Science Literature

Predation in the Early Cambrian is demonstrated by the occurrence of borings or drill holes in shelly fauna. Confirmatory evidence has been adduced from gut content analyses, although these evidences could also be attributed to scavenging. However, predation involving shell breakage or crushing has been documented previously only from the late Ordovician. This situation has changed with the published research on Lower Cambrian lingulate brachiopods.

"Here we present the first report of repaired damage to linguliform brachiopod shells caused by durophagous shell-crushing, which is exquisitely recorded from exceptionally preserved specimens in the early Cambrian Wulongqing Formation (Guanshan fauna), Kunming, China. The healed fractures on specimens with preserved thin pedicles unambiguously suggest failed predation attempts. Although they cannot be linked to any specific predators, this record shows that early Cambrian brachiopods experienced predation pressure by unspecified durophagous predators, probably anomalocariids."

Slab containing fossils of Diandongia pista (Source here)

The organism being studied is Diandongia pista, one of the most common species of its type in the Lower Cambrian of South China. It was first studied as a member of the Chengjiang fauna (which is assigned to Stage 3 of the Lower Cambrian). A total of 1150 specimens have been collected for research purposes, none of which have been damaged by predation. The newly reported research is based on a study of fossils of Diandongia pista from the overlying Guanshan fauna (assigned to Stage 4 of the Lower Cambrian). 273 specimens have been collected, 13 of which have non-lethal predatory shell damage and subsequent shell repair. The hypothesis that these features do not signify predation has been examined and rejected.

"However, the possibility of non-predatory causes can be directly rejected with certainty in the specimens described here; the shell damages illustrated from Diandongia cannot have been formed by marginal abrasion of the shell, and the damaged areas are not restricted to anterolateral marginal edge of shell. Evidently, the healed injuries are steeply U or V-shaped, cutting vertically across multiple concentric growth lines, contradicting to the proposed marginal gashes."

These evidences show that the earliest-known record of repaired predatory attack on brachiopods can be assigned to Stage 3 of the Lower Cambrian. The authors infer that predation levels were comparatively very low in the earliest phases of Cambrian brachiopods. Adaptive changes associated with predation are suggested to increase with time, but it is only in the Great Ordovician Biodiversification that it becomes important.

"[T]he emergence and subsequent diversification of smashing/crushing attacks throughout the Cambrian may be the precursor to an Early Palaeozoic marine revolution. Such marine revolutions, attributed to durophage radiations, have been recognized in the Early Cretaceous, and in the Middle Devonian. By the Early Ordovician, brachiopod communities were very different from their Late Cambrian counterparts, and increasing predation pressure from durophages may have been a contributing factor to the proliferation of calcitic-shelled brachiopods in the Ordovician, and possibly to the subsequent infaunal habit of some linguliform taxa, as phosphatic shells were likely insufficient to protect against attacks."

Whilst agreeing with this conclusion of the authors, a design perspective does allow some additional conclusions to be reached. Three of these are outlined below.

1. Drivers of change in the Lower Cambrian. This point is concerned with the Cambrian Explosion and possible drivers of diversification. Darwinian concepts of the struggle for survival have featured strongly in attempts to explain the Cambrian Explosion. However, although predator-prey roles are to be found, the evidence linking these with adaptive change is lacking. Last December, an alternative ecological framework for interpreting this part of the fossil record was discussed (go here for the 4th in the series). The authors introduce their paper by pointing out the importance of this issue in the minds of evolutionary biologists, but then they go on to show evidence that for shell-crushing predation, this selection force was non-existent before Stage 4 of the Lower Cambrian. Some re-evaluation of drivers affecting the Cambrian Explosion is therefore needed.

"Predation has been considered as one of the driving and selective forces in evolution, and thus plays an important macroevolutionary role in marine environments. Coevolution between organisms and their predators had led to an intensification of the struggle for existence and increased complexity of organisms during the Cambrian explosion interval."

2. Evidence of repair mechanisms co-exist with the first appearance of durophagous shell damage. The process of growing a mollusc shell is a remarkable phenomenon in itself, but mechanisms for repairing damaged shells should be regarded as an additional level of complexity. The fossil record does not provide us with a 3-stage story (i.e. no predation by shell breaking - predation without repair - predation accompanied by repair). The Chengjiang fauna document 'no predation by shell breaking'. The "slightly younger" Guanshan fauna documents 'predation accompanied by repair'. This leaves open the issue of the origin of the repair mechanism - did it evolve rapidly under the influence of natural selection? - is this a pointer to design?

