A Cambrian Meraspid Cluster: Evidence of Trilobite Egg Deposition in a Nest Site

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PALAIOS, 2019, v. 34, 254–260 Research Article DOI: http://dx.doi.org/10.2110/palo.2018.102

A CAMBRIAN MERASPID CLUSTER: EVIDENCE OF TRILOBITE EGG DEPOSITION IN A NEST SITE

1 2 DAVID R. SCHWIMMER AND WILLIAM M. MONTANTE 1Department of Earth and Space Sciences, Columbus State University, Columbus, Georgia 31907-5645, USA 2Tellus Science Museum, 100 Tellus Drive, Cartersville, Georgia, 30120 USA email: [email protected]

ABSTRACT: Recent evidence confirms that trilobites were oviparous; however, their subsequent embryonic development has not been determined. A ~ 6cm2 claystone specimen from the upper Cambrian (Paibian) Conasauga Formation in western Georgia contains a cluster of .100 meraspid trilobites, many complete with librigenae. The juvenile trilobites, identified as Aphelaspis sp., are mostly 1.5 to 2.0 mm total length and co-occur in multiple axial orientations on a single bedding plane. This observation, together with the attached free cheeks, indicates that the association is not a result of current sorting. The majority of juveniles with determinable thoracic segment counts are of meraspid degree 5, suggesting that they hatched penecontemporaneously following a single egg deposition event. Additionally, they are tightly assembled, with a few strays, suggesting that the larvae either remained on the egg deposition site or selectively reassembled as affiliative, feeding, or protective behavior. Gregarious behavior by trilobites (‘‘trilobite clusters’’) has been reported frequently, but previously encompassed only holaspid adults or mixed-age assemblages. This is the first report of juvenile trilobite clustering and one of the few reported clusters involving Cambrian trilobites. Numerous explanations for trilobite clustering behavior have been posited; here it is proposed that larval clustering follows egg deposition at a nest site, and that larval aggregation may be a homing response to their nest.

INTRODUCTION for the present study that the specimens figured by Gunther and Gunther (1981) are all holaspids of typical mature sizes. Among the modes of trilobite occurrences are massed individuals on New material from the upper Cambrian of Georgia, USA, includes a ‘‘ ’’ single bedding planes which are generically termed trilobite clusters small claystone slab with clustered, complete, juvenile trilobites (Fig. 1). (Whittington 1997b; Brett et al. 2012). Within the concept of trilobite Most of these juveniles are of meraspid degree 5 (Chatterton and Speyer clusters are two distinct categories representing entirely different types of 1997; Fusco et al. 2011; Shen et al. 2014), in a multiply oriented association (Speyer and Brett 1985): molt clusters, which are shed exuviae, assemblage, indicating that clustering was not caused by current sorting. and body clusters, which are associated intact or associated organisms. We hypothesize that this clustering primarily resulted from homing Molt clusters likely result from sedimentary and fluid dynamics operating behavior at the egg-deposition site, since it is now confirmed that trilobites on the relatively low density, high surface-to-volume sclerites, whereas did, indeed, produce eggs that were likely deposited in external masses body clusters may reflect multiple causes. Clustered complete- or partial (Hegna et al. 2017). organisms may indicate involuntary physical processes resulting in mass preservation, such as extreme bottom currents causing immuration AGE AND GEOLOGICAL SETTING (smothering) or rapid onset of benthic anoxia. Alternatively, clustered intact trilobites have been proposed to reflect conscious behaviors by living The specimen in study, CSUC-2016-1, comes from the Conasauga organisms (Speyer and Brett 1985; Karim and Westrop 2002) including Formation in Murray County, northwestern Georgia (Fig. 2). This site is at protective, feeding, and reproductive associations. Voluntary and involun- the crossing of Tibbs Bridge Road (here referred as the TBR site) on the tary processes may be combined in the events producing trilobite clusters, Conasauga River, at the east river bank. The locality and site paleontology since some involuntary event (e.g., anoxia) must have killed trilobites that has been detailed in Schwimmer and Montante (2012): in brief, the had clustered voluntarily. lithology is tan-to-light gray, planar, flaggy bedded, carbonate-free Trilobite clusters, both molt- and body-types, are reported from early claystone, with approximately 4.0 m thickness exposed in the riverside through mid-Paleozoic deposits ranging from late Cambrian through Late outcrop. The claystone contains abundant, typically complete specimens of Devonian, but the majority are reported from Ordovician deposits (e.g., the ptychopariid trilobite species Aphelaspis brachyphasis Palmer 1962, Chatterton and Fortey 2008; Fatka and Budil 2014). Cambrian trilobite along with a sparse fauna of agnostoids representative of the global clusters are frequently observed in the field, especially in mudstone Glyptagnostus reticulatus Biozone (Geyer and Shergold 2000 Peng et al. deposits (Schwimmer and Montante 2012), but have rarely been described. 2004, 2009). The trilobite assemblage constrains the age of the site (Fig. 3) The only such published report comes from the Wheeler Shale in Utah to the Paibian Stage of the Furongian Series, coinciding with the North (Gunther and Gunther 1981), where ubiquitous Elrathia kingii and American Steptoean Series and, informally, the lowest unit of the upper Asaphiscus wheeleri specimens occur in dense associations. It is notable Cambrian (Peng et al. 2004).

