Paleolimulus, an early limuline (Xiphosurida), from Pennsylvanian± LagerstaÈtten of and taphonomic comparison with modern Limulus

LOREN E. BABCOCK, DANIEL F. MERRIAM AND RONALD R. WEST

Babcock, L.E., Merriam, D.F. & West, R.R. 2000 09 15: Paleolimulus, an early limuline (Xiphosurida), from Pennsylvanian±Permian LagerstaÈtten of Kansas and taphonomic comparison with modern Limulus. Lethaia, Vol. 33, pp. 129±141. Oslo. ISSN 0024- 1164.

The Pennsylvanian±Permian horseshoe crab Paleolimulus signatus (Beecher), incorpor- ating as a junior synonym P. avitus Dunbar, is one of the earliest species of the Limuli- na (Xiphosurida). Some specimens from Kansas, USA, are exceptionally well preserved, retaining intact book gills and appendages. Indistinct, bilobed burrowing traces of vari- able width occur in association with some examples of P. signatus and may have been produced by that . Based on actualistic taphonomic experiments on Limulus polyphemus, ancient horseshoe crabs and other having non-mineralized exoskeletons are inferred to have become pliable soon after death or moulting, and to have disarticulated slowly prior to burial. Extreme compression, wrinkling, and loose folding of sclerites are attributed to burial of a pliable exoskeleton. Slow preburial dis- articulation partly accounts for the exceptional preservation of Paleolimulus remains. Also relevant for the exceptional preservation of these arthropods was burial in estuar- ine, tidal ¯at, or lacustrine environments. Because of ¯uctuating salinity and possibly dessicating conditions, these settings were limiting to scavengers, burrowers, and some microbes that could potentially disarticulate or decompose xiphosurid remains. & , horseshoe crab, Kansas, Pennsylvanian, Permian, taphonomy, Xiphosurid.

Loren E. Babcock, Department of Geological Sciences, The Ohio State University, Colum- bus, Ohio 43210, USA [[email protected]]; Daniel F. Merriam, Kansas Geological Sur- vey, The University of Kansas, Lawrence, Kansas 66047, USA [[email protected]]; Ronald R. West, Department of , Thompson Hall, Kansas State University, Man- hattan, Kansas 66506, USA [[email protected]]; 1st December, 1999; revised 22 March, 2000.

Horseshoe crabs, or xiphosurids, are a small clade of are Xaniopyramis from the upper Mississippian (or chelicerate arthropods that have a stratigraphic record Lower ; Namurian) of England (Siveter ranging from the Palaeozoic to the present. Approxi- & Selden 1987), and Valloisella from the middle mately 30 valid genera and approximately 50 species of Pennsylvanian (or Upper Carboniferous; Westphalian these are known, of which four species, B) of England (Dix & Jones 1932; Anderson & belonging to three genera, are extant. Horrocks 1995). Understanding the evolution of The purposes of this paper are: (1) to clarify the limulines depends on elucidation of the relationships concept of the type species of Paleolimulus from upper among species attributed to Paleolimulus and other Palaeozoic rocks of Kansas (Figs 1, 2); and (2) to early forms. In this context, we provide new informa- provide information on taphonomic conditions lead- tion on the type species of Paleolimulus that will help ing to the exceptional preservation of specimens in facilitate comparison with early species. those rocks. Paleolimulus has been recognized (Berg- The second important aspect of this work is that it stroÈm 1975; Stùrmer 1952, 1955; Fisher 1982, 1984; represents a further step in our understanding of the Waterston 1985; Selden & Siveter 1987; Anderson & taphonomic conditions under which non-mineralized Horrocks 1995; Anderson & Selden 1997) as one of the arthropods are preserved as fossils. Lack of a miner- earliest limulines, or so-called modern horseshoe alized cuticle, possession of a rather generalized crabs. A putative Paleolimulus from Wales indicates arthropod morphology, and occurrence of morpho- that this genus dates from the Middle Pennsylvanian logically similar living and fossil representatives, (or Upper Carboniferous; Westphalian B; BergstroÈm, makes these xiphosurids excellent subjects for under- 1975; Fisher 1984; Waterston 1985; Anderson & standing taphonomic processes affecting various non- Horrocks 1995). Other early examples of limulines mineralized arthropods. Actualistic experiments can 130 Loren E. Babcock et al. LETHAIA 33 (2000)

Fig. 1. Map showing localities of Permian (1±3) and Pennsylvanian (4) Paleolimulus signatus in northeastern Kansas, USA. County names (Riley, Pottawatomie, Dickinson, and Wabaunsee) are indicated on the map. Locality 1, 4.8 km west of Stockdale, Riley County, is uncertain because the original town no longer exists. Locality 2 is on the east side of Tuttle Lake, Pottawatomie County. Locality 3 is about 4 km south± southeast of Elmo, Dickinson County. Locality 4 is 3.2 km southwest and 3.2 km south of Maple Hill, Wabaunsee County.

