A CAMBRIAN MERASPID CLUSTER: EVIDENCE OF TRILOBITE …

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

DAVID R. SCHWIMMER1AND WILLIAM M. MONTANTE2

1Department of Earth and Space Sciences, Columbus State University, Columbus,

Georgia 31907-5645, USA2Tellus Science Museum, 100 Tellus Drive, Cartersville, Georgia, 30120 USA

email: [email protected]

ABSTRACT: Recent evidence confirms that trilobites were oviparous; however, their subsequent embryonicdevelopment has not been determined. A ~ 6 cm2 claystone specimen from the upper Cambrian (Paibian)Conasauga Formation in western Georgia contains a cluster of .100 meraspid trilobites, many complete withlibrigenae. The juvenile trilobites, identified as Aphelaspis sp., are mostly 1.5 to 2.0 mm total length and co-occur inmultiple 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 thoracicsegment counts are of meraspid degree 5, suggesting that they hatched penecontemporaneously following a single eggdeposition event. Additionally, they are tightly assembled, with a few strays, suggesting that the larvae either remainedon 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 thefew reported clusters involving Cambrian trilobites. Numerous explanations for trilobite clustering behavior havebeen posited; here it is proposed that larval clustering follows egg deposition at a nest site, and that larval aggregationmay be a homing response to their nest.

INTRODUCTION

Among the modes of trilobite occurrences are massed individuals on

single bedding planes which are generically termed ‘‘trilobite clusters’’

(Whittington 1997b; Brett et al. 2012). Within the concept of trilobite

clusters are two distinct categories representing entirely different types of

association (Speyer and Brett 1985): molt clusters, which are shed exuviae,

and body clusters, which are associated intact or associated organisms.

Molt clusters likely result from sedimentary and fluid dynamics operating

on the relatively low density, high surface-to-volume sclerites, whereas

body clusters may reflect multiple causes. Clustered complete- or partial

organisms may indicate involuntary physical processes resulting in mass

preservation, such as extreme bottom currents causing immuration

(smothering) or rapid onset of benthic anoxia. Alternatively, clustered

intact trilobites have been proposed to reflect conscious behaviors by living

organisms (Speyer and Brett 1985; Karim and Westrop 2002) including

protective, feeding, and reproductive associations. Voluntary and involun-

tary processes may be combined in the events producing trilobite clusters,

since some involuntary event (e.g., anoxia) must have killed trilobites that

had clustered voluntarily.

Trilobite clusters, both molt- and body-types, are reported from early

through mid-Paleozoic deposits ranging from late Cambrian through Late

Devonian, but the majority are reported from Ordovician deposits (e.g.,

Chatterton and Fortey 2008; Fatka and Budil 2014). Cambrian trilobite

clusters are frequently observed in the field, especially in mudstone

deposits (Schwimmer and Montante 2012), but have rarely been described.

The only such published report comes from the Wheeler Shale in Utah

(Gunther and Gunther 1981), where ubiquitous Elrathia kingii and

Asaphiscus wheeleri specimens occur in dense associations. It is notable

for the present study that the specimens figured by Gunther and Gunther

(1981) are all holaspids of typical mature sizes.

New material from the upper Cambrian of Georgia, USA, includes a

small claystone slab with clustered, complete, juvenile trilobites (Fig. 1).

Most of these juveniles are of meraspid degree 5 (Chatterton and Speyer

1997; Fusco et al. 2011; Shen et al. 2014), in a multiply oriented

assemblage, indicating that clustering was not caused by current sorting.

We hypothesize that this clustering primarily resulted from homing

behavior at the egg-deposition site, since it is now confirmed that trilobites

did, indeed, produce eggs that were likely deposited in external masses

(Hegna et al. 2017).

AGE AND GEOLOGICAL SETTING

The specimen in study, CSUC-2016-1, comes from the Conasauga

Formation in Murray County, northwestern Georgia (Fig. 2). This site is at

the crossing of Tibbs Bridge Road (here referred as the TBR site) on the

Conasauga River, at the east river bank. The locality and site paleontology

has been detailed in Schwimmer and Montante (2012): in brief, the

lithology is tan-to-light gray, planar, flaggy bedded, carbonate-free

claystone, with approximately 4.0 m thickness exposed in the riverside

outcrop. The claystone contains abundant, typically complete specimens of

the ptychopariid trilobite species Aphelaspis brachyphasis Palmer 1962,

along with a sparse fauna of agnostoids representative of the global

Glyptagnostus reticulatus Biozone (Geyer and Shergold 2000 Peng et al.

