taxon superficially resembles in outline and parallels in being an early predatory form. This is combined with -pterus (φτερός – wing), the epithet typically applied to eurypterid genera. The species name refers to Decorah in Winneshiek County, Iowa, where the material originates.

Material

Holotype: SUI 139941, prosomal ventral plate and proximal podomeres of prosomal appendage II. Paratypes: SUI 102857, SUI 139913, SUI 139917, SUI 139920, SUI 139924, SUI 139926, SUI 139931, SUI 139933, SUI 139935–139936, SUI 139945, SUI 139948, SUI 139953, SUI 139955–139956, SUI 139961, SUI 139965, SUI 139969, SUI 139979, SUI 139983–139984, SUI 139998–139999, SUI 140003, SUI 140008, SUI 140014. Additional Material: SUI 139912, SUI 139914–139916, SUI 139918–139919, SUI 139921–139923, SUI 139925, SUI 139927–139930, SUI 139932, SUI 139934, SUI 139937–139940, SUI 139942–139944, SUI 139946–139947, SUI 139949–139952, SUI 139954, SUI 139957–139960, SUI 139962–139964, SUI 139966–139968, SUI 139970–139978, SUI 139980–139982, SUI 139985–139997, SUI 140000–140002, SUI 140004–140007, SUI 140009–140013, SUI 140015–140061. Numerous fragmentary specimens in the University of Iowa Paleontology Repository.

Horizon and locality

Middle Ordovician (Darriwilian) Winneshiek Lagerstätte, Winneshiek Shale Formation, Winneshiek County, Iowa, USA.

Diagnosis

Megalograptidae retaining a single pair of spines on third podomere of prosomal appendages III; appendage V short with serrated distal margin of podomeres; prosomal ventral plates widening anteriorly; lateral margins of podomere VI-7 and VI-8 with small serrations; VI-7 with anterior rounded projection; pretelson lacking posterolateral expansion; telson xiphous, margin laterally ornamented with scales.

Description

The large number of fragmentary specimens of exuviae allows an almost complete description of the external morphology of the animal. The only structures that are not represented in the material are the prosomal shield and metastoma. A number of specimens represent juvenile instars (see discussion below).

The lack of specimens of the prosomal shield precludes any knowledge of the lateral eyes and ocelli, or of carapace cuticular ornamentation. The prosomal ventral plate is known from 12 specimens (Fig. 1) ranging from 60 mm to at least 144 mm in length and 62 mm to at least 122 mm in width (Table 1). The ventral plate is of Erieopterus-type, consisting of a single plate covering the anterior and lateral portion of the ventral carapace, as in modern horseshoe crabs. The ventral plate extends into a large rostrum anteriorly where no prosomal appendages could insert (Figs. 1c, f and 2); the appendages were attached to the soft ventral integument rather than the sclerotized ventral plates [14]. The appendages in SUI 139979 (Fig. 2) and the holotype SUI 139941 (Fig. 3) appear to have been displaced onto the rostrum during ecdysis as the animal pulled itself through the gap between the ventral plate and the carapace. The rostrum accounts for approximately half of the length of the ventral plates and has a shallow anterior indentation similar to that in Waeringopterus [54, 55] and Eusarcana [56, 57]. The rostral region of the ventral plate is drawn out posteriorly into a linguoid projection (Fig. 1d, e, f) which extends back between the prosomal appendage insertions (Fig. 3). A similar posterior process is present in Erieopterus [58] and would likely have projected between the chelicerae in life; this structure may be homologous to the ‘triangular area’ noted by Størmer [41] and Lamsdell [45]. Laterally, the ventral plate narrows evenly towards the posterior of the carapace (Fig. 1h) before terminating in an expanded lobe (Fig. 1a, f). The postero-lateral edge of this lobe folds dorsally over itself (SUI 139978, Fig. 1b) and this likely represents the posterior locking mechanism by which the ventral plate folds over onto the prosomal shield [14]. The lobed posterior outline of the ventral plate suggests that the carapace may have projected into genal facets, as the posterior margins of eurypterid ventral plates usually correspond closely to the posterior morphology of the prosomal shield [14]. Two specimens (Fig. 4) preserve fine cuticular details of the ventral plate and the ventral integument of the prosoma. The ventral plate cuticle is thick and preserved in a darker color with terrace lines and a row of scales along the interior margin, while the integument is flexible and covered in dense conical setae.