be conducted on living representatives and applied to Late Palaeozoic xiphosurids from fossil representatives that seem to have similar Kansas taphonomic histories. Results of such experiments, as shown here, can have important implications for Six genera and eight species of real or supposed the morphologic and even systematic interpretation of xiphosurids were named from specimens collected non-mineralized arthropods from Palaeozoic Lager- from Permian rocks of eastern Kansas (Beecher 1904; staÈtten. In addition to demonstrating that some non- Dunbar 1923; Tasch 1961, 1963, 1964; Babcock 1990). mineralized arthropod exoskeletons are resistant to This number is mitigated, however, by the subsequent preburial disarticulation, these experiments demon- reassignment of ®ve genera and ®ve species described strate that distortion without breakage of the exoske- by Tasch (1961, 1963, 1964) from the Wellington leton is related to pliability of that exoskeleton after Formation (Middle Permian) to other arthropod death or moulting. Specimens showing similar distor- groups (BergstroÈm 1975). One species described by tion have been illustrated from most Palaeozoic Tasch (1961) from the Wellington Formation at Konservat-LagerstaÈtten representing aquatic environ- Annelly, Harvey County, Pringlia leonardensis, appears ments (e.g. see Schram 1979; Conway Morris et al. from published illustrations (Tasch 1961, pl. 98, ®gs. 1982; Baird et al. 1985; Mikulic et al. 1985; Whitting- 4a, b) to be a genuine xiphosurid. ton 1985; Hou & BergstroÈm 1990; Robison 1990; The valid horseshoe crab genus Paleolimulus Dun- Babcock & Chang 1997). bar, 1923, was described using P. avitus Dunbar, 1923 Repositories. ± Fossil specimens cited herein are (Fig. 3D±G), from the Wellington Shale (Middle deposited in the following institutions: Peabody Permian) at `Insect Hill', near Elmo, Dickinson Museum of Natural History, Yale University, New County, as the type species. Prestwichia signata Haven, Connecticut (YPM); U.S. National Museum of Beecher, 1904 (Fig. 3A±C), which was previously Natural History, Washington, D.C. (USNM); Uni- described from the Fort Riley Member of versity of Kansas Natural History Museum (Division the Barneston Limestone (Lower Permian) near of Invertebrate Paleontology), Lawrence, Kansas Stockdale, Riley County, was transferred to Paleoli- (KUMIP); and Orton Geological Museum, The Ohio mulus by Dunbar (1923). As revised herein, P. signatus State University, Columbus, Ohio (OSU). is a senior synonym of P. avitus. LETHAIA 33 (2000) An early limuline from Pennsylvanian±Permian LagerstaÈtten of Kansas 131

of this species (YPM 35153) was collected from a Mazon Creek-type assemblage (Braidwood biota) in the Lawrence Formation (Douglas Group; Upper Pennsylvanian) near Lawrence, Douglas County. Other evidence of the existence of xiphosurids in Kansas is in the form of trace fossils. Bandel (1967) reported trails putatively produced by xiphosurids (= Kouphichnium) from the Tongonoxie Sandstone Member of the Stranger Formation (Douglas Group; Upper Pennsylvanian), near Ottawa, Franklin County. Here, we illustrate an indistinct, bilobed trace fossil (Fig. 4I), one of many at locality 4 (Figs 1, 2). The traces are inferred to have been produced by xipho- surids (presumably P. signatus), although they do not exactly conform in shape to either Kouphichnium or Protolimulus traces described from other localities and ages (e.g. Caster 1938; Malz 1964; Hardy 1970; Miller 1982; Chisholm 1985; Babcock et al. 1995). The bilobed ploughing traces are similar to bilobed traces produced by modern Limulus in wet, soupy mud (Fig. 5). Of particular note is the general lack of distinct scratch marks left by the appendages in both the ancient and modern traces. Indistinct impressions may be the result of burrowing into a relatively ¯uid- rich sediment.

Geographic and stratigraphic occurrences

Paleolimulus is known from four localities in Kansas (Fig. 1). A generalized stratigraphic column, showing productive intervals, is given in Fig. 2. Additional stratigraphic information concerning the units from which P. signatus has been collected was provided by Zeller (1968). Locality 1. ± Fort Riley Limestone Member of the Barneston Limestone (Chase Group; Lower Permian); from about 4.8 km (3 miles) west of Stockdale, Riley County (Beecher 1904). This is the type locality for P. signatus (USNM 483405; casts YPM 26319, 26319A). The location of this site relative to present-day Fig. 2. Generalized stratigraphic section for part of the Upper geography is uncertain because Stockdale no longer Pennsylvanian to Middle Permian of northeastern Kansas (modi- exists. According to an early gazetteer (Gannett 1898), ®ed from Zeller 1968). Localities are in these stratigraphic units: 1, Stockdale was a post of®ce and station on the Blue Fort Riley Limestone Member of the Barneston Limestone (Chase Group); 2, Crouse Limestone (); 3, Elmo Valley and Manhattan Railway in Grant Township, Limestone Member of the Wellington Formation (Sumner Group); Riley County, altitude 340 m. The most likely locality and 4, Pony Creek Shale Member of the Wood Siding Formation is west of the Stockdale Recreation Area (near Tuttle (Wabaunsee Group). Approximate stratigraphic thickness repre- sented: 3750 m. Creek Lake) in the south half of sec. 33, T. 8 S., R. 7 E., Riley County (Oldsburg SW, Kansas 7.5' topographic quadrangle, U.S. Geological Survey 1964, photore- In addition to taxa named from Kansas, one vised 1978). euproopid xiphosurid, Euproops danae (Meek & Worthen) also occurs in the state. A studied specimen Locality 2. ± Crouse Limestone (Council Grove Group; 132 Loren E. Babcock et al. LETHAIA 33 (2000)

