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. 2015 Sep 1:15:169.
doi: 10.1186/s12862-015-0443-9.

The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa

James C Lamsdell  1 , Derek E G Briggs  2   3 , Huaibao P Liu  4 , Brian J Witzke  5 , Robert M McKay  6
Affiliations

The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa

James C Lamsdell et al. BMC Evol Biol. .

Abstract

Background: Eurypterids are a diverse group of chelicerates known from ~250 species with a sparse Ordovician record currently comprising 11 species; the oldest fully documented example is from the Sandbian of Avalonia. The Middle Ordovician (Darriwilian) fauna of the Winneshiek Lagerstätte includes a new eurypterid species represented by more than 150 specimens, including some juveniles, preserved as carbonaceous cuticular remains. This taxon represents the oldest described eurypterid, extending the documented range of the group back some 9 million years.

Results: The new eurypterid species is described as Pentecopterus decorahensis gen. et sp. nov.. Phylogenetic analysis places Pentecopterus at the base of the Megalograptidae, united with the two genera previously assigned to this family by the shared possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of all spines except the pair on the penultimate podomere of appendage V, and an ornamentation of guttalate scales, including angular scales along the posterior margin of the dorsal tergites and in longitudinal rows along the tergites. The morphology of Pentecopterus reveals that the Megalograptidae are representatives of the derived carcinosomatoid clade and not basal eurypterids as previously interpreted.

Conclusions: The relatively derived position of megalograptids within the eurypterids indicates that most eurypterid clades were present by the Middle Ordovician. Eurypterids either underwent an explosive radiation soon after their origination, or earlier representatives, perhaps Cambrian in age, remain to be discovered. The available instars of Pentecopterus decorahensis suggest that eurypterids underwent extreme appendage differentiation during development, a potentially unique condition among chelicerates. The high degree of appendage specialization in eurypterids is only matched by arachnids within chelicerates, supporting a sister taxon relationship between them.

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Figures

Fig. 1
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
Fig. 2
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
Fig. 3
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
Fig. 4
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
Fig. 5
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
Fig. 6
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
Fig. 7
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. F1 = fixed finger, F2 = free finger. Scale bars = 10 mm
Fig. 8
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
Fig. 9
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
Fig. 10
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
Fig. 11
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
Fig. 12
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
Fig. 13
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
Fig. 14
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
Fig. 15
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
Fig. 16
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
Fig. 17
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
Fig. 18
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
Fig. 19
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
Fig. 20
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)
Fig. 21
Fig. 21
Result of the phylogenetic analysis. Single most parsimonious tree. Bootstrap branch support values are shown above the node in italics, with Bremer support values in bold within parentheses. Jackknife values are displayed beneath each node. The position of Pentecopterus decorahensis is highlighted in red. Arachnids are not included in the analysis
Fig. 22
Fig. 22
Evolutionary tree of the major clades of Eurypterina. Stratigraphic ranges of clades are shown by triangles, those of individual genera and species by boxes. Dashed black lines represent inferred ghost ranges. Solid gray lines show the tree topology
Fig. 23
Fig. 23
Pentecopterus decorahensis, SEM images of cuticular structures. a SUI 139963, scales on juvenile appendage. b SUI 139951, scales and setae on coxa of appendage III. c SUI 140020, scales on mesosomal tergite. d SUI 139951, scale and seta on appendage III coxa. e SUI 140031, scale on mesosomal tergite showing base of seta projecting from beneath the posterior margin. f SUI 140054, follicle on mesosoma with base of seta. Scale bars for a, b, e = 100 μm, c = 1 mm, d, f = 50 μm
Fig. 24
Fig. 24
Pentecopterus decorahensis, prosomal appendage cuticle. a SUI 140049, podomere III of appendage IV showing fringe of setae around margin. b SUI 140049, podomere of appendage IV showing distal fringe of setae. c SUI 140051, podomere of appendage V, part and counterpart. d SUI 140019, podomere of juvenile appendage showing distal denticulations and moveable spine. e SUI 140050, portion of podomere cuticle. Scale bars = 1 mm
Fig. 25
Fig. 25
Ontogenetic changes in prosomal appendages of Pentecopterus decorahensis from juvenile to adult (coxa not shown). a Juvenile appendages II–V. b Adult appendage II. c Adult appendage III. d Adult appendage IV. e Adult appendage V. Homologous structures are color coded: red = distal denticulations, green = moveable ventral spines, blue = fixed lateral spines
Fig. 26
Fig. 26
Appendage specialization and differentiation among chelicerate clades. a Pycnogonids (sea spiders). b Horseshoe crabs. c Chasmataspidids. d Spiders. e Scorpions. f Whip scorpions (vinegaroons). g Stylonurine eurypterids. h Eurypterine eurypterids. Somites are labeled 0–VI; ‘m’ indicates the position of the mouth. Appendages are color coded by morphology and function: green = locomotory, red = modified for reproduction, blue = tactile/sensory function, yellow = raptorial, purple = modified for a swimming/digging. For Megalograptidae, darker shades of yellow and green indicate limbs with a raptorial and locomotory function respectively but with a different morphology to other limbs in the taxon with a similar function. Where sexual dimorphism is present the condition in the female is shown on the left and the male on the right
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