The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa
© Lamsdell et al. 2015
Received: 18 May 2015
Accepted: 30 July 2015
Published: 1 September 2015
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.
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.
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.
Eurypterids are a monophyletic group of Paleozoic aquatic arthropods which represent the first major radiation of chelicerates: some 250 species are known from marine to freshwater environments . Eurypterids are relatively common components of Silurian and Devonian Lagerstätten where conditions favor the preservation of their unmineralized cuticle . They are distinctive Paleozoic arthropods, with a fossil record previously known to extend from the Sandbian (Late Ordovician) to the Wuchiapingian (Permian) . The Ordovician record of eurypterids is sparse, however, and the majority of occurrences reported in the literature have been shown to be either misidentifications of other taxa or pseudofossils . Currently, 11 species of Ordovician eurypterid are known, falling into two ecological categories: larger active predators from Laurentia [4–6] and more basal demersal forms from Gondwana and Avalonia [7–9].
Here, we describe a new species of megalograptid eurypterid, Pentecopterus decorahensis gen. et sp. nov., from the Middle Ordovician (Darriwilian) Winneshiek Lagerstätte of Decorah, Iowa [10, 11], extending the stratigraphic range of Eurypterida back some 9 million years. The material is exceptionally preserved as organic cuticle remains providing remarkably complete information on the overall morphology as well as details of the microstructure. In addition to abundant adults a limited number of juvenile specimens are present, revealing ontogenetic changes within the species.
This new taxon is important in expanding our limited knowledge of eurypterid cuticular structures. While Holm’s spectacular material of Eurypterus cuticle from the Silurian of the Baltic (Saareema) has received attention [12–15], most previous papers have simply reported the occurrence of preserved cuticle [16–19], although there have been a few studies of cuticular structure [20, 21] and chemistry [2, 22]. Ontogenetic data are available for basal eurypterids [23–26]; Pentecopterus is the first derived taxon to provide evidence of development. Here we describe the new species and place it within a phylogenetic framework, discussing its significance for the early evolution and postembryonic development of eurypterids.
The majority of the specimens described here were collected from the upper 4 m section of the Winneshiek Shale which was excavated from its only outcrop near Decorah, northeastern Iowa, in 2010. Other samples were collected from blocks eroded during flooding, which are assumed to have been sourced from the uppermost 2–3 m. The material yielded over 5,000 fossil specimens (n = 5,354) of which about 6.6 % are eurypterid remains. This number excludes cuticular fragments too small to provide information on the morphology of the eurypterid. Arthropods, which also include phyllocarids (7.9 %) and other bivalved taxa (1.6 %) , make up the dominant and most diverse invertebrate group of the Winneshiek fauna. All material described here is accessioned in the Paleontology Repository, Department of Earth and Environmental Sciences, University of Iowa.
Specimens were prepared by using water to disaggregate the matrix and steel periodontal probes and bin angled chisels to remove matrix from the cuticle. Specimens less than 20 mm in dimension were photographed using a Leica DFC420 digital camera attached to a Leica MZ16 stereomicroscope: cuticle free of the matrix was illuminated with light transmitted through the microscope stage. Specimens larger than 20 mm were photographed using a Canon EOS 60D digital camera with a Canon EF-S 60 mm f/2.8 Macro USM lens; cuticle free of the matrix was illuminated with transmitted light generated from a Huion L42 LED light pad. All specimens were imaged dry and with normal light. Image cropping and leveling was carried out using Adobe Photoshop CS5, and interpretive drawings were prepared with Adobe Illustrator CS5, on a MacBook Pro running OS X.
Geological setting and preservation
In 2005, geologists of Iowa Geological Survey discovered an unusual fossil fauna from the Winneshiek Shale in northeastern Iowa. This fauna is characterized by abundant well-preserved fossils including conodonts, arthropods, possible jawless fish, algae, and plant materials, and represents a new fossil Lagerstätte . Based mainly on the conodont taxa present, the Winneshiek fauna is dated as Middle Ordovician (Darriwilian: 467.3 – 458.4 Ma) in age [10, 11].