Fig. 3. &A±H. Paleolimulus signatus (Beecher). &A. Restored prosoma, plastoholotype of Prestwichia signata Beecher (original of Beecher 1904: ®g. 1); locality 1; YPM 26319, Â1. &B. Slightly restored prosoma, same specimen as in A showing appearance of the specimen about the time of Beecher's (1904) work; YPM 26319A, Â1. &C. Holotype prosoma (specimen from which those in A and B were cast), an internal mould, as it appears today; pieces from the right side have been lost (cf. B); note the moulds of the intestinal diverticula; USNM 483405; Â1. &D. Small, articulated exoskeleton preserved as an internal mould, and broken across the left side and the anterior part of the telson; impressions of most of the prosomal appendages are preserved but the gills seem to have been lost; lectotype of Paleolimulus avitus Dunbar (original of Dunbar 1923: ®g. 6); locality 3; YPM 26324; Â4. &E. Counterpart of specimen in D (original of Dunbar 1923: ®gs. 4, 5), an external mould showing impressions of some of the appendages; locality 3; YPM 26324; Â1. &F. Strongly wrinkled prosoma, internal mould?; paralectotype of Paleolimulus avitus Dunbar (original of Dunbar 1923: ®g. 3); locality 3; YPM 26317; Â4. &G. Prosoma, internal mould?, paralectotype of Paleolimulus avitus Dunbar (original of Dunbar 1923, p. 449); YPM 26325; Â2. &H. Broken opisthosoma, retaining both sclerites, collected by C.O. Dunbar; locality 3; YPM 35116; Â2.5.

Lower Permian); from Carnahan Creek Recreation Geological Survey 1964, photorevised 1978). One Area on the east side of Tuttle Creek Lake, west side of specimen, USNM 484412, was collected from this site. sec. 23, T. 8 S., R. 7 E., Pottawatomie County; Tuttle Creek Dam, Kansas 7.5' topographic quadrangle (U.S. Locality 3. ± Insect Hill Konservat-LagerstaÈtte. Elmo LETHAIA 33 (2000) An early limuline from Pennsylvanian±Permian LagerstaÈtten of Kansas 133

