Behavioural asymmetry in Devonian trilobites

Jin

et al. 2021

LET

It is important to understand moulting behaviours in trilobites through different growth stages. Most studies have focused on patterns of moulting in trilobites based on a few exuviae. The present study is based on 177 disarticulated specimens of Arthricocephalites xinzhaiheensis from the Balang Formation (Cambrian Series 2, Stage 4, Guizhou Province, South China). The abundance of disarticulated assemblages allows the distinction between carcasses and exuviae to be made with confidence and enables a detailed exploration of moulting behaviours. About 96.6% of exuviae of both meraspides and holaspides of Ar. xinzhaiheensis show a disarticulated cranidium characterizing Henningsmoen's configuration. The trilobites commonly used the cephalic sutures and cephalothoracic joint for moulting, involving lateral rotation or inversion of the cranidium to create a wide exuvial gape. Only one specimen displays the Somersault configuration characterized by an overturned lower cephalic unit and the data also show that trilobites employing the cephalic sutures to create an anterior exuvial gape are rare in holaspides. Though some specimens are similar to Salter's configuration with an inverted cephalon with respect to the thoracopygon, the specimens show closed cephalic sutures and thus dissociated or inverted cephala are likely to represent disturbed carcasses. Several other species of oryctocephalid trilobites display Henningsmoen's configuration indicating that exuviation by disarticulated cranidium to enlarge the exuvial gape was prevalent in the family from Cambrian Series 2 Stage 4 to the succeeding Wuliuan Stage of the Miaolingian Series.

Section: Discussionmentioning

confidence: 99%

“…First, the opening of cephalic sutures (facial, rostral, connective and hypostomal sutures) to provide an aperture for exuviation and to free the vital soft parts of the cephalon (Henningsmoen 1975; Speyer 1985; Whittington 1997; Rustán et al . 2011; Zong & Gong 2017; Drage et al . 2018b).…”

Section: Methodsmentioning

confidence: 99%

Moulting in the Cambrian oryctocephalid trilobiteArthricocephalites xinzhaiheensisfrom Guizhou Province, South China

Jin

et al. 2021

LET

“…There is countless examples of local asymmetry in Animals, including asymmetry in Humans [19]. An asymmetry in Animals is, e.g., developed in trilobites relatively early after the emergence of the bilateral symmetry [20]. The asymmetry in a lobster with a cutter and a crusher is established during a lobsters evolution and appears with both the crusher to the left and to the right.…”

Section: Symmetries In Animals and Plantsmentioning

confidence: 99%

The Emergence of the Bilateral Symmetry in Animals: A Review and a New Hypothesis

Toxværd

2021

Symmetry

Most biological organisms exhibit different kinds of symmetry; an Animal (Metazoa), which is our Darwinist ancestor, has bilateral symmetry, and many plants exhibit rotational symmetry. It raises some questions: I. How can the evolution from an undifferentiated cell without bilateral symmetry to a complex biological organism with symmetry, which is based on asymmetric DNA and enzymes, lead to the bilateral symmetry? II. Is this evolution to an organism with bilateral symmetry obtained by other factors than DNA and enzymatic reactions? The existing literature about the evolution of the bilateral symmetry has been reviewed, and a new hypothesis has been formulated based on these reviews. The hypothesis is that the morphogenesis of biosystems is connected with the metabolism and that the oscillating kinetics in the Glycolysis have played a role in the polarity of the biological cells and in the establishment of the bilateral symmetry in Animals.

“…For example, some trilobites shed their shells by hiding in empty shells or burrows of other animals ( Davis, Fraaye & Holland, 2001 ; Chatterton, Collins & Ludvigsen, 2003 ; Chatterton & Fortey, 2008 ; Zong, Fan & Gong, 2016 ), and even molted infaunally ( Rustán et al., 2011 ), reflecting the hiding behavior of trilobites. Some phacopids may also have exhibited asymmetric behaviors during molting ( Zong & Gong, 2017 ). Other trilobites collectively shed their shells, which may have been related to molting–mating behavior ( Speyer & Brett, 1985 ; Speyer, 1990 ).…”

Section: Introductionmentioning

confidence: 99%

Coupled exuviae of the OrdovicianOvalocephalus(Pliomeridae, Trilobita) in South China and its behavioral implications

Zong

2020

PeerJ

Ecdysis was a vital process during the lives of trilobites. In addition to preserving the morphological changes in trilobite ontogeny, the preservation of its action often captured interesting behavioral information. Abundant exuviae of Ovalocephalus tetrasulcatus are preserved in the Ordovician strata in central Hubei, China, and some of them are arranged with two or three together end to end or superimposed. The preserved patterns and burial conditions indicate that these specimens were caused by the active behavior of trilobites. It is speculated that these exuvial clusters were formed by two or three trilobites in line to molt; that is, after one trilobite finished molting, other trilobites molted in front of, behind, or overlying the previously molted shells. This ecdysis strategy is interpreted as related to the postulated herding behavior of some trilobites, representing a behavioral response of the trilobites to choose a nearby safe zone during some risky life activities.