The Winneshiek Shale is an 18–27 m thick greenish brown to dark grey laminated sandy shale [28, 29]. It overlies an unnamed stratigraphic unit of thick massive breccia and is in turn disconformably overlain by the St. Peter Sandstone [10, 11]. The Winneshiek Shale crops out only in one locality which is mostly submerged by the Upper Iowa River near Decorah. Bore hole data indicate that the total thickness of the Winneshiek Shale is about 18 m at the outcrop locality, but only the upper 4 m was systematically collected during the excavation.
The Winneshiek Shale is confined to a circular basin about 5.6 km in diameter in the Decorah area. Multiple lines of geological evidence indicate that the circular basin originated from a meteorite impact [11, 29]. The shape and dimension of the impact structure have recently been established by aerial geophysical surveys conducted by the U.S. Geological Survey, and the crater has been named the Decorah Impact Structure. Paleogeographic and paleoenvironmental studies indicate that the crater was located in marginal to nearshore marine conditions, with low-oxygen and possibly brackish water, within tropical southern Laurentia [11, 30, 31]. Rhythmic sandy laminations may indicate a local tidal influence [11, 30, 31]. The Winneshiek fauna is dramatically different from a normal marine shelly fossil fauna, indicating that the restricted environment was inhospitable to typical marine taxa [10, 11].
The eurypterid material comprises partially disarticulated individuals preserved as organic cuticle within fine shale laminations. The cuticle is red- to yellow-brown in color and does not fluoresce under UV light, even though the cuticle of both scorpions [32, 33] and xiphosurids (J. Lamsdell pers. obs.), which bracket eurypterids phylogenetically [34, 35], is known to do so. This lack of fluorescence may be original or due to diagenetic change: previous studies of eurypterid cuticle have found it to be almost identical in structure to that of xiphosurids , although we know of no other attempts to determine whether eurypterid cuticle fluoresces. The specimens exhibit patterns typical of eurypterid exuviae in late stages of disarticulation , including isolated ventral plates, tergites, and prosomal appendages, and form two-dimensional compressions of the dorsal and ventral surfaces with a fine sediment infill; internal soft tissue is not preserved. Eurypterids have been hypothesized to molt en masse [38–40], and accumulations of molts have been reported from a number of sheltered, marginal marine environments [26, 41–43], suggesting that the specimens are exuviae that accumulated within the Decorah crater during molting.
SUI, University of Iowa Paleontology Repository, Iowa City, Iowa, USA.
Eurypterid terminology largely follows Tollerton  for morphology of the carapace, lateral eyes, prosomal appendages, metastoma, genital appendage, opisthosomal differentiation, telson, and marginal ornamentation; however, the terminology for the ventral plate follows Tetlie et al. . Terminology for prosomal structures and cuticular sculpture, and the labeling of the appendages, follows Selden . Minor modifications to the terminology used in these papers follows Lamsdell .
The phylogenetic analysis presented herein is based on an expanded version of the matrix of Lamsdell and Selden . Sampling of the Carcinosomatoidea is increased, with ten species included for the carcinosomatid, megalograptid, and mixopterid clades in addition to the three sampled previously. The genus Alkenopterus Størmer, 1974 is also included for the first time, as it has been shown to be a basal representative of the Eurypterina  rather than a stylonurine as previously interpreted . Only one carcinosomatoid genus was omitted: Eocarcinosoma Caster and Kjellesvig-Waering, 1964, which is known from a single small carapace. The new matrix consists of 158 characters coded for 74 taxa. Additional file 1 includes the matrix and the character descriptions; it has also been deposited in the online MorphoBank database  under the project code p2116 and can be accessed from http://morphobank.org/permalink/?P2116.