Table 1 Pentecopterus decorahensis prosomal ventral plate measurements Full size table

The sole near-complete specimen of the ventral plate (Fig. 1f, g) reveals that the general outline of the carapace was quadrate with a large anterior rostrum, i.e., an elongate trapezoidal outline. A clear marginal rim is present in some specimens (Fig. 2).

The most commonly preserved morphological features, excluding tergite fragments, are the prosomal appendages. All six appendages are represented, and a number of juvenile appendages are known. Appendages II–V are homonomous in juveniles (Fig. 5), each podomere bearing a single pair of ventral moveable spines and a pair of elongate fixed lateral spines projecting distally a length almost equal to that of the succeeding podomere. Each podomere is strongly denticulated distally towards its ventral edge. These juvenile appendages are densely ornamented with guttalate (droplet-shaped) scales (Fig. 6a) which are relatively larger and more closely spaced than in adult individuals (Fig. 6b, d, e). Appendage VI is similar to that in adults but appears to be relatively longer (see description of Appendage VI below).

The postoral appendages (II–VI) are differentiated from one another in adult specimens. Appendage I, the preoral chelicera (Fig. 7), comprises three segments: a non-spiniferous peduncle, a fixed finger, and a free finger; only six examples are known, all from adults. The peduncle of the chelicera (Fig. 7c) is approximately equal in length to the fixed ramus (Table 2), while the free ramus is about half this length (Fig. 7a, b). The free ramus terminates in a distal hook (Fig. 7d) which overlaps the termination of the fixed ramus (Fig. 7e). Neither ramus bears denticles (Fig. 7f). The second and third prosomal appendages are oriented anteriorly rather than ventrally, as shown by the rotation of the proximal podomeres, and bear enlarged armature, suggesting that their primary use was in prey capture. The morphology of appendage II is evidenced by seven specimens (Fig. 8), all but one of them adult (Table 3). Appendage II is relatively short with no more than seven podomeres but it is nonetheless robust and spinous. The coxa extends dorsally over the proximal podomeres of the endopod (Fig. 8c), increasing the area of the limb insertion into the body wall compared to that in most eurypterids, thereby effectively buttressing the appendage. Several coxae preserve a moveable endite (Fig. 8d). The paired ventral moveable spines on each podomere are conical and heavily sclerotized (Fig. 8a). The paired lateral fixed spines of podomeres four to six are enlarged compared to the width of the podomere (Fig. 8e, f). The paired lateral spines of the fourth podomere are angled ventrally (Fig. 3), extending in length almost to the distal termination of the appendage, and are serrated along the inner margin (Fig. 8b). Appendage III, in contrast, which is known from 17 specimens (Fig. 9), is a relatively simple raptorial limb (Fig. 9a, b), essentially similar but larger than the juvenile appendage (Table 4). The coxa is broad but has a narrow gnathobasic surface (Fig. 9a, c, i). The second podomere of the limb is modified to allow for greater rotation, with a wide, crescent-shaped distal aperture (Fig. 9c, j). The appendage armature is distinctly different from that of juvenile limbs; the paired ventral spines are largely reduced in size relative to the podomere width (Fig. 9d, n, o) although the lateral spines are enlarged and elongated (Fig. 9e, k, l), increasing in length through podomeres four to six. The penultimate podomere is long and circular in cross section, largely lacking in armature. The terminal podomere is a short, curved spine (Fig. 9f, g, h, m). Both these distalmost podomeres are usually obscured in lateral view by the massively elongate lateral spines of the sixth podomere (Fig. 9b).