Limestone Member of the Wellington Formation habitats expected of xiphosurids (e.g. Rudloe 1979; (Sumner Group, Middle Permian); from Insect Hill, Miller 1982; Pickett 1984; Fisher 1984; Baird et al. about 4 km (2.5 miles) south±southeast of Elmo, 1985; Chisholm 1985; Boucot 1990; Babcock et al. 1 Dickinson County; center of SE4 sec. 21 T. 16 S., R. 2 1995). However, paleolimulids from Carboniferous E., Elmo, Kansas 7.5' topographic quadrangle (U.S. coal swamp settings have been inferred to have been Geological Survey 1964, photorevised 1985). The distinctly marine in habitat (Schram 1979; Fisher syntypes of P. avitus (YPM 26324, 26317, 26325) as 1984), compared to euproopids, which are interpreted well as at least 12 additional specimens collected by as nearer the brackish water-freshwater-partly terres- C.O. Dunbar (YPM 16156, 35116, among others; see trial end of the spectrum of environments represented Raymond, 1944), were collected from this locality. The in coal swamps (Fisher 1984). Permian paleolimulids specimen illustrated by Tasch (1963, pl. 173, ®g. 1) similarly were inferred (Fisher 1984) to have tended from quarry debris was from this locality and bed toward marine habitats. Oddly, other marine-in¯u- (Elmo III, bed 1? of Tasch 1963). enced Pennsylvanian or Permian Konservat-LagerstaÈt- ten (Mapes & Mapes 1988; Maples & Schultze 1988) in Locality 4. ± Pony Creek Shale Konservat-LagerstaÈtte. the same general region of the USA have not yielded Pony Creek Shale Member of the Wood Siding Paleolimulus. Formation (Wabaunsee Group; Upper Pennsylva- nian); 3.2 km (2 miles) southwest and 3.2 km (2 miles) south of Maple Hill, Wabaunsee County; along the north section line of sec. 35, T. 11 S., R. 12 E., Preservational circumstances and Maple Hill, Kansas 7.5' topographic quadrangle (U.S. taphonomy Geological Survey 1953, photorevised 1978). Numer- ous specimens, including USNM 465528, 484407- Xiphosurid diversity and preservation. ± The fossil 484411, YPM 170019-170024, KUMIP 286276- record of xiphosurids, although it extends to the early 286281, and OSU 46358-46363, have been collected Palaeozoic, is rather meagre (Fisher 1984). Xiphosur- from limestone concretions and lenses at this site. ids are one of the least diverse groups of arthropods Depositional environments and xiphosurid habitats.± known from fossil remains, and they seem to have had Among the localities yielding Paleolimulus in Kansas, a genuine low diversity of species through time (see only localities 3 and 4 have yielded multiple speci- Fisher 1984). The accepted diversity of present-day mens, so it is for these localities that the interpretation xiphosurids world-wide is four species, Carcinoscor- of depositional environments is most meaningful for pius rotundicauda, Limulus polyphemus, Tachypleus understanding the habitat preferences of Paleolimulus. gigas, and T. tridentatus (Sekiguchi & Sugita 1980; The insect beds of the Wellington Formation (locality Fisher 1984). A ®fth described species, T. hoeveni 3) were evidently deposited in a lacustrine setting (Pocock 1902) has not been recognized by most (Dunbar 1923; Brooks 1957; Tasch 1958, 1961, 1963); workers (Waterman 1958; Fisher 1984). This is a adjacent rocks, which are rich in conchostracan marked departure from the diversity of most other crustaceans (Tasch 1958, 1961, 1963), represent fresh- arthropod groups, which together have a present-day water lake or brackish water lagoonal deposits diversity numbering in the millions of species, and (Dunbar 1923; Brooks 1957). which account for more than 75% of living animal Xiphosurids from the Pony Creek Shale (locality 4) species (Hickman et al. 1982). occur within a succession of strata that has been The low observed diversity of ancient xiphosurids interpreted (West & Matsumoto 1989) to represent a probably is enhanced by two additional factors. One marine-to-non-marine transition. Our observations factor is the lack of a mineralized exoskeleton, which of paired, mm-scale, light-dark laminations in the limits their preservation potential. The second factor is Pony Creek Shale Member at the site suggest tidally the role of habitat in limiting the preservation of taxa. in¯uenced sedimentation (see Feldman et al. 1993) for Marginal-marine through brackish water, freshwater, the beds yielding Paleolimulus and the xiphosurid and partly terrestrial settings, which represent the trace fossils. The Pony Creek Shale at this locality is depositional environments where most fossil xipho- interpreted to have been either an estuary or tidal ¯at. surid remains occur (Fisher 1979; Pickett 1984), Walking or burrowing traces of xiphosurids, in generally are not well represented in the stratigraphic particular, are usually associated with tidal-¯at litho- record. facies (Caster 1938; Brooks 1957; Miller 1982; Babcock Despite the rather sporadic stratigraphic occurrence et al. 1995), although they are not restricted to them. of horseshoe crabs, specimens from some localities are An estuarine, tidal-¯at, or lacustrine habitat for P. so well preserved that we can obtain from them an signatus is compatible with the general range of excellent morphological understanding of not just the 134 Loren E. Babcock et al. LETHAIA 33 (2000)

Fig. 4. &A±H. Paleolimulus signatus (Beecher) in different stages of disarticulation. &A. External mould of specimen in stage 1 of disarticulation, retaining appendages but lacking gills; note the split along the anterolateral margin of the prosoma; locality 3; YPM 16156; Â4. &B. Internal mould of specimen apparently buried before reaching stage 1 of disarticulation, preserved in a small limestone concretion; the telson and the posterior part of the posterior opisthosomal sclerite were outside the concretion and not preserved or collected; note the book gills attached to the opisthosoma; locality 4; USNM 484407; Â1.25. &C. Internal mould of specimen in stage 2 of disarticulation, in which the book gills have been lost and most of the appendages have been broken along joints; the anterior part of the telson is attached to the posterior opisthosomal sclerite but most of the telson was not collected; locality 4; USNM 465528; Â1.25. &D. Internal mould of specimen in stage 3 of disarticulation, in which the telson has been lost; little remains of the appendages; note loose folding of the exoskeleton, especially along the opisthosoma; also note the joint between anterior and posterior abdominal sclerites (arrow); locality 4; USNM 484408; Â1. &E. External mould of specimen in stage 3 of disarticulation (the un®gured part of the specimen, which preserves only the most posterior part of the specimen, demonstrates that the telson was disarticulated); locality 4; USNM 484409; Â1. &F. Internal mould? of separate prosoma in stage 4 of disarticulation; note strong wrinkling of the exoskeleton; locality 4; USNM 484410; Â1. &G. Internal mould of separate prosoma in stage 4 of disarticulation; note remains of the intestinal diverticula; locality 4; USNM 484411; Â0.75. &H. Internal mould of separate opisthosoma, retaining both sclerites, in stage 4 of disarticulation; locality 2; USNM 484412; Â1. &I. Incomplete ploughing trace from an inferred tidal-¯at lithofacies; occurs in association with, and thought to have been produced by, P. signatus; locality 4; USNM 484413; Â1. LETHAIA 33 (2000) An early limuline from Pennsylvanian±Permian LagerstaÈtten of Kansas 135