The analysis was performed using TNT  (made available with the sponsorship of the Willi Hennig Society) employing random addition sequences followed by tree bisection-reconnection (TBR) branch swapping (the mult command in TNT) with 100,000 repetitions with all characters unordered and of equal weight. Jackknife , Bootstrap  and Bremer  support values were calculated in TNT; the ensemble Consistency, Retention and Rescaled Consistency Indices were calculated in Mesquite 3.02 . Bootstrapping was performed with 50 % character resampling for 5,000 repetitions, and jackknifing by using simple addition sequence and tree bisection-reconnection branch swapping for 5,000 repetitions with 33 % character deletion.
This article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSID for this publication is: urn:lsid:zoobank.org:pub:6E58DCAD-B5A8-4552-98B7-FA5585A20499. The journal is identified by ISSN 1471–2148, and has been archived and is available from the following digital repositories: PubMed Central, LOCKSS, INIST, and Koninklijke Bibliotheek.
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.
Pentecopterus decorahensis sp. nov.
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.
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.
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.
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).
Pentecopterus decorahensis prosomal ventral plate measurements
Main body length
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).
Pentecopterus decorahensis cheliceral measurements
(podomere 3): Free finger, 14/5.
(podomeres 1–3): Peduncle, 15/8. Fixed finger, 17/7. Free finger, 9/4.
(podomeres 1–3): Peduncle, 10a/4a. Fixed finger, 23a/11. Free finger, 8a/4.
(podomeres 1–3): Peduncle, 28/11. Fixed finger, 28/12. Free finger, 14/5.
(podomeres 2–3): Fixed finger, 13/5. Free finger, 4/2.
(podomeres 2–3): Fixed finger, 28/12. Free finger, 15/5.
Pentecopterus decorahensis prosomal appendage II measurements
(podomere 6): 6, 14a/11.
(podomeres 3–7): 3, 15a/6a. 4, 15/6a. 5, 23/10. 6, 21/10. 7, 33/7.
(podomeres 2–5): 2, 15/23. 3, 14/19. 4, 20/18. 5, 10a/7a.
(podomeres 3–7): 3, 2a/4. 4, 3/4. 5, 3/3. 6, 2/2. 7, 2/1.
(podomere 1): Coxa, 9a/5.
(podomeres 1–3): Coxa, 21/25. 2, 9/14. 3, 7a/12.
(podomere 1): Coxa, 23a/12.
Pentecopterus decorahensis prosomal appendage III measurements
(podomeres 1–8): Coxa, 25a/52. 2, 11/24. 3, 16/19. 4, 16/19. 5, 19/17. 6, 19/14. 7, 12/7. 8, 8/4.
(podomere 6): 6, 18a/17.
(podomere 1): Coxa, 22a/21a.
(podomeres 7–8): 7, 20/8. 8, 18/5.
(podomeres 2–6): 2, 15/27. 3, 19/23. 4, 16/20. 5, 22/15. 6, 18a/13.
(podomeres 7–8): 7, 17/7. 8, 18/5.
(podomeres 1–8): Coxa, 28a/30. 2, 11/21. 3, 21/20. 4, 14/19. 5, 20/13. 6, 19/11. 7, 18/8. 8, 5a/4.
(podomeres 1–5): Coxa, 27/23. 2, 12/17. 3, 14/13. 4, 10/12. 5, 15/8.
(podomeres 1–5): Coxa, 35/21a. 2, 17/26. 3, 21/22. 4, 15/18. 5, 21/14.
(podomeres 2–8): 2, 15/19. 3, 18/22. 4, 16/19. 5, 22/14. 6, 22/23. 7, 15/8. 8, 22/7.
(podomeres 1–8): Coxa, 7a/9a. 2, 4/6. 3, 5/6. 4, 4/5. 5, 4/5. 6, 4/4. 7, 4/3. 8, 4/2.
(podomeres 4–8): 4, 2a/4. 5, 4/4. 6, 4/3. 7, 5/3. 8, 3/2.