Published Online: May 2019 Copyright Ó 2019, SEPM (Society for Sedimentary Geology) 0883-1351/19/034-254

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FIG. 1.—Overall view of CSUC-16-1, showing meraspid cluster on right and associated shed holaspid sclerites of Aphelaspis brachyphasis.

The lithology of the fossil site is noteworthy for the discussion to follow since the relatively light-colored, flaggy-bedded claystones indicate deposition in quiet marine water, below storm wave base but not in slope to bathyal marine depths. The absence of carbonates in the sediment suggests that these deposits formed distal to the ooid shoals and algal buildups which are typically associated with peritidal to mid-shelf Cambrian marginal deposits (e.g., Palmer 1971; Pfiel and Read 1980; Hasson and Haase 1988). Alternatively, these sediments may have accumulated on the proximal shelf during a siliclastic-dominated phase of Conasauga sedimentation (Astini et al. 2000). In either case, the absence of evident sand and silt in the sediment shows significant distances of the study site from the Cambrian inter- and subtidal nearshore environment. At the TBR site there are abundant, intact Aphelaspis preserved without evidence of mechanical disturbance or preferred orientation (Fig. 4A). Since opisthoparian ptychopariid trilobites detached the librigenae during ecdysis, occurrences of significant numbers of complete individuals, and the absence of numerous isolated librigenae in the deposit, indicates rapid death of individuals (Henningsmoen 1975; Whittington 1997b). Among explanations for rapid death and preservation of clusters of complete trilobites are obrution (rapid burial) and anoxia (Seilacher et al. 1985; Brett et al. 2012). The Conasauga unit in study does not show bedding evidence of density cascades (Brett et al. 2012) or other sedimentary indications suggestive of obrution events. Anoxia is the most plausible and frequently documented mechanism to explain the occurrence of numerous dead

FIG. 2.—Locality map of northwest Georgia and vicinity, showing the Tibbs benthic organisms without signiﬁcant sedimentary disturbance (Gill et al. Bridge Road (TBR) site (asterisk) on the Conasauga River. Outline color shows the 2011; Woods et al. 2011). Anoxia resulting from enhanced burial of approximate outcrop of Cambrian strata in the Conasauga River Valley. organic carbon (Li et al. 2018) is a plausible result of the onset of a SPICE

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TABLE 1.—Tallies of meraspids in CSUC-2016-1 with intact librigenae and/or determinable meraspid degree.

113 identifiable discrete individuals 27 meraspids with attached librigenae 29 meraspids identifiable of degree 5 2 meraspids identifiable of degree 6 1 meraspid identifiable of degree 2 1 meraspid of degrees 3 or 4

away from the cluster, which is also oriented ventral side up. Since they are of proportional size and orientation, both shed thoracic and librigenal sclerites may come from the same adult A. brachyphasis individual. Three additional complete degree 5 meraspids are preserved in the matrix adjacent to the pygidial end of the holaspid sclerites, along with two smaller meraspids, one of apparent degree 2 and a second of indeterminable degree.