2004, 2009). The trilobite assemblage constrains the age of the site (Fig. 3)

to the Paibian Stage of the Furongian Series, coinciding with the North

American Steptoean Series and, informally, the lowest unit of the upper

Cambrian (Peng et al. 2004).

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

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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

benthic organisms without significant sedimentary disturbance (Gill et al.

2011; Woods et al. 2011). Anoxia resulting from enhanced burial of

organic carbon (Li et al. 2018) is a plausible result of the onset of a SPICE

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

FIG. 2.—Locality map of northwest Georgia and vicinity, showing the Tibbs

Bridge Road (TBR) site (asterisk) on the Conasauga River. Outline color shows the

approximate outcrop of Cambrian strata in the Conasauga River Valley.

UPPER CAMBRIAN MERASPID TRILOBITE CLUSTERP A L A I O S 255


(Steptoean Positive Carbon Isotope Excursion) event (Saltzman et al. 1998,

2000), which coincides with the early Paibian age of the specimen in study.

MATERIALS AND METHODS

The specimen in study, CSUC-2016-1 (Fig. 1), is a small (~ 6 cm2),

claystone slab with an iron-stained surface layer containing a cluster of at

least 105 recognizable meraspid trilobites identified as juvenile Aphelaspis

sp. The meraspids are present as a thin, iron-stained surface layer on the

claystone, with much of their demarcation due to the iron-oxide coloration.

Because of their low relief on the claystone surface, and dependence on

color for elucidation, the specimen was photographed uncoated (Fig. 1).

Likewise, subsequent SEM images here (Fig. 4B, 4C) were taken without

sputter coating to protect the surface color.

The meraspid cluster is adjacent to and partly overlaps shed holaspid

trilobite sclerites: a partial thorax and pygidium, and a single separated

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

Chatterton 2005) but cannot be assigned to species given the limited

available specific comparative data for Aphelaspis ontogenetic stages. The

associated meraspid material occupies a single bedding plane on the

specimen surface, with the larval trilobites largely confined to a small area

of approximately 32 3 28 mm. One edge of the sample with tightly

clustered individuals is sharply broken, suggesting that more individuals

were present originally. Figure 1 shows that a portion of the surface

occupied by the meraspids incorporates the shed trilobite librigena:

approximately one-fifth of the meraspids occupy the ventral surface of the

free cheek. A few additional meraspids—at least three evident with several

poorly determinable—are preserved on the surface of the thoracic sclerites,

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

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

88% of those in the sample preserved sufficiently to determine their

number of thoracic segments (Table 1). Meraspid degree 5 is approxi-

mately mid-way through the ontogeny (Fusco et al. 2011) of typical

Aphelaspis species, which have 13 thoracic segments in holaspids. Among

the many additional specimens where the thoracic segments cannot be

counted reliably, most are of comparable size to the degree 5 individuals.

The clustered meraspids lie in multiple axial orientations with respect to

the surface of CSUC-2016-1 (Fig. 4C), and a relatively small number are

oriented ventral-side up, including at least one specimen with intact ventral

sclerites (rostral plate and hypostome; Fig. 4D). Overall observation of the

claystone specimen (Fig. 1), combining multiple meraspid orientations and

intact individuals, indicates that clustering of the meraspids was not a result

of selective winnowing and sorting by high-energy benthic processes.

Discussion to follow will present and evaluate the hypothesis that this

meraspid cluster represents voluntary association of related individuals,

clustering at their original egg-deposition site.

DISCUSSION

Trilobite Egg Brooding and Deposition

Trilobite embryology has previously received limited discovery and

study, whereas the ontogeny of post-hatch trilobites has been widely

studied and generally understood for more than 160 years (e.g., Barrande

1852; Beecher 1893). This difference in information naturally results from

the absence of hard tissue in early developmental stages from egg up to

protaspis, versus the abundant sclerotized protaspid and meraspid fossils in

the record. The embryology of Paleozoic arthropods other than trilobites

has been reported for several groups preserved in Lagerstatten, and notably

from arthropods with sclerotized or partially sclerotized carapaces. Caron

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).