(podomere 6): 6, 11a/13.
(podomeres 1–4): Coxa, 34/23. 2, 7/15. 3, 8/15. 4, 6a/16.
(podomeres 7–8): 7, 9a/9. 8, 21/5.
(podomeres 6–8): 6, 15/14. 7, 13/10. 8, 12a/6.
(podomere 1): Coxa, 34/31.
(podomeres 6–8): 6, 4a/5. 7, 15/6. 8, 8/4.
Pentecopterus decorahensis prosomal appendage IV measurements
(podomeres 5–8): 5, 11/11. 6, 10/10. 7, 10/8. 8, 21/5.
(podomere 8): 8, 21/5.
(podomere 4): 4, 12a/10.
(podomere 3): 3, 13/6.
(podomere 3): 3, 16/7.
(podomeres 6–8): 6, 7a/7a. 7, 12/10. 8, 13a/4.
(podomeres 1–2): Coxa, 43/18a. 2, 22a/17a.
(podomere 1): Coxa, 14a/13.
(podomeres 3–8): 3, 5/5. 4, 4/5. 5, 5/4. 6, 4/3. 7, 2/2. 8, 4/2.
(podomeres 5–8): 5, 5/4. 6, 4/3. 7, 5/2. 8, 3/2.
(podomeres 1–3): Coxa, 28/17. 2, 13/15. 3, 8a/12.
(podomeres 2–3): 2, 30a/21. 3, 25a/20.
(podomeres 6–8): 6, 4a/7a. 7, 9/9. 8, 12a/6.
Pentecopterus decorahensis prosomal appendage V measurements
(podomeres 8–9): 8, 16/9. 9, 10/5.
(podomeres 1, 6–9): Coxa, 19a/11a. 2, −/−. 3, −/−. 4, −/−. 5, −/−. 6, 6a/10. 7, 10/8. 8, 10/7. 9, 7/3.
(podomere 1): Coxa, 30a/21.
(podomeres 1–5): Coxa, 7a/10a. 2, 6/4a. 3, 8/8. 4, 6/7. 5, 6/6.
(podomeres 6–9): 6, 10/8. 7, 9/6. 8, 4/4. 9, 4a/3.
(podomeres 1–9): Coxa, 10a/12a. 2, 4a/8. 3, 2/7. 4, 5/7. 5, 5/7. 6, 5/6. 7, 6/5. 8, 6/3. 9, 4/2.
(podomere 1): Coxa, 28/15.
(podomeres 4–6): 4, 20/17. 5, 12/17. 6, 14a/13.
(podomeres 6–9): 6, 5a/10. 7, 7/8. 8, 7/6. 9, 11/4.
(podomeres 3–4): 3, 11a/12. 4, 15/11.
(podomeres 4–5): 4, 11a/17. 5, 9/10.
(podomeres 4–5): 4, 12/12. 5, 10a/12.
(podomeres 2–9): 2, 6/18. 3, 5/16. 4, 12/15. 5, 9/14. 6, 10/12. 7, 14/10. 8, 7/6. 9, 8/4.
(podomeres 4–7): 4, 20/18a. 5, 13/21. 6, 19/18. 7, 16a/14.
(podomeres 2–7): 2, 3/6a. 3, 3/10. 4, 7/9. 5, 5/7. 6, 7/7. 7, 7/6.
Pentecopterus decorahensis prosomal appendage VI measurements
(podomere 7): 7, 42/22.
(podomere 7): 7, 52a/27a.
(podomeres 7–9): 7, 43/24. 7a, 6/7. 8, 13/12. 9, 3/2.
(podomere 1): Coxa, 40a/22a.
(podomeres 1–3): Coxa, 23a/32a. 2, 10/31. 3, 11/25.
(podomere 6): 6, 17a/14a.
(podomeres 5–7): 5, 7a/13. 6, 8/17. 7, 28a/15.