ANALYSIS

The quality of preservation of the clustered meraspid individuals is variable, but 27 are sufficiently delimited (Table 1) to show that they are preserved with intact librigenae and pygidia (Fig. 4B). In most prior reports, the meraspids of Aphelaspis are observed with detached librigenae (Palmer 1962; Lee and Chatterton 2005), indicating these were shed during ontogenetic ecdysis, as with adults. As previously noted here, the presence of complete Aphelaspis with intact librigenae indicates these were rapidly killed trilobites rather than accumulated exuviae (Chatterton and Speyer 1997), suggesting that the massed meraspids here comprise a death FIG. 3.—Correlation of the Aphelaspis fauna in the Conasauga Formation at the TBR site (asterisk). Global stratigraphy based largely on Peng et al. (2009). assemblage. In addition to observation that many meraspids in the cluster are intact, it is also notable that 29 individuals are of meraspid degree 5, comprising (Steptoean Positive Carbon Isotope Excursion) event (Saltzman et al. 1998, 88% of those in the sample preserved sufficiently to determine their 2000), which coincides with the early Paibian age of the specimen in study. number of thoracic segments (Table 1). Meraspid degree 5 is approximately mid-way through the ontogeny (Fusco et al. 2011) of typical MATERIALS AND METHODS Aphelaspis species, which have 13 thoracic segments in holaspids. Among the many additional specimens where the thoracic segments cannot be The specimen in study, CSUC-2016-1 (Fig. 1), is a small (~ 6cm2), counted reliably, most are of comparable size to the degree 5 individuals. claystone slab with an iron-stained surface layer containing a cluster of at The clustered meraspids lie in multiple axial orientations with respect to least 105 recognizable meraspid trilobites identified as juvenile Aphelaspis the surface of CSUC-2016-1 (Fig. 4C), and a relatively small number are sp. The meraspids are present as a thin, iron-stained surface layer on the oriented ventral-side up, including at least one specimen with intact ventral claystone, with much of their demarcation due to the iron-oxide coloration. sclerites (rostral plate and hypostome; Fig. 4D). Overall observation of the Because of their low relief on the claystone surface, and dependence on claystone specimen (Fig. 1), combining multiple meraspid orientations and color for elucidation, the specimen was photographed uncoated (Fig. 1). intact individuals, indicates that clustering of the meraspids was not a result Likewise, subsequent SEM images here (Fig. 4B, 4C) were taken without of selective winnowing and sorting by high-energy benthic processes. sputter coating to protect the surface color. Discussion to follow will present and evaluate the hypothesis that this The meraspid cluster is adjacent to and partly overlaps shed holaspid meraspid cluster represents voluntary association of related individuals, trilobite sclerites: a partial thorax and pygidium, and a single separated clustering at their original egg-deposition site. librigena, possibly all from a single individual. The shed holaspid sclerites in the sample are identified as Aphelaspis brachyphasis Palmer 1962. The meraspids are identifiable as Aphelaspis sp. (Palmer 1962; Lee and DISCUSSION Chatterton 2005) but cannot be assigned to species given the limited Trilobite Egg Brooding and Deposition available specific comparative data for Aphelaspis ontogenetic stages. The associated meraspid material occupies a single bedding plane on the Trilobite embryology has previously received limited discovery and specimen surface, with the larval trilobites largely confined to a small area study, whereas the ontogeny of post-hatch trilobites has been widely of approximately 32 3 28 mm. One edge of the sample with tightly studied and generally understood for more than 160 years (e.g., Barrande clustered individuals is sharply broken, suggesting that more individuals 1852; Beecher 1893). This difference in information naturally results from were present originally. Figure 1 shows that a portion of the surface the absence of hard tissue in early developmental stages from egg up to occupied by the meraspids incorporates the shed trilobite librigena: protaspis, versus the abundant sclerotized protaspid and meraspid fossils in approximately one-fifth of the meraspids occupy the ventral surface of the the record. The embryology of Paleozoic arthropods other than trilobites free cheek. A few additional meraspids—at least three evident with several has been reported for several groups preserved in Lagerstätten,and notably poorly determinable—are preserved on the surface of the thoracic sclerites, from arthropods with sclerotized or partially sclerotized carapaces. Caron