Table 2 Pentecopterus decorahensis cheliceral measurements Full size table

Table 3 Pentecopterus decorahensis prosomal appendage II measurements Full size table

Table 4 Pentecopterus decorahensis prosomal appendage III measurements Full size table

The fourth to sixth prosomal appendages are shorter than the second and third and oriented ventrally. Appendage IV is known from 13 specimens (Fig. 10), the majority of which are isolated individual podomeres (Table 5). The most complete specimen (Fig. 10d) is attached to a portion of the carapace with a marginal rim, indicating that the marginal rim extended at least midway back along the carapace. As in appendages II and III, the coxa extends distally along the coxa-body junction (Fig. 10j), and ancillary rows of spinose hairs surround the gnathobases (Fig. 10i). Appendage IV is short (Fig. 10a), with fixed lateral spines extending parallel to the limb axis (Fig. 10c, d). The distal denticulations on each podomere are greatly developed and randomly oriented (Fig. 10e, g), forming an expanded (swollen) surface surrounding the base of the moveable ventral spines (Fig. 10b, f, h). The armature of Appendage V, which is known from 15 specimens (Figs. 11 and 12) (Table 6), is much less pronounced. The coxa bears a narrow gnathobasic surface with multiple rows of small teeth (Fig. 11g). The second podomere is curved ventrally (Fig. 11h, j, k). The margins of the distal podomeres are denticulate (Fig. 11c, e). The ventral spines are strongly reduced on all podomeres (Fig. 11d, f, l), as are the lateral spines on all but the penultimate podomere (Figs. 11b and 12a); the termination is trifurcate, made up of a terminal and two lateral spines (Fig. 11i). The overall morphology of Appendage V is slender compared to the more anterior appendages. This morphology of the fifth limb is similar to that of the equivalent appendage in Megalograptus [5] and Eurypterus [14], which is thought to have a ‘balancing’ function [4], but the podomeres are relatively shorter in adults (Fig. 11a) and the appendage flexes ventrally at the third podomere (Fig. 12b).

Table 5 Pentecopterus decorahensis prosomal appendage IV measurements Full size table

Table 6 Pentecopterus decorahensis prosomal appendage V measurements Full size table

Appendage VI, known from 24 specimens, expands distally into a paddle with an unusual morphology (Figs. 13 and 14) (Table 7). The expanded coxal gnathobasic surface is differentiated from the main body of the coxa by a narrow ‘neck’ (Fig. 13b). The gnathobase bears 16–18 teeth (Fig. 13a, c). The proximal podomeres of appendage VI are short and homonomous, distally serrated with a rounded ventral projection (Fig. 13d). The fourth podomere is longer than the third or fifth (Fig. 14c). The sixth podomere bears an enlarged ventral projection and is also expanded dorsally, resulting in a blade- or leaf-like shape (Fig. 13f, g). The distal margin of the sixth podomere bears elongate serrations that extend behind the ventral projection (Fig. 14i, k); podomere seven inserts into the region demarcated by these serrations (Fig. 14h), in line with the attachment point for the fifth podomere (Figs. 13e and 14e). The distal morphology of the sixth podomere is unusual compared to that in the equivalent appendage of other eurypterids; the anterior expansion allows for greater flexibility of the distal paddle (Fig. 14c). The seventh podomere is elongate and curved ventrally (Fig. 14d, j, m) with a serrated anterior margin and rounded distal projection (Fig. 14a). Podomere 7a is a small triangular element located ventrally on the distal margin of the seventh podomere and less than half its width (Fig. 14o), which overlaps podomere 8. The eighth podomere is oval, narrowing somewhat distally, and bears small lateral serrations (Fig. 14n) while the ninth podomere is short and narrow and inserts in a small recess in the posterior margin of podomere eight (Fig. 14b, f). In contrast to the scales that are present on the more proximal podomeres (Figs. 6f and 14l), podomeres 7–9 are ornamented with follicles, which increase in density towards the margins (Fig. 14e, g), and represent the insertions of sensory setae.