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

D.R. SCHWIMMER and W.M. MONTANTE256 P A L A I O S


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.



and Vannier (2016) observed brooded eggs within the anterior carapace

structure in Waptia fieldensis, a crustacean-like arthropod from the middle

Cambrian (Miaolingian Series) Burgess Shale in British Columbia. Duan

et al. (2014) described eggs attached to the appendages of a small bradoriid

arthropod Kunmingella douvillei from the early Cambrian Chengjiang

Lagerstatte in South China. Ostracods had egg-pouches within their

carapaces (Fortey and Hughes 1998; Siveter et al. 2014, and pyritized

ostracods from the Ordovician in New York contain the oldest known with

evidence of eggs within the pouches (Siveter et al. 2014). Putative

Paleozoic arthropod egg occurrences outside of a specific host have been

reported from a middle Cambrian mudstone deposit in South China (Lin et

al. 2006); however, it is not possible to attribute those eggs to specific

arthropods.

It has been generally assumed that trilobites reproduced sexually with

production of fertilized eggs, and the site of egg formation is generally

assumed to have been in the cephalon (Chatterton and Speyer 1997). It was

further argued that females of presumed dimorphic trilobite species with

preglabellar swellings produced or stored their eggs in those sites (Fortey

and Hughes 1998; McNamara et al. 2009), analogous to the brood pouches

observed in ostracods and some crustaceans. However, Sundberg (1999)

observed that some species of ptychopariids with preglabellar swellings do

not appear to be dimorphic, suggesting that the feature is not unique to

females, and by implication, not an egg-related structure. Hegna et al.

(2017) recently confirmed the presence of multiple eggs, averaging ~ 200llength, in pyritized specimens of Ordovician Triarthrus eatoni (an olenid

species that lacks preglabellar swelling). This discovery provides strong

evidence that at least some trilobites did produce masses of eggs, and that

those eggs were produced or stored, before extrusion, in the genal regions

of the cephalon.

Arthropods are arguably the most environmentally diverse extant animal

phylum and show equally diverse hatchling behavior. Despite positive

evidence of trilobite egg production, it has not been determined previously

whether trilobite eggs were brooded by various means of attachment to the

parent, as observed in many extant malacostracan crustaceans (Richer and

Scholtz 2001), or were excreted on marine benthic sediment surfaces in

masses, as with xiphosurans (Hong 2011). A common brooding behavior

observed among various extant clades of chelicerates (e.g., many

arachnids), along with many mandibulate arthropod clades (e.g., some

crustaceans, many hexapods), involves masses of eggs deposited on

specific types of surfaces or in egg cases. Post-hatch larval arthropods may

abandon egg-deposition sites immediately after hatching: e.g., as with

xiphosurans (Hong 2011); whereas others may remain in deposition sites

or in association with parents until well advanced in ontogeny: e.g., as with

some crustaceans (Thiel 2000) and common spiders and insects that brood

in cocoons. The behavior of trilobites with respect to egg deposition and

post-hatch behavior has been, to date, undocumented.

Larval Mobility and Gregarious Behavior

The ventral structures of larval trilobites are known in many groups

(Chatterton and Speyer 1997; Whittington 1997a). Based on the presence

of ventral appendages and by analogy with extant larval arthropods, it is

evident that trilobites were mobile from the earliest protaspid stage onward.

Since protaspids were both small and lacked distinct segmentation (thus,

lacking thoracic appendages), it is generally assumed most were planktonic

(Chatterton and Speyer 1997; Fortey and Owens 1999). During the

transition to meraspid stages trilobite larvae added thoracic segments at

variable rates (Shen et al. 2014), with each new segment assumed to have

paired biramous appendages as in holaspids. The addition of thoracic

segments would suggest that meraspids were increasingly mobile beyond

the protaspis, and, with greater size and additional ventral appendages,

would be better adapted to benthic habitats (Lin and Yuan 2008). The

feeding behavior of Cambrian meraspid ptychopariid trilobites is

undetermined (Fortey and Owens 1999); however, given that they had

numerous ventral appendages and natant hypostomes (Fig. 4D) in common

with holaspids, it is likely that they behaved in a similar manner, which is

commonly assumed to include vagrant detritus feeding (Whittingon

1997b).

Following assumptions about the mobility of meraspids, the clustering

of .100 meraspids largely of degree 5 in the present specimen indicates

that these mobile organisms associated voluntarily, if alive, or involuntarily

by mechanical processes if dead or dying (Paterson et al. 2008; Gutierrez-

Marco et al. 2009). Involuntary association would most plausibly be

caused by higher-energy, benthic flow regimes, such as winnowing by

strong bottom currents or soft-sediment flows. However, there are multiple

observations arguing against the interpretation that CSUC-2016-1

represents dead organisms associated involuntarily.