Fig. 5. Ploughing traces produced by extant Limulus polyphemus in wet mud, St. Catherines Island, Georgia, USA. Note indistinct scratch marks left by the appendages. Centimetre scale is on camera lens cap.

dorsal surface, but also the appendages and book gills Actualistic taphonomic experiments.±Paleolimulus (Dunbar 1923; Raymond 1944; herein). In a few signatus specimens from the Pennsylvanian±Permian Konservat-LagerstaÈtten such as the Solnhofen Lime- of eastern Kansas represent the full range of tapho- stone of Germany (Barthel 1978; Barthel et al. 1990), nomic states from individual, disarticulated sclerites the Insect Hill deposit (Dunbar 1923, 1924; Carpenter to articulated exoskeletons that retain the ventral 1930; Raymond 1944) of Kansas, and the Pony Creek appendages and gills (Fig. 4). To better understand the Shale of Kansas (herein), specimens are also moder- early taphonomic history (necrolysis through biostra- ately abundant. This apparent paradox seems to be tinomy) of these fossils, comparative actualistic explainable partly by a slow preburial disarticulation experiments were performed on Limulus polyphemus. rate of xiphosurid exoskeletons, and partly by the Most of these results were reported earlier (Babcock & exclusion or near-exclusion of scavengers, sediment Chang 1997), but additional information is presented bioturbators, and effective microbial decomposers here. The results show that preburial disarticulation of from some postburial depositional settings that yield xiphosurid remains (Fig. 6) is a relatively slow process, xiphosurid remains (see Barthel 1978; Barthel et al. a discovery that is in general agreement with actualistic 1990; Babcock & Chang 1997). Fluctuating salinity data on other marine arthropods and polychaete conditions (Babcock & Chang 1997; Babcock 1998) worms (Allison 1986, 1988; Plotnick 1986; Plotnick and possibly desiccating conditions are probably et al. 1988; Allison & Briggs 1991a, b; Briggs & Kear mostly responsible for limiting these organisms from 1993). Disarticulation of Limulus, however, seems to preburial and postburial environments; anoxic sedi- proceed at a slower rate than disarticulation of ments could have limited further destructive previously studied arthropods. organisms from postburial environments. High sedi- Comparative taphonomic experiments were per- mentation rates, which are characteristic of some formed on small specimens of the Holocene L. tidally in¯uenced environments that are potential polyphemus. Specimens were approximately 7±13 cm xiphosurid-bearing LagerstaÈtten (Feldman et al. long, which is the general size range of most observed 1993; Babcock et al. 1995; herein), may also partly P. signatus. Three different experiments were con- account for the exceptional preservation of unminer- ducted on dead specimens kept in oxygenated, alized, chitinous xiphosurid exoskeletons. arti®cial seawater inoculated with bacteria. Specimens 136 Loren E. Babcock et al. LETHAIA 33 (2000)

mimics incomplete enrollment. Usually, the prosoma ¯ops forward a little compared to the opisthosoma. Internal soft parts of L. polyphemus decay in seven days in oxygenated, inoculated, arti®cial seawater, but sclerites tend to remain articulated for a longer time whether the carcass is left undisturbed or is vigorously tumbled in seawater. Tumbled specimens show perforation of the ventral exoskeleton and loss of some book gills (Fig. 6A), beginning with the posterior ones, after about 7 days. Thus, after this time, a carcass effectively becomes a sedimentary particle that is hydrodynamically similar to, if not indistinguishable from, a moulted exoskeleton. Vigorously tumbled specimens have most of their appendages broken after 10 days (Fig. 6B), and the telson detaches after 14 days (Fig. 6C). Finally, the prosoma separates from the opisthosoma after 27±40 days. (Fig. 6D). At a tumbling rate of 3 km/day, this is the equivalent of being transported approximately 81±120 km in a unidirectional current system. Major sclerites of undisturbed (untumbled) specimens can remain articulated in normally oxygenated seawater for at least 64 days under experimental conditions when scavengers and sediment bioturbators are excluded.