Table 7 Pentecopterus decorahensis prosomal appendage VI measurements Full size table

The mesosoma is represented by a large number of specimens, the majority fragments of tergites (Figs. 15 and 16). Eleven specimens comprise relatively complete tergites or tergites in series (Table 8). The majority of specimens are incomplete laterally, but those that preserve the margins show no evidence of a lateral division (i.e., trilobation) (Fig. 15h) and they lack epimera (Fig. 15a, b). Some specimens are very large (Fig. 15g, i) but the width of these specimens does not greatly exceed the maximum carapace width as estimated from the ventral plate, suggesting that the eurypterid had a gracile outline. The length of the tergites increases evenly posteriorly (Fig. 15c, e, f) with the exception of the first tergite which is markedly reduced, as in other eurypterids [34]. The ornamentation of the mesosoma comprises a mixture of narrow and broad lunate scales interspersed with some enlarged guttalate scales (Fig. 16c, e, f, j, k, l, n, o, p, q). These guttalate scales, which are highly sclerotized, are arranged in three to four imperfect longitudinal rows along the center of each tergite (Fig. 16h, i). Follicles also occur across the cuticular surface (Figs. 15d and 16m). A smooth articulating facet extends across the anterior margin of each tergite (Fig. 16g, i). A pattern similar to desiccation cracks is evident on some specimens (Fig. 16b, d) and is likely due to taphonomic drying and shrinking of the cuticle; similar structures are found in carcasses of modern horseshoe crabs (Fig. 16a).

Table 8 Pentecopterus decorahensis mesosoma tergite measurements Full size table

Only a single specimen each of the genital operculum and genital appendage represent the ventral mesosomal structures. The operculum (Fig. 17b) is incomplete with a total preserved width of 72 mm, of which 55 mm is made up of the intact left lobe, and a length of 33 mm. The right lobe is too poorly preserved to show any details but the left lobe preserves a triangular deltoid plate, 15 mm long by 15 mm wide. The operculum preserves no evidence of an anterior opercular plate, nor of a suture or difference in ornamentation demarcating median and posterior plates. The genital appendage specimen consists of a single joint with a bilobate termination (Fig. 17a): it is unclear whether it represents a complete type B appendage or the distal joint of a type A appendage. The specimen is 24 mm long and 14 mm wide proximally, and the reconstructed distal width is 20 mm. A median suture line is present and a narrow doublure 2 mm wide runs along the lateral margins, expanding to 7 mm at the distal lobes. The genital appendage is ornamented proximally by small, outwardly-oriented scales, which give way distally to a dense covering of setal follicles.

The metasoma, which is comprised of the six posterior opisthosomal segments, is represented by seven specimens (Table 9) but only one of these preserves multiple articulated segments (Fig. 18a). The metasomal segments show no abrupt differentiation from those of the mesosoma, instead narrowing evenly from the seventh or eighth opisthosomal segment. The length of the metasomal segments decreases gradually to the pretelson, which is approximately 50 % longer than the preceding segment. The pretelson does not expand laterally and there is no evidence of an attachment point for cercal blades; this, combined with their absence in the material, indicates that such structures were lacking. The posterior margin of each metasomal segment is dentate (Fig. 18b, f), which distinguishes them from the mesosomal segments. The ornamentation of the metasomal and mesosomal segments is otherwise identical, the dorsal portion of both bearing a median row of highly sclerotized, enlarged scales (Fig. 18c, e). The ventral surface of the metasomal segments bears a similar ornamentation to the dorsal, with narrow angular scales in the anterior portion grading posteriorly into broad lunules with occasional chevrons. The ventral ornamentation is much more dense than the dorsal, however, and lacks the median row of enlarged scales (Fig. 18b, d). The metasomal segments show no evidence of epimera.

Table 9 Pentecopterus decorahensis metasoma tergite and telson measurements Full size table

The telson is represented by two specimens (Fig. 19) and is xiphous, with a length/width ratio of between 2.5 and 3.0 (Table 9). The telson appears to lack a median ridge or keel (Fig. 19a, b), and is sparsely ornamented with broad lunules. The lateral margins are ornamented by heavily sclerotized scales similar to those forming median rows on the opisthosomal tergites (Fig. 19c).