Involuntary association of the meraspids in this assemblage is

contradicted, first, by their generally complete makeup with attached

librigenae and ventral sclerites (Fig. 4B, 4D). This indicates that the

assemblage represents intact (presumably live) organisms at the time of

association, rather than assembled molted sclerites (Lin and Yuan 2008), or

admixed complete individuals and molts. Live organisms were denser than

molted sclerites, and were thus less likely to be winnowed by bottom

currents. Second, observing that the axial (i.e., antero-posterior) orienta-

tions of the meraspids are diverse with no strongly preferred direction

(Figs. 1, 4C), this contradicts the argument of current sorting, assuming

that a sorted assemblage would have an axial trend oriented with the

current direction. Third, the meraspids occupy a relatively flat plane,

without the evidence of mass roll-up that would be evident if the

association resulted from cascades of soft-sediment sufficient to kill and

accumulate live benthic organisms (Brett et al. 2012). Lastly, the

sedimentary bedding of the specimen slab and the surrounding clayshale

rock shows only planar bedding, indicating that the depositional

environment was of relatively low-energy.

Hypotheses for Voluntary Clustering Behavior

If we reject the argument that the meraspid cluster in CSUC-2016-1

consists of accumulated molts or an involuntary assemblage, the logical

conclusion is that it formed by voluntary gregarious behavior, either

instinctive or conscious. Such behavior has been considered inherent in

trilobites (Speyer and Brett 1985; Karim and Westrop 2002), attributable to

five general behavioral categories: protection, molting, mating, feeding,

and egg deposition. These behavioral assumptions are based, in part, by

analogy with extant limulid xiphosurans, which are commonly observed to

form mass associations in intertidal environments where they molt,

copulate, and deposit eggs, in great numbers (Hong 2011).

Considering protective behavior, it is notable that horseshoe crab mass

assemblies are also scenes of mass feeding by predators, and therefore the

gregarious behavior of nearshore-assembling xiphosurans is not necessar-

ily protective. With respect to trilobite assemblages for protection, various

examples of aligned and zig-zag trilobite clusters have specifically been

argued as such. Linear clusters have been considered evidence that

trilobites occupied worm burrows, generally assumed to be pre-existing

(Chatterton and Fortey 2008; Gutierrez-Marco et al. 2009), which would

clearly represent voluntary protective behavior. Clustered trilobites have

also been observed in non-linear, tunneling associations in carbonate firm

grounds (Cherns et al. 2006), where the trilobites are inferred to be the

burrowing agents. Additional protective behavior has been inferred where

trilobites were found clustered under sheltering objects: e.g., Fatka and

Budil (2014) reported a cluster of six small harpetids sheltering under a

large asaphid pygidium.

We cannot absolutely reject the argument for the association of clustered

juveniles in CSUC-2016-1 as protective behavior; however, the relatively

flat surface of the claystone specimen, with no evidence of burrowing or



other cryptic behavior, indicates that the clustered meraspids were exposed

on the benthic environment. This observation suggests that the protection

hypothesis for gregarious behavior in the present specimen is not

supported.

Two additional categories of voluntary trilobite clustering, mating and

molting, can be summarily rejected by the composition of the specimen in

study. The meraspid cluster in CSUC-2016-1, by definition consisting of

immature individuals, was obviously not formed for mating. We can

likewise reject the hypothesis of clustering for mass molting with this

specimen because of the observed absence of shed meraspid librigenae and

the large number of intact individuals.

Trilobite feeding as a proximal cause for clustering has been proposed

by Kimmig and Pratt (2018) in the Drumian-age Ravens Throat River

Lagerstatte, where it was observed that trilobites and other apparent

coprovores were clustered in association with preserved fecal masses. In

this association, the fauna that clustered around the coprolites include

mixed sizes, taxa, and orientation of trilobites, as well as non-trilobite

arthropods and hyoliths. It is notable that the clustered trilobites include

ptychoparioids and agnostoids, which are both assumed detritivores

(Fortey and Owens 1999). Also notable in the same Lagerstatte is

significant evidence of bioturbation (Pratt and Kimmig 2019), which

contradicts a dysoxic sedimentary environment and differs from the

situation evident for CSUC-2016-1.