FIG. 4.—A) Aphelaspis brachyphasis holaspids from TBR site. The intact sclerites and iron oxide rings around individuals suggests that these were whole-body specimens at the time of burial (ﬁde Schwimmer and Montante 2007). B) Representative individuals in the meraspid cluster, showing attached librigenae and complete dorsal exoskeletons. C) Representative clustered meraspids in CSUC-2016-1, showing multiple axial orientations. D) Ventral view of complete meraspid showing attached rostral plate and natant hypostome.

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and Vannier (2016) observed brooded eggs within the anterior carapace undetermined (Fortey and Owens 1999); however, given that they had structure in Waptia fieldensis, a crustacean-like arthropod from the middle numerous ventral appendages and natant hypostomes (Fig. 4D) in common Cambrian (Miaolingian Series) Burgess Shale in British Columbia. Duan with holaspids, it is likely that they behaved in a similar manner, which is et al. (2014) described eggs attached to the appendages of a small bradoriid commonly assumed to include vagrant detritus feeding (Whittingon arthropod Kunmingella douvillei from the early Cambrian Chengjiang 1997b). Lagerstätte in South China. Ostracods had egg-pouches within their Following assumptions about the mobility of meraspids, the clustering carapaces (Fortey and Hughes 1998; Siveter et al. 2014, and pyritized of .100 meraspids largely of degree 5 in the present specimen indicates ostracods from the Ordovician in New York contain the oldest known with that these mobile organisms associated voluntarily, if alive, or involuntarily evidence of eggs within the pouches (Siveter et al. 2014). Putative by mechanical processes if dead or dying (Paterson et al. 2008; Gutiérrez- Paleozoic arthropod egg occurrences outside of a specific host have been Marco et al. 2009). Involuntary association would most plausibly be reported from a middle Cambrian mudstone deposit in South China (Lin et caused by higher-energy, benthic flow regimes, such as winnowing by al. 2006); however, it is not possible to attribute those eggs to specific strong bottom currents or soft-sediment flows. However, there are multiple arthropods. observations arguing against the interpretation that CSUC-2016-1 It has been generally assumed that trilobites reproduced sexually with represents dead organisms associated involuntarily. production of fertilized eggs, and the site of egg formation is generally Involuntary association of the meraspids in this assemblage is assumed to have been in the cephalon (Chatterton and Speyer 1997). It was contradicted, first, by their generally complete makeup with attached further argued that females of presumed dimorphic trilobite species with librigenae and ventral sclerites (Fig. 4B, 4D). This indicates that the preglabellar swellings produced or stored their eggs in those sites (Fortey assemblage represents intact (presumably live) organisms at the time of and Hughes 1998; McNamara et al. 2009), analogous to the brood pouches association, rather than assembled molted sclerites (Lin and Yuan 2008), or observed in ostracods and some crustaceans. However, Sundberg (1999) admixed complete individuals and molts. Live organisms were denser than observed that some species of ptychopariids with preglabellar swellings do molted sclerites, and were thus less likely to be winnowed by bottom not appear to be dimorphic, suggesting that the feature is not unique to currents. Second, observing that the axial (i.e., antero-posterior) orienta- females, and by implication, not an egg-related structure. Hegna et al. tions of the meraspids are diverse with no strongly preferred direction (2017) recently confirmed the presence of multiple eggs, averaging ~ 200l (Figs. 1, 4C), this contradicts the argument of current sorting, assuming length, in pyritized specimens of Ordovician Triarthrus eatoni (an olenid that a sorted assemblage would have an axial trend oriented with the species that lacks preglabellar swelling). This discovery provides strong current direction. Third, the meraspids occupy a relatively flat plane, evidence that at least some trilobites did produce masses of eggs, and that without the evidence of mass roll-up that would be evident if the those eggs were produced or stored, before extrusion, in the genal regions association resulted from cascades of soft-sediment sufficient to kill and of the cephalon. accumulate