Remarks

Despite the fragmentary nature of the material, the comprehensive representation of the morphology allows Pentecopterus to be reconstructed (Fig. 20). The taxon bears a number of similarities to Megalograptus [5], including the typical megalograptid guttalate ornamentation and a number of features of the prosomal appendages, notably the randomly-oriented armature on the distinctly swollen podomeres of appendage IV and the narrow gnathobase bearing multiple rows of small teeth on the coxa of appendage V, but it is nonetheless distinguished from Megalograptus by a number of characters (see Phylogenetic Affinities). Pentecopterus, like Megalograptus, bears rows of enlarged scales running down the center of the opisthosomal tergites but, unlike the scales in Megalograptus, those in Pentecopterus are not situated on pronounced ridges. Pentecopterus exhibits a number of morphological features unique within eurypterids, including the carapace shape and aberrant morphology of the sixth podomere of appendage VI. Pentecopterus is also unusual in the presence of lateral scales on the telson, a feature otherwise only observed in pterygotids [59]. However, the droplet-shaped guttalate scales in Pentecopterus differ from the angular chevron-type in pterygotids and the two conditions are likely convergent.

Fig. 20 Pentecopterus decorahensis, reconstruction of adult. a Dorsal view. b Genital operculum. c Ventral view of carapace and prosomal ventral plate. d Ventral view of prosoma. The appendages are shown rotated in lateral view; in life, appendages IV–VI would be oriented so that the anterior edge (in IV) or the posterior edge (in V and VI) of the limb as in the reconstruction would face ventrally. The form of the median and lateral eyes and metastoma are hypothetical and based on ancestral state reconstructions using the phylogenetic matrix and topology. Scale bars = 10 cm (maximum size) Full size image

The size of Pentecopterus, from carapace anterior to telson posterior, can be inferred from individual fragmentary specimens based on the reconstruction. Most of the limb specimens indicate a total length of 75–100 cm, while the juvenile specimens indicate lengths of around 10–15 cm; some large tergites suggest lengths of up to 170 cm. This makes Pentecopterus the largest known megalograptid and by extension the largest known Ordovician eurypterid: at 85 cm, the average size of Pentecopterus outstrips the largest records of Megalograptus ohioensis, which ranges 49–78 cm in length [5]. Previous reports of megalograptids in excess of 200 cm in length are based on two fragmentary tergites of Megalograptus shideleri which Caster and Kjellesvig-Waering [5] considered to be derived from a giant individual based on the dimensions of cuticular scales. One of these tergites of M. shideleri, however, does not exceed 30 mm in length, suggesting a total body length of no more than 56 cm, far short of the sizes attained by Pentecopterus. Specimens of both Pentecopterus and Megalograptus show that scale size varies across the exoskeleton, with some scales being much larger than the average scale size.

Some features of Pentecopterus lend themselves to interpretations of the functional morphology and possible mode of life of the eurypterid. The second podomere of limbs II and III is modified to allow for greater rotation which, combined with the massive elongation of the ventrally-oriented spines, suggests that these limbs were angled forward in life and were involved primarily in prey capture rather than locomotion. The fourth to sixth prosomal appendages are shorter than the second and third, and oriented ventrally; their morphology suggests that they served a locomotory function resulting in a hexapodous gait. Ichnological evidence indicates that eurypterids adopted either a hexapodous [60–62] or octopodous [62, 63] mode of locomotion, although some trackways evidence a hexapodous gait with occasional transitions to octopodous locomotion [64, 65]. The interpretation of the gait of Pentecopterus as hexapodous is supported by trackways of the closely related taxon Mixopterus which exhibit a hexapodous gait [66]. The swimming capabilities of Pentecopterus, however, are difficult to determine. The sixth appendage is expanded into a paddle with an unusual morphology: the sixth podomere is drawn out and overlaps podomere seven laterally in much the same way as ‘podomere’ 7a overlaps podomere eight. This overlap likely increases the degree of movement possible at the podomere joint, as well as the surface area of the paddle, as has been hypothesized for ‘podomere’ 7a [14]. The enlarged denticulations on the distal margins of the podomeres would serve to lock them in place and reduce the degree of antero-posterior flexure of the paddle during forward and back strokes. This, combined with the increased paddle surface area, indicates that Pentecopterus was capable of swimming, although it has been suggested that the paddle of some eurypterids may have had a digging function [26, 45] and such a role for the paddle of Pentecopterus cannot be ruled out. Pentecopterus lacks the cercal blades that occur in Megalograptus, where they have been interpreted as functioning as a biological rudder, like the pterygotid telson [67]. Thus Pentecopterus may have been a less able swimmer than Megalograptus.