Taphonomic comparison. ± In Holocene Limulus and Pennsylvanian±Permian Paleolimulus, ®ve distinct stages of progressive disarticulation are recognizable Fig. 6. &A±D. Diagrammatic line drawings showing Limulus (Figs 4, 6). These stages, as indicated here, can be polyphemus (Linnaeus) in different stages of disarticulation. &A. Stage 1 of disarticulation; most gills have detached from the ventral equated with an elapsed time of decay and disarticula- side. &B. Stage 2 of disarticulation; most legs have been broken tion in Limulus, and, by inference, in Paleolimulus. from the ventral side and many marginal spines have been lost. They bracket the time of ®nal burial of Paleolimulus or &C. Stage 3 of disarticulation; the telson has separated from the opisthosoma. &D. Stage 4 of disarticulation; the opisthosoma has other xiphosurid remains relative to the time of death. separated from the prosoma. Stage 0: Specimen is fully articulated. Stage 1: Most of the book gills become detached (Figs 4A, 6A). Stage 1 is reached in about 7 days if the were: (1) tumbled shortly after death (to simulate specimen is tumbled and after 53 days if the specimen transport in a nearshore marine or estuarine setting); is untumbled. (2) tumbled beginning one week after death (following Stage 2: Most of the legs become broken (Fig. 4C). decay of all internal soft parts); or (3) left undisturbed Stage 2 is reached in 10±18 days if the specimen is in a tank. The tumbling of specimens was carried out tumbled, and after 62 days if the specimen is in an ordinary lapidary tumbler with inoculated untumbled. arti®cial seawater and no sediment; specimens were Stage 3: The telson becomes detached (Figs 4D, 4E, tumbled at a rate equivalent to transport of about 3 6C). Stage 3 is reached after 14 days; it is not always km/day. recognizable as distinct from stage 4 because all of the A small, freshly dead specimen of L. polyphemus major sclerites may disarticulate at nearly the same ¯oats for a few hours to two days. The carcass initially time. curls somewhat as rigor mortis begins but becomes Stage 4: The prosoma and opisthosoma become entirely limp or pliable in 1±48 hours. If handled, the detached (Figs 4F±H, 6D). Stage 4 is reached between exoskeleton can be compressed and wrinkled or bent about 27 and 40 days if the specimen is tumbled, and to a considerable extent without rupturing. The joints after 64 days (quickly following stage 2 or 3) if the are soft, and major sclerites ¯op about loosely. When specimen is untumbled. allowed to settle through water and come to rest on Pliability of Limulus exoskeletons in water also the sediment, the major tagmata (prosoma, opistho- seems to have parallels among ancient specimens. All soma, and telson) can fold loosely in a way that specimens of P. signatus illustrated here (Fig. 3, 4A±H) LETHAIA 33 (2000) An early limuline from Pennsylvanian±Permian LagerstaÈtten of Kansas 137