live benthic organisms (Brett et al. 2012). Lastly, the Arthropods are arguably the most environmentally diverse extant animal sedimentary bedding of the specimen slab and the surrounding clayshale phylum and show equally diverse hatchling behavior. Despite positive rock shows only planar bedding, indicating that the depositional evidence of trilobite egg production, it has not been determined previously environment was of relatively low-energy. whether trilobite eggs were brooded by various means of attachment to the parent, as observed in many extant malacostracan crustaceans (Richer and Hypotheses for Voluntary Clustering Behavior Scholtz 2001), or were excreted on marine benthic sediment surfaces in masses, as with xiphosurans (Hong 2011). A common brooding behavior If we reject the argument that the meraspid cluster in CSUC-2016-1 observed among various extant clades of chelicerates (e.g., many consists of accumulated molts or an involuntary assemblage, the logical arachnids), along with many mandibulate arthropod clades (e.g., some conclusion is that it formed by voluntary gregarious behavior, either crustaceans, many hexapods), involves masses of eggs deposited on instinctive or conscious. Such behavior has been considered inherent in specific types of surfaces or in egg cases. Post-hatch larval arthropods may trilobites (Speyer and Brett 1985; Karim and Westrop 2002), attributable to abandon egg-deposition sites immediately after hatching: e.g., as with five general behavioral categories: protection, molting, mating, feeding, xiphosurans (Hong 2011); whereas others may remain in deposition sites and egg deposition. These behavioral assumptions are based, in part, by or in association with parents until well advanced in ontogeny: e.g., as with analogy with extant limulid xiphosurans, which are commonly observed to some crustaceans (Thiel 2000) and common spiders and insects that brood form mass associations in intertidal environments where they molt, in cocoons. The behavior of trilobites with respect to egg deposition and copulate, and deposit eggs, in great numbers (Hong 2011). post-hatch behavior has been, to date, undocumented. Considering protective behavior, it is notable that horseshoe crab mass assemblies are also scenes of mass feeding by predators, and therefore the Larval Mobility and Gregarious Behavior gregarious behavior of nearshore-assembling xiphosurans is not necessar- ily protective. With respect to trilobite assemblages for protection, various The ventral structures of larval trilobites are known in many groups examples of aligned and zig-zag trilobite clusters have specifically been (Chatterton and Speyer 1997; Whittington 1997a). Based on the presence argued as such. Linear clusters have been considered evidence that of ventral appendages and by analogy with extant larval arthropods, it is trilobites occupied worm burrows, generally assumed to be pre-existing evident that trilobites were mobile from the earliest protaspid stage onward. (Chatterton and Fortey 2008; Gutiérrez-Marco et al. 2009), which would Since protaspids were both small and lacked distinct segmentation (thus, clearly represent voluntary protective behavior. Clustered trilobites have lacking thoracic appendages), it is generally assumed most were planktonic also been observed in non-linear, tunneling associations in carbonate firm (Chatterton and Speyer 1997; Fortey and Owens 1999). During the grounds (Cherns et al. 2006), where the trilobites are inferred to be the transition to meraspid stages trilobite larvae added thoracic segments at burrowing agents. Additional protective behavior has been inferred where variable rates (Shen et al. 2014), with each new segment assumed to have trilobites were found clustered under sheltering objects: e.g., Fatka and paired biramous appendages as in holaspids. The addition of thoracic Budil (2014) reported a cluster of six small harpetids sheltering under a segments would suggest that meraspids were increasingly mobile beyond large asaphid pygidium. the protaspis, and, with greater size and additional ventral appendages, We cannot absolutely reject the argument for the association of clustered would be better adapted to benthic habitats (Lin and Yuan 2008). The juveniles in CSUC-2016-1 as protective behavior; however, the relatively feeding behavior of Cambrian meraspid ptychopariid trilobites is flat surface of the claystone specimen, with no evidence of burrowing or