(podomere 6): 6, 20/42.
(podomeres 8–9): 8, 17/8. 9, 1/1.
(podomeres 5–6): 5, 7a/5a. 6, 10a/16a.
(podomere 6): 6, 18/48.
(podomere 7): 7, 68/30.
(podomeres 1–3): Coxa, 18a/26a. 2, 6/19. 3, 19/18.
(podomeres 7–9): 7, 74a/35. 7a, −/−. 8, 25a/20a. 9, 10/4.
(podomeres 1–7): 1, 5a/5a. 2, 10/13a. 3, 13/14a. 4, 21a/13a. 5, 8/14. 6, 11/30. 7, 26a/21.
(podomere 1): Coxa, 35a/29a.
(podomere 7): 7, 36a/17.
(podomeres 8–9): 8, 27a/21. 9, 6/4.
(podomere 6): 6, 14/17a.
(podomere 1): Coxa, 21a/20a.
(podomeres 6–7): 6, 6/9. 7, 10a/10.
(podomeres 5–6): 5, 3a/18. 6, 10/14a.
(podomeres 7–9): 7, 23a/18a. 7a, −/−. 8, 29a/18. 9, 4/3.
(podomeres 6–7a): 6, 7/14. 7, 33/13. 7a, 3/3.
Pentecopterus decorahensis mesosoma tergite measurements
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.
Pentecopterus decorahensis metasoma tergite and telson measurements
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).
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 . 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  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 . 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 . 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 . Thus Pentecopterus may have been a less able swimmer than Megalograptus.
Megalograptidae resolves as a basal clade of Carcinosomatoidea, which also includes the families Mixopteridae and Carcinosomatidae, the superfamily being united by an anterior carapace projection, enlargement of the forward appendages, elongation of the spines on appendage III, an anterior rather than ventral orientation of prosomal appendages II and III, and the presence of thickened and highly sclerotized spines on appendages II–IV. This grouping was first proposed by Størmer  and was considered to be a relatively derived clade of eurypterids by Novojilov ; however, in their revision of the group, Caster and Kjellesvig-Waering  considered Megalograptus to be an extremely primitive eurypterid while regarding it as related to the carcinosomatoids. The hypothesis that Megalograptus exhibits ‘primitive’ (i.e., plesiomorphic) characteristics, combined with its early stratigraphic occurrence, led Tetlie  to consider megalograptids as a clade distinct from carcinosomatoids occupying a basal position within the eurypterids, while carcinosomatoids remained as a relatively derived group having attained a number of shared characteristics with megalograptids through convergence. As such, Megalograptus has been considered to represent the plesiomorphic condition within eurypterids for a number of characters, including the occurrence of the opisthosomal preabdominal/postabdominal constriction at the sixth and seventh segment as opposed to the seventh and eighth , the possession of an extra podomere on prosomal appendages II–V , and the morphology of the prosomal ventral plates . Nevertheless no phylogenetic analysis has retrieved megalograptids in such a basal position: Megalograptus has either been excluded from consideration due to its mix of plesiomorphic, derived and autapomorphic characteristics  or has resolved as part of a derived clade of carcinosomatoids [9, 26, 45]. The analysis presented herein provides strong support for Megalograptidae as part of a carcinosomatoid clade. Furthermore, the presence of Pentecopterus at the base of the megalograptids reveals that a number of supposedly plesiomorphic characters in Megalograptus are either reversals or derived conditions. A single character in particular, the lack of a modified distal margin of the sixth podomere of the swimming leg, was used to infer a basal position for Megalograptus . The morphology of the sixth appendage of Pentecopterus is somewhat unusual and resembles that of Strobilopterus  but the distal margin of its sixth podomere is clearly modified, indicating that the unmodified condition in Megalograptus is a reversal. Similarly, the unique arrangement of the prosomal ventral plates in Megalograptus appears to be derived from the anterior carapace projection folding ventrally over the Erieopterus-type ventral plates of Pentecopterus. Pentecopterus also demonstrates that the fifth ‘balancing’ limb of Megalograptus is independently derived from that of Eurypterus, as the adult morphology of appendage V in Pentecopterus exhibits the reduction of spines but lacks the elongated, tubular podomeres of Megalograptus and Eurypterus.
