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 Downloaded from https://pubs.geoscienceworld.org/sepm/palaios/article-pdf/34/5/254/4707636/i0883-1351-34-5-254.pdf by David R. Schwimmer on 22 May 2019 PALAIOS UPPER CAMBRIAN MERASPID TRILOBITE CLUSTER 255 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 Downloaded from https://pubs.geoscienceworld.org/sepm/palaios/article-pdf/34/5/254/4707636/i0883-1351-34-5-254.pdf by David R. Schwimmer on 22 May 2019 256 D.R. SCHWIMMER and W.M. MONTANTE PALAIOS 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