other cryptic behavior, indicates that the clustered meraspids were exposed penecontemporaneous hatching event. The variables of trilobite hatching on the benthic environment. This observation suggests that the protection and ontogeny are very poorly constrained: we have no data about the hypothesis for gregarious behavior in the present specimen is not synchronicity of individual egg hatch, nor on the length of time between supported. each ontogenetic instar. Since the thoracic segment number is highly Two additional categories of voluntary trilobite clustering, mating and variable among trilobite clades, the number of additional thoracic segments molting, can be summarily rejected by the composition of the specimen in produced by each molt is also unknown (Fusco et al. 2011). Given these study. The meraspid cluster in CSUC-2016-1, by definition consisting of unknown variables, the span of time between hatching and the meraspid immature individuals, was obviously not formed for mating. We can degree of an individual trilobite is indeterminate; however, the assemblage likewise reject the hypothesis of clustering for mass molting with this of so many juveniles of a single meraspid degree, as above, would be specimen because of the observed absence of shed meraspid librigenae and improbable as a random event. the large number of intact individuals. Combining the evidence of a suitable localized firm ground afforded by Trilobite feeding as a proximal cause for clustering has been proposed shed sclerites, and the association with many apparently same-age, mobile by Kimmig and Pratt (2018) in the Drumian-age Ravens Throat River larvae, we argue that CSUC-2016-1 represents a cluster of meraspids that Lagerstätte, where it was observed that trilobites and other apparent returned to their egg-deposition site on the sclerites in response to a coprovores were clustered in association with preserved fecal masses. In homing instinct. This may simultaneously represent a feeding assemblage this association, the fauna that clustered around the coprolites include as well as a form of herd-protection behavior. The locality where this mixed sizes, taxa, and orientation of trilobites, as well as non-trilobite specimen was obtained contains abundant individuals of Aphelaspis arthropods and hyoliths. It is notable that the clustered trilobites include brachyphasis at all stages of development (Schwimmer and Montante ptychoparioids and agnostoids, which are both assumed detritivores 2012), and there are other specimens from the TBR site which may (Fortey and Owens 1999). Also notable in the same Lagerstätte is represent additional occurrences of larval clustering. However, no other significant evidence of bioturbation (Pratt and Kimmig 2019), which specimens collected thus far have sufficient preservation to document contradicts a dysoxic sedimentary environment and differs from the whether the trilobite material evident on sedimentary surfaces comprises situation evident for CSUC-2016-1. intact meraspids or current-winnowed comminuted sclerites. Therefore, We cannot categorically reject feeding in relationship to clustering in the CSUC-2016-1 is to date the only documentable meraspid trilobite cluster. Conasauga specimen in study; however, below we propose that the primary cause of the meraspid association in CSUC-2016-1 relates to the fifth ACKNOWLEDGMENTS assumption presented for voluntary clustering: egg deposition. As a farther extension of this argument, it is proposed that the association of a We thank colleagues at Columbus State University: Elizabeth Klar, for SEM predominantly same-age assemblage of larvae represents homing to their imaging, and Clinton Barineau and Diana Ortega-Ariza, for assistance in brood site. This argument is also supported by the sedimentology of the graphics work. We also thank Jeff Scovill, Scovill Photography, for work on the Conasauga depositional environment represented by CSUC-2016-1. overall assemblage figure. Thomas Hegna, Western Illinois University, provided Previous observations about voluntary, gregarious behavior by trilobites valuable information about trilobite eggs for this project. Brian Pratt and have largely involved adults or mixed-age assemblages. Some reports of Gabriel Mańgano provided editorial suggestions which greatly improved the trilobite associations include smaller taxa—e.g., the eodiscid Pagetia (Lin manuscript. We also thank Olda Fatka and Per Ahlberg for their careful and and Yuan 2008) and the harpetid Eoharpes (Fatka and Budil 2014)—but helpful technical reviews of the manuscript. none have specifically addressed only associated juveniles. Given evidence that trilobites do produce masses of eggs (Hegna et al. REFERENCES

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