Implications for early eurypterid evolution
It has been suggested that eurypterids originated in Gondwana, as the Ordovician eurypterids from Gondwana are basal forms with poor dispersal abilities, whereas those from Laurentia are more derived forms capable of active swimming . As a swimming form, like other megalograptids , Pentecopterus is consistent with this pattern. All early Laurentian eurypterids are relatively large predators, either megalograptids  or waeringopterids . The taxonomically restricted nature of the Laurentian fauna may be due to a limited influx of early colonists from Gondwana, whereas other eurypterid groups arrived in the early Silurian.
Eurypterid cuticular structures
Numerous isolated follicles with broken setal bases are preserved on the opisthosomal cuticle surface (Fig. 23f); these follicles are also prevalent on the prosomal appendages, where they appear to be concentrated in the ventral region of the podomeres (Fig. 12). Setal density is particularly high on the paddle of the sixth appendage where it increases towards the paddle margin (Fig. 14e) forming a fringe of setae similar to that in brachyuran swimming crabs . In brachyurans these setae are stiff and function to expand the surface area of the paddle during swimming . The follicles in eurypterids are smaller than in brachyurans and the setae may have had a sensory function.
Setae are also preserved on the integument surrounding the prosomal ventral plate (Fig. 4b, c). Here they take the form of very fine hairs covering the cuticle on the ventral surface of the prosoma where the appendages and ventral plate insert. Similar setation is present in modern horseshoe crabs . The ventral plate itself bears a row of scales along its inside margin (Fig. 4a) and terrace lines (Fig. 4b) across the remainder of the plate. Terrace lines have been reported in a number of other eurypterids [14, 45, 82] and are prevalent in trilobites  and decapod crustaceans [84, 85]. Setae often insert along the terrace lines in trilobites , and such setae were likely sensory in both trilobites and eurypterids [45, 86].
Appendage differentiation and ontogeny
The smallest eurypterid specimens (SUI 139963 and SUI 139965, Fig. 5) exhibit an appendage morphology different from that of the larger individuals. They bear a series of homonomous appendages but the armature, which consists of a pair of moveable ventral spines, a pair of elongate fixed lateral spines, and a denticulate distal margin on each podomere, shows marked similarities to that in the large Pentecopterus specimens. All the appendages share a distinctive ornament of guttalate scales, which are relatively larger and more densely spaced in the smaller individuals. The number of scales in the smallest specimens is similar to that in the larger ones, although the distance between them increases. The occurrence of a relatively small ventral plate (Fig. 1f, g) confirms that small individuals of Pentecopterus occur at the locality, even though no small specimens showing the morphology of the larger appendages are present. The similarities in armature and ornamentation, combined with the absence of any other morphologically distinct eurypterid material in the fauna, suggests that all the available Winneshiek eurypterid material represents the same species and that SUI 139963 and SUI 139965 are juvenile individuals of Pentecopterus decorahensis.
Juveniles have been reported for only a handful of eurypterid species [4, 25, 26, 70, 87]. In-depth analysis of allometric trends between instars has only been attempted for Eurypterus remipes [23–25], Hardieopterus (?) myops , Adelophthalmus luceroensis , and Strobilopterus proteus . Such a detailed study is not possible for Pentecopterus, as there are only two readily identifiable juveniles. Pentecopterus does, however, reveal a degree of ontogenetic change in appendage armature previously undocumented in eurypterids. Change in the morphology of the sixth appendage paddle has been described in Strobilopterus princetonii  and an allometric decrease in relative appendage length was noted in Drepanopterus pentlandicus  and Strobilopterus proteus , but the configuration of the appendage armature does not vary between instars in any of these taxa. Pentecopterus, in contrast, undergoes change in appendage armature as well as an allometric shift in appendage proportions.