"[T]he healed fractures show sets of distinctive drape-like arches of shell repairs, which markedly differs from the marginal concentric secretion of shell complements on scars in the mollusk Marocella described by Skovsted et al. (2007) from the Mernmerna Formation and Oraparinna Shale of South Australia, thus implying that a wider range of shell repair mechanisms evolved in different Cambrian shelly organisms of concentric accretionary growth."

3. When lingulids experienced selection forces associated with predation, they did respond, but remained lingulids. The exquisite preservation of the fossils has allowed soft tissue preservation: the pedicle used to anchor the animal in sediment. Lingulids subsequent to the Cambrian have a muscular pedicle and thick shells, as is appropriate for a burrowing infaunal lifestyle. However, the Lower Cambrian lingulids are different. The pedicles are more delicate and are suggestive of a semi-infaunal lifestyle "with only the pedicle buried in deep sediments for its anchorage and the anterior marginal shell margin tilted upwards". Thus, the Lower Cambrian lingulids were vulnerable to predation and selection pressures favoured animals that buried deeper into the sediments. By the Ordovician, this change was complete, and it never changes again in the fossil record. The stasis in Lingula was noted by Charles Darwin in Chapter XI of On the Origin of Species. He wrote: "The Silurian Lingula differs but little from the living species of this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly." (Source here) The case of Lingula is therefore a fascinating one: an abrupt appearance in the Lower Cambrian with a semi-infaunal lifestyle; durophagous-style predation by Stage 4 of the Lower Cambrian; adaptive change to a burrowing infaunal lifestyle followed by stasis (earning the extant Lingula the right to be called a living fossil). This case study illustrates well the empirical evidence relating to the capabilities of natural selection acting on natural variations. There is no sign of the evolution of complexity, but only the fine-tuning of existing complexity.

There are implications here for the study of evolution. The research community needs to be more aware of the limitations of proposed mechanisms (in particular, the Darwinian mechanisms of mutation and natural selection). It is not good enough to point to industrial melanism or Galapagos finches and claim that here is evolution in action! These examples simply do not address the challenge of building biological complexity. As researchers become more aware of the limitations, it will become apparent that the most appropriate context for analysing natural selection is not evolutionary biology - but ecology.

First record of repaired durophagous shell damages in Early Cambrian lingulate brachiopods with preserved pedicles
Zhifei Zhang, Lars E. Holmer, Sean P. Robson, Shixue Hu, Xiangren Wang and Haizhou Wang.
Palaeogeography, Palaeoclimatology, Palaeoecology, 302, (March 2011) 206-212 | doi:10.1016/j.palaeo.2011.01.010

Durophagy, the macro-predatory consumption of hard-shelled organisms, has been proposed as an important driving and selective force ("arms race") responsible for the explosive advent of Cambrian skeleton-bearing animals. Nevertheless, the direct evidence of durophagous predationis mostly restricted to borings or drill holes in skeletons at around the Ediacaran–Cambrian transition. In contrast, pre-ingestive breakage or crushing of shell, another important type of durophagous predation evidence, is very rarely fossilized. Here we present the first evidence of durophagous shell-breaking in an exceptionally preserved pedunculate lingulate brachiopod from the Lower Cambrian Wulongqing Formation (Series 2, early Stage 4), Yunnan, southern China. The repaired shells of Diandongia pista all have elongate (up to 36 mm) pedicles that demonstrate that they survived the failed predation and remained in situ. The bite embayment shows three sets of distinctive drape-like convex arcs of shell repairs, suggesting that the specific drape-like ornamentation usually seen in lingulate shells could be taken as reparative responses to shell damage and malformation. Discovery of sublethal shell damage demonstrates that durophagous predators may have caused an increasing predation pressure on brachiopods since the Canglangpuian Stage (Series 2, Stage 4). In contrast there are no records of durophagous shell-breaking recognized from thousands of Chengjiang (Series 2, Stage 3) specimens of D. pista with fully developed organization of tissues and organ system. It is therefore assumed that that predation was of little importance to the earliest evolution of Cambrian lingulates.