Understanding locomotion in trilobites by means of three-dimensional models.

Trilobites were one of the first animals on Earth to leave their imprints on the seafloor. Such imprints represent behavioral traces related to feeding or protection, in both cases implying different types of locomotion. Modeling how trilobites moved is essential to understand their evolutionary history and ecological impact on marine substrates. Herein, locomotion in trilobites is approached by means of three-dimensional models, which yielded two main gait types. These two gaits reflect basic behaviors: burrowing and walking. This model reveals that trilobites could change their gait and consequently increase rapidly their speed varying the amplitude of the metachronal wave, a change independent from their biological structure. Fast increases in speed enhanced the protection of trilobites against predators and sudden environmental crises. The trilobite body pattern constrained their gaits, controlled by the distance between the pair of legs and between legs in a same segment.

Developmental and functional controls on enrolment in an ancient, extinct arthropod.

Three-dimensional models reveal how the mechanics of exoskeletal enrolment changed during the development of a model organism for insights into ancient arthropod development, the 429-million-year-old trilobite Aulacopleura koninckii. Changes in the number, size and allocation of segments within the trunk, coupled with the need to maintain effective exoskeletal shielding of soft tissue during enrolment, necessitated a transition in enrolment style about the onset of mature growth. During an earlier growth phase, enrolment was sphaeroidal, with the venter of the trunk fitting exactly against that of the head. In later growth, if lateral exoskeletal encapsulation was to be maintained trunk length proportions did not permit such exact fitting, requiring an alternative, non-sphaeoridal enrolment style. Our study favours the adoption of a posture in later growth in which the posterior trunk extended beyond the front of the head. This change in enrolment accommodated a pattern of notable variation in the number of mature trunk segments, well known to characterize the development of this species. It suggests how an animal whose early segmental development was remarkably precisely controlled was able to realize the marked variation in mature segment number that was related, apparently, to life in a physically challenging, reduced oxygen setting.

Life cycle evolution in the trilobites Balangia and Duyunaspis from the Cambrian Series 2 (Stage 4) of South China.

The evolution process can be reconstructed by tracking the changes in the dynamic characters of life cycles. A number of related trilobites from the Cambrian of South China provide additional information for the study of trilobite evolutionary patterns, which has been hampered by previous incomplete fossil record though. Here, Balangia and Duyunaspis represent related Cambrian oryctocephalid trilobites from South China, are comprehensively discussed over the ontogeny, and the results show that, from B. balangensis via D. duyunensis to D. jianheensis, their exoskeletal morphology shows a directional evolution. Based on the direction of evolutionary changes in the development of Balangia and Duyunaspis, we speculate that Duyunaspis likely evolved from Balangia instead of Balangia evolved from Duyunaspis, as was previously assumed. This inference is also supported by the phylogenetic tree. This research provides not only a better understanding of the mechanisms of evolution in trilobites, but also new insights for the relationship between developmental evolutionary changes and phylogeny in trilobites.

Fluid dynamic simulation suggests hopping locomotion in the Ordovician trilobite Placoparia.

Colonization of the water column by animals occurred gradually during the early Palaeozoic. However, the morphological and functional changes that took place during this colonization are poorly understood. The fossil record provides clear evidence of animals that were well adapted for swimming near the seafloor or in the open ocean, but recognising transitional forms is more problematic. Trilobites are a good model to explore the colonization of marine ecosystems. Here, we use computational fluid dynamics (CFD) to test between competing functional hypotheses in the Ordovician trilobite Placoparia. The CFD simulations exhibits hydrodynamics that promote detachment from the seafloor but also promote return to the seafloor following detachment, this is compatible with hopping locomotion. The results suggest that Placopara was not able to swim, but its hydrodynamics allowed it to hop long distances. This is consistent with the fossil record, as some ichnofossils show evidence of hopping. This type of locomotion could be useful to avoid predators as an escape mechanism. In addition, CFD simulation shows how the morphology of Placoparia is adapted to protect anterior appendices of the trunk and generate a ventral vortex that send food particles directly to the trilobite mouth. Adaptations in Placoparia allowed the first steps to evolved a new ecological habitat and consequently nektonization during the GOBE.

Synchronized moulting behaviour in trilobites from the Cambrian Series 2 of South China.

The study of moulting behaviour in the fossil record is relatively well known in arthropods and this is especially true for trilobites. Nevertheless, while studies focusing on the style of moulting in social and semi-social groups of modern animals (e.g. arthropods) are common, very few works investigate moulting adaptations in deep time. Here we report a trilobite assemblage from the Cambrian Series 2 "Tsinghsutung" Formation of South China. Around 850 specimens were used for this study from three different levels across one section near Balang (SE Guizhou Province, South China). These levels preserve numerous trilobite clusters in some cases containing around 400 individual specimens. Up to four species have been found in these clusters, but two species are more common. Trilobite clusters bear a high percentage of disarticulated specimens that we interpret as moults. Additionally, measurements of bioclast orientation and the dorsoventral attitude suggests very quiet water conditions followed by rapid burial events, prior to scavenger disturbance. Together, this indicates that the fossil assemblages were a result of a biological phenomenon rather than mechanical processes, allowing us to interpret the position of the fossil parts as different moulting configurations. Since the trilobite assemblage seems to be in situ, the large number of exuviae suggests a local place of migration. This was triggered by the need for group protection while moulting, which is suggestive of gregarious behaviour, possibly synchronized. These trilobites from the Cambrian Epoch 2, Age 4 constitute one of the earliest known gregarious community of trilobites and has important implications for understanding the ecology of this group during their emergence in the Cambrian.

A new high-resolution 3-D quantitative method for analysing small morphological features: an example using a Cambrian trilobite.

Taphonomic processes play an important role in the preservation of small morphological features such as granulation or pits. However, the assessment of these features may face the issue of the small size of the specimens and, sometimes, the destructiveness of these analyses, which makes impossible carrying them out in singular specimen, such as holotypes or lectotypes. This paper takes a new approach to analysing small-morphological features, by using an optical surface roughness (OSR) meter to create a high-resolution three-dimensional digital-elevation model (DEM). This non-destructive technique allows analysing quantitatively the DEM using geometric morphometric methods (GMM). We created a number of DEMs from three populations putatively belonging to the same species of trilobite (Oryctocephalus indicus) that present the same cranidial outline, but differ in the presence or absence of the second and third transglabellar furrows. Profile analysis of the DEMs demonstrate that all three populations show similar preservation variation in the glabellar furrows and lobes. The GMM shows that all populations exhibit the same range of variation. Differences in preservation are a consequence of different degrees of cementation and rates of dissolution. Fast cementation enhances the preservation of glabellar furrows and lobes, while fast dissolution hampers preservation of the same structures.

Humble origins for a successful strategy: complete enrolment in early Cambrian olenellid trilobites.

Trilobites are typified by the behavioural and morphological ability to enrol their bodies, most probably as a defence mechanism against adverse environmental conditions or predators. Although most trilobites could enrol at least partially, there is uncertainty about whether olenellids-among the most phylogenetically and stratigraphically basal representatives-could perform this behaviour because of their poorly caudalized trunk and scarcity of coaptative devices. Here, we report complete-but not encapsulating-enrolment for the olenellid genus Mummaspis from the early Cambrian Mural Formation in Alberta, the earliest direct evidence of this strategy in the fossil record of polymerid trilobites. Complete enrolment in olenellids was achieved through a combination of ancestral morphological features, and thus provides new information on the character polarity associated with this key trilobite adaptation.