Fig. 7. &A, B. Limulus polyphemus (Linnaeus); Holocene. &A. Dorsal view of exoskeleton showing position of fused joint in the opisthosoma (arrow), which is thought to be homologous with the joint between the two opisthosomal sclerites of Paleolimulus signatus (cf. Fig. 4D); Âl. &B. Dorsal view of prosoma showing impressions of the intestinal diverticula on the internal surface of the dorsal side; Â1.25. show some post-mortem distortion. Distortion ranges signatus are compressed to some extent. Upon from simple compression (Fig. 4H) through minor moderate compression in soft sediment, a pliable (Figs 3G, 4E) or major (Fig. 4F) wrinkling, to folding prosoma is expected to show enhancement of the (Fig. 4D) of the exoskeleton. With one exception (Fig. lobation because the exoskeleton originally would 4A), distortion has occurred without obvious ruptur- have been corrugated at the boundaries between the ing of the exoskeletal margin. Evidently, illustrated P. lobes (cf. Pickett 1984, pl. 56, ®g. 1). There is no reason signatus specimens (with the possible exception of the to think, however, that the lobes were more strongly one in Fig. 4A) had pliable exoskeletons at the time of expressed on living specimens of P. signatus that they burial, just as small L. polyphemus specimens do if they are on L. polyphemus. are kept in water until burial. The exoskeleton of L. polyphemus becomes brittle upon desiccation, and Comparison with the taphonomy of other arthropods.± dorsoventrally oriented pressure can cause rupturing The preburial disarticulation rates of present-day at the margin. The large marginal rupture on the xiphosurids (Babcock & Chang 1997; herein) and anterolateral part of the prosoma of the specimen in other arthropods (Allison 1986, 1988; Allison & Briggs Fig. 4A suggests that it was at least partly dried out at 1991a, b) are relatively slow. Actualistic experiments the time of burial. help us to more accurately specify at what rates Variable compression of a pliable exoskeleton in arthropod exoskeletons disarticulated after death (cf. water is here interpreted to be responsible in part for Speyer & Brett 1985, 1986; Speyer 1987), and, in the expression of lobation on the intra-opthalmic general, those rates are somewhat slower than pre- areas of the prosoma of P. signatus (see Dunbar 1923; viously assumed. Aquatic arthropods seem to dis- Raymond 1944). Dunbar (1923) considered the articulate more slowly than do echinoderms kept in lobation of this area to be the most distinguishing warm water (see Kidwell & Baumiller 1990; Baumiller character of the genus Paleolimulus. Although the & Ausich 1992). This demonstrates that not all intra-ophthalmic area of the prosoma of Paleolimulus organisms having multipart skeletons can be con- is undoubtedly lobed, just as the homologous area in strued as equivalent in terms of disarticulation rates. extant Limulus is lobed (Fig. 7A, B), available speci- The relatively slow disarticulation rates of present- mens show considerable variability in that character day aquatic arthropods have important implications (cf. Figs 3C, 3F, 3G, 4D±G). All known specimens of P. for the fossil record of arthropods. Prior to burial, 138 Loren E. Babcock et al. LETHAIA 33 (2000) some ancient arthropods may have remained articu- Type species. ± Paleolimulus avitus Dunbar, 1923, by lated or partly articulated for somewhat longer lengths original designation (Dunbar 1923, p. 444). of time than earlier assumed. Arthropods that have mineralized exoskeletons (e.g. trilobites and ostra- Paleolimulus signatus (Beecher, 1904) codes) are more abundant in the stratigraphic record Figs 3,4 as a whole, and articulated or partly articulated specimens are moderately abundant in places. Miner- 1904 Prestwichia signata Beecher, p. 24, ®g. 1. alized sclerites of arthropods are held together at joints 1923 Paleolimulus avitus Dunbar - pp. 444±451, ®gs. by unmineralized cuticle. Slow preburial disarticula- 1±6. tion rates in environments having few scavengers, low rates of bioturbation, and possibly low rates of 1923 Paleolimulus signatus (Beecher) ± Dunbar, p. 450, microbial decay (in some situations related to ¯uctu- Fig. 8. ating salinity), can help to explain the abundant 1944 Paleolimulus avitus Dunbar ± Shimer & Shrock, occurrence of articulated arthropods in some places. p. 707, Pl. 300, ®gs. 1, 2. Pliability of the sclerites or joints of arthropods soon after death also has important implications for 1952 Palaeolimulus [sic] avitus Dunbar ± Stùrmer, ®g. the fossil record of aquatic arthropods, particularly as 1j. it affects our interpretation of life habits. Among 1955 Paleolimulus avitus Dunbar ± Stùrmer, pp. P21, arthropods lacking a mineralized exoskeleton, post- P22, Fig. 15C, 16,1. mortem or post-moulting distortion has been dis- cussed for various arthropods including xiphosurids 1963 Paleolimulus avitas [sic] Dunbar ± Tasch, p. 1247, (herein) and naraoiids (Babcock & Chang 1997), and pl. 173, ®g. 1. the phenomenon probably affected a wide variety of 1987 Paleolimulus [sp.] Robison ± Fig. 13.46, A. non-mineralized arthropods from LagerstaÈtten depos- its. As examples among arthropods having miner- 1991 Paleolimulus [sp.] Eldredge ± pl. 90. alized sclerites, agnostoid trilobites have been inferred 1991 Paleolimulus avitus Dunbar ± Novozhilov, Fig. to have lived in an enrolled condition (Robison 1972; 1183. MuÈller & Walossek 1987), yet articulated specimens usually occur outstretched (e.g. Robison 1982, 1984). Types. ± Holotype prosoma of P. signatus, USNM The outstretched condition of these animals (which 483405, from locality 1; plaster casts, YPM 26319, could be misinterpreted as a life posture) may be 26319A. Lectotype of P. avitus (selected here), part and attributable to post-mortem or post-moulting plia- counterpart, YPM 26324; paralectotypes, YPM 26317, bility of the cuticle at the arthrodial membranes. 26325, all from locality 3. Pliability of the arthrodial membranes also raises questions about the ¯exed condition in which Discussion. ± The holotype prosoma of P. signatus was trilobites such as calymenids may occur (e.g. Whit- originally illustrated by only a restored plaster cast tington 1992; Mikulic 1994; Babcock 1997). As (Beecher 1904, ®g. 1; reillustrated by Dunbar 1923, suggested by Mikulic (1994), they may be preserved Fig. 8). The actual fossil was not illustrated, and it in a moulting posture. It is also possible that some probably remained for years in the collection of J.W. moults or carcasses were deposited in slightly ¯exed Beede. We have identi®ed an unlabeled specimen, position following transport by currents. apparently the holotype, in the collections of Kansas State University. The specimen has been transferred to the U.S. National Museum of Natural History (USNM Systematic palaeontology 48305; Fig. 3C). Although broken along pre-existing cracks, the pattern of anastomosing lines and cracks in the fossil is identical with those in the YPM casts (cf. Order Xiphosurida Latreille, 1802 Fig. 3A, B.) The original fossil is a large prosoma Suborder Limulina Richter & Richter, 1929 (emend. preserved as an internal mould. Examination of it (Fig. BergstroÈm 1975) 3C) and the slightly restored cast (Fig. 3B) shows that not only were the lateral regions of the prosoma Superfamily Paleolimuloidea Anderson & Selden, restored on the illustrated cast (Fig. 3A), a part of the 1997 posterior margin was also restored. Grooves on the Family Paleolimulidae Raymond, 1944 dorsal surface seem to have been subtly enhanced by mechanical preparation of Beecher's illustrated plaster Genus Paleolimulus Dunbar, 1923 cast. LETHAIA 33 (2000) An early limuline from Pennsylvanian±Permian LagerstaÈtten of Kansas 139