Appendage morphology is used as a key diagnostic feature for assigning eurypterid species to higher clades  and differences in armature would normally be considered indicative of a separate species. Our demonstration that at least some eurypterid species undergo changes in armature morphology during ontogeny reinforces the importance of including ontogenetic data when defining and describing taxa . Division of form and function in the prosomal appendages has been considered a significant character in defining eurypterid higher taxa, and is thought to display strong evolutionary signal [41, 44]. A lack of appendage differentiation could result in juvenile eurypterids, like that of Pentecopterus, being placed in more basal eurypterid clades if their juvenile nature was not recognized.
The newly described eurypterid Pentecopterus decorahensis from the Winneshiek Lagerstätte is the earliest described representative of the group, pushing our knowledge of Eurypterida back some 9 million years to the Darriwilian in the Middle Ordovician. Pentecopterus shows clear affinities with megalograptids, a highly distinct group of large predatory eurypterids known solely from the Ordovician of North America. Inclusion of the taxon in an expanded phylogenetic analysis of Eurypterida resolves Pentecopterus as basal within the megalograptid clade, which is itself part of the relatively derived carcinosomatoids. Pentecopterus reveals that a number of characteristics thought to link megalograptids with basal Eurypterina, such as the absence of a modified distal margin to the sixth podomere of the paddle, represent reversals in Megalograptus. Meanwhile, characters that were regarded as indicative of affinities between Megalograptus and Eurypterus, such as modification of the fifth prosomal appendage into a ‘balancing’ limb, are due to convergence. The occurrence of derived eurypterid clades in the Middle Ordovician indicates that Eurypterida either have a longer evolutionary history than previously recognized, extending back into the Cambrian, or underwent an explosive radiation following an Ordovician origin. This would also push back the origin of arachnids, which likely have a sister-group relationship with eurypterids [34, 35] from the early Silurian to at least the Middle Ordovician.
As well as informing on broader evolutionary trends, numerous specimens of Pentecopterus reveal the patterning and structure of the cuticular ornament, including scales and setal insertions, allowing for direct comparison with exceptionally preserved material of Silurian age Eurypterus . This allows for potential exploration of general properties of eurypterid cuticle and ornamentation, as well as revealing the position of sensory setae and the form of podomere articulations. The identification of juvenile specimens of Pentecopterus provides evidence for an unusual degree of postembryonic appendage differentiation in eurypterids. Initial studies utilizing eurypterid phylogeny  and exquisitely preserved morphological features  such as those reported here are already beginning to place eurypterids within an ecological framework, and this diverse arthropod group represents a promising source for macroevolutionary and macroecological studies.
Availability of supporting data
The data set supporting the results of this article is available in the MorphoBank repository, http://morphobank.org/permalink/?P2116, and is also included within the article and its additional file.
The research was funded by NSF USA (EAR 0921245 and 0922054). We are grateful to Tiffany Adrain of the University of Iowa Paleontology Repository, Department of Earth and Environmental Sciences, for specimen curation. Robert Rowden, Tom Marshall, Caroline Davis, Charles Monson and Donald Campbell assisted with the excavation. Marc Spencer, Julia McHugh, Eric Wilberg, Matthew Tibbits, Huijuan Zou, Jeff Matzke, Kathlyn McVey, David McKay, Jon Hansen, Brian Neale, Caitlin Kuempel, Elizabeth Greaves, Kelli Parson, Marta Behling, Bass Dye and Travis Maher helped with sample preparation and fossil identification in the laboratory. David Marshall furnished references on cuticle structure, Stefan Nicolescu provided access to ultraviolet lighting, and Emma Locatelli assisted with Scanning Electron Microscopy. Carolin Haug and two anonymous referees provided comments that greatly improved the manuscript during review.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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