We cannot categorically reject feeding in relationship to clustering in the

Conasauga specimen in study; however, below we propose that the primary

cause of the meraspid association in CSUC-2016-1 relates to the fifth

assumption presented for voluntary clustering: egg deposition. As a farther

extension of this argument, it is proposed that the association of a

predominantly same-age assemblage of larvae represents homing to their

brood site. This argument is also supported by the sedimentology of the

Conasauga depositional environment represented by CSUC-2016-1.

Previous observations about voluntary, gregarious behavior by trilobites

have largely involved adults or mixed-age assemblages. Some reports of

trilobite associations include smaller taxa—e.g., the eodiscid Pagetia (Lin

and Yuan 2008) and the harpetid Eoharpes (Fatka and Budil 2014)—but

none have specifically addressed only associated juveniles.

Given evidence that trilobites do produce masses of eggs (Hegna et al.

2017), it is obvious that the eggs would have to be either deposited

somewhere or brooded somatically by the mother, either by external

attachment as in some crustaceans, or internally, as in some hexapods.

The well-sorted claystone matrix of Conasauga specimen CSUC-2016-1

indicates that the environment of deposition was a soft-sediment, benthic

setting. Such an environment would not be suitable generally for

deposition of arthropod eggs, since the bottom conditions would be

inherently unstable and eggs could be smothered by any disturbance of the

bottom surface. We observe in the Conasauga specimen that the meraspid

cluster is located on and adjacent to the larger shed librigena and thorax of

a holaspid Aphelaspis, which would comprise a local firm ground on the

soft sediment, providing a suitable egg deposition site. We hypothesize,

therefore, that these sclerites comprise the egg deposition site that

originated this meraspid assemblage. It is recognized that this hypothesis

assumes there was some form of homing or kin-affiliative behavior in

meraspid trilobites which would explain how mobile, and likely

planktonic, individuals re-associated after some developmental stages.

Absent direct modern analogs of trilobites, or explicit information about

drift currents and conditions in upper Cambrian shelf environments, that

remains indeterminable. Nevertheless, the association of so many same-age

meraspids demands a non-random explanation.

CONCLUSIONS

The largely similar developmental stage for most of the meraspids in

CSUC-2016-1 is the strongest evidence that they were the result of a

penecontemporaneous hatching event. The variables of trilobite hatching

and ontogeny are very poorly constrained: we have no data about the

synchronicity of individual egg hatch, nor on the length of time between

each ontogenetic instar. Since the thoracic segment number is highly

variable among trilobite clades, the number of additional thoracic segments

produced by each molt is also unknown (Fusco et al. 2011). Given these

unknown variables, the span of time between hatching and the meraspid

degree of an individual trilobite is indeterminate; however, the assemblage

of so many juveniles of a single meraspid degree, as above, would be

improbable as a random event.

Combining the evidence of a suitable localized firm ground afforded by

shed sclerites, and the association with many apparently same-age, mobile

larvae, we argue that CSUC-2016-1 represents a cluster of meraspids that

returned to their egg-deposition site on the sclerites in response to a

homing instinct. This may simultaneously represent a feeding assemblage

as well as a form of herd-protection behavior. The locality where this

specimen was obtained contains abundant individuals of Aphelaspis

brachyphasis at all stages of development (Schwimmer and Montante

2012), and there are other specimens from the TBR site which may

represent additional occurrences of larval clustering. However, no other

specimens collected thus far have sufficient preservation to document

whether the trilobite material evident on sedimentary surfaces comprises

intact meraspids or current-winnowed comminuted sclerites. Therefore,

CSUC-2016-1 is to date the only documentable meraspid trilobite cluster.

ACKNOWLEDGMENTS

We thank colleagues at Columbus State University: Elizabeth Klar, for SEM

imaging, and Clinton Barineau and Diana Ortega-Ariza, for assistance in

graphics work. We also thank Jeff Scovill, Scovill Photography, for work on the

overall assemblage figure. Thomas Hegna, Western Illinois University, provided

valuable information about trilobite eggs for this project. Brian Pratt and

Gabriel Mangano provided editorial suggestions which greatly improved the

manuscript. We also thank Olda Fatka and Per Ahlberg for their careful and

helpful technical reviews of the manuscript.

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Received 19 November 2018; accepted 27 April 2019.



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