Table 1. Measurements of cardiac lobe length (CL), total prosomal The opisthosoma of Paleolimulus is divided into length (PL), and posterior cardiac lobe width (CW) on a two sclerites (e.g. Figs 3H, 4D, E, H). Previously, the plastoholotype of Paleolimulus signatus (YPM 26319A) and the lectotype (YPM 26324) and paralectotypes of P. avitus (YPM smaller posterior sclerite was homologized with the 26317, 26325). Ratios of cardiac lobe length to total prosomal transverse area at the anterior of the telson of Limulus length (CL:PL) and posterior cardiac lobe width to total prosomal (Raymond 1994; but see Anderson & Selden 1997). length (CW:PL) are also given. Based on new collections, we concur with Anderson & Specimen no. PL CL CW CL:PL CW:PL Selden (1997) and interpret the posterior opisthoso- mal sclerite (`posterior axial lobe' of Anderson & YPM 26319A 45 35 ca. 17 1.29 2.65 YPM 26324 6.5 4.3 2.1 1.51 3.10 Selden 1997) as homologous with the posterior part of YPM 26317 7.3 5.3 3.0 1.38 2.43 the opisthosoma of Limulus. Evidence for this inter- YPM 26325 15.0 10.5 5.5 1.43 2.73 pretation is the presence in L. polyphemus of a distinct, although subtle, fused joint between a larger anterior area and a smaller posterior area of the opisthosoma Dunbar (1923) erected Paleolimulus avitus using (Fig. 7A). This fused joint is presumed to be specimens from the Insect Hill LagerstaÈtte (locality 3). homologous with the joint between the two opistho- Three syntypes (Fig. 3D±G) were identi®ed; the most somal sclerites of P. signatus (Fig. 4D). The larger complete of these (Fig. 3D, E; original of Dunbar 1923, anterior sclerite of P. signatus contains six segments, ®gs. 4±6) is here selected as the lectotype of P. avitus. the smaller posterior segment two segments. The Characters used to distinguish P. avitus from P. anterior part of the opisthosoma (forward of the signatus seem to be trivial and attributable to fused suture) of L. polyphemus contains six segments individual or ontogenetic variation, rather than to and the posterior part two segments. species differences; for this reason, P. avitus is regarded Anastomosing lines on the prosoma of P. signatus as a junior synonym of P. signatus. Dunbar (1923, p. are interpreted to be impressions left in the exoske- 450) stated that P. signatus differs from P. avitus in leton by intestinal diverticula. The anastomosing lines, being of larger size; having less pronounced ophthal- which closely resemble the pattern of the intestinal mic ridges, a more slender cardiac lobe, and a longer diverticula impressed on the ventral side of the dorsal cardiac lobe. The three-fold size difference between surface in extant L. polyphemus (Fig. 7B) are well Beecher's specimen and Dunbar's specimens reason- expressed in the holotype (Fig. 3A±C). Another ably can be attributed to ontogenetic differences. specimen of P. signatus (Fig. 4G) shows a mosaic of Ophthalmic ridges are de®ned slightly better in polygonal bumps over the surface of the internal Dunbar's (1923) specimens of P. avitus than in mould left as the result of the dissolution of the Beecher's (1904) specimen of P. signatus. Judging anastomosing lines. from the new collections (e.g. Fig. 4D±G), distinctness of the ophthalmic ridges is largely taphonomically Occurrence. ± Upper Pennsylvanian to Middle Per- related, with comparatively more compressed speci- mian of northeastern Kansas; Middle Pennsylvanian of mens exhibiting more poorly de®ned ophthalmic northeastern Illinois. ridges. Comparison of the length of the cardiac lobe to total prosomal length (Table 1) indicates that the Acknowledgements. ± We thank R.D. White (Peabody Museum, cardiac lobe is proportionately shorter, although only Yale University) for lending xiphosurid specimens for study, and slightly so, in Beecher's specimen (YPM 26319A) than for providing much valuable information about the specimens. The late A.S. Horowitz (Indiana University) provided information in any of Dunbar's specimens. This difference is about J.W. Beede's collections. H.R. Feldman (Shell Oil Company) attributable to ontogenetic or individual variation. helped with some ®eld work. C.G. Maples (Indiana University) Finally, comparison of the width of the cardiac lobe at provided helpful information about Upper Pennsylvanian±Per- mian stratigraphy of Kansas. G.J. Wasserman (Ohio State the posterior margin to total prosomal length (Table University) printed many of the photographs used herein. Betty 1) indicates that the proportionate width of that Heath (Ohio State University) keyboarded the manuscript. Karen character in Beecher's specimen falls well within the Tyler (Ohio State University) drafted some ®gures. The manuscript has bene®ted from reviews by R.M. Feldmann (Kent State range of variation of Dunbar's specimens. University) and three anonymous reviewers. This work was partly Paleolimulus signatus has been well described (as P. supported by grants from the Kansas Geological Survey and the avitus) by Dunbar (1923), Raymond (1944), and U.S. National Science Foundation (EAR-9405990) to Babcock. Anderson & Selden (1997). New information concerns the book gills, the opisthosoma, and the intestinal diverticula. Some new specimens (e.g. Fig. 4B) show References mouldically preserved book gills. The book gills do not Allison, P.A. 1986: Soft-bodied animals in the fossil record; the differ substantially from the homologous characters of role of decay in fragmentation during transport. Geology 14, present-day Limulus. 979±981. 140 Loren E. Babcock et al. LETHAIA 33 (2000)

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