Stage-specific feeding strategies – 520 million years ago
In many animal species, larvae and adults occupy different ecological niches, and do not compete for the same resources. An LMU team has now uncovered evidence that the earliest ancestors of spiders and scorpions exploited the strategy.
It was serendipity rather than forethought that drew the attention of Drs. Yu Liu, Carolin Haug and Joachim Haug from Biology Department II at LMU to an unusually small specimen of a species of chelicerates as they were sorting through material recovered from a fossil Lagerstätte in Chengjiang in southwestern China. The fossils from this site, which has been intensively studied since the 1980s, are on the order of half a billion years old. The material excavated from a shale formation of that age is so diverse – and much of it is so well preserved – that it provides a detailed picture of animal diversity in the early Cambrian. This was the period in which many of the major invertebrate groups we know today first evolved.
Very many of the species that have been found in Chengjiang represent early forms of arthropods, the superdiverse group which includes insects or crustaceans. Thus, fossil samples attributable to the species Leanchoilia illecebrosa, on which the LMU team has done a great deal of work, are regarded as an early marine chelicerate. Chelicerates comprise, for example, modern spiders, scorpions and mites, but also one of the most famous “living fossils”, the horseshoe crab Limulus polyphemus. The finds of L. illecebrosa that the LMU researchers had previously examined were between 2 and 4 cm in length. The newly discovered one measures only 8 mm.
Nevertheless, the LMU paleontologists were able to assign the new specimen unequivocally to the same species. Together with colleagues based in China and the US, they have now reported their findings in the journal Nature Communications. Despite its minute size, they were able to identify several morphological features that are characteristic of L. illecebrosa: The anterior region or head of the new specimen bears four pairs of legs and the trunk region is made up of eleven segments. Furthermore, its posterior end (the so-called telson) has a highly characteristic dagger-like shape and bears spines, and it also displays a particular type of head shield, and possesses branched legs whose morphology is specific for L. illecebrosa. Liu, Haug and Haug therefore concluded that it represents a larval stage of that species.
In all of the characteristic morphological traits, the team found striking differences in between the larval and mature adult forms. In chelicerates, the first pair of appendages is shaped like pincers and serves as raptorial “jaws”. In the larva, certain regions of this structure, which ends in a multipartite claw, bear stiff hairs. The researchers interpret these structures as a particle-feeding apparatus, which trapped food particles suspended in the water and transported them into the oral cavity. In the adults, in contrast, the claw, which now bears an additional hook-shaped spine, functioned as a true claw – and could deal with much larger prey items than the juveniles could handle. “This type of stage-specific morphological differentiation is quite common in modern ecosystems,” says Carolin Haug, as it provides a significant selective advantage: It enables immature and adult representatives of the same species to occupy and exploit different ecological niches, and therefore precludes intraspecific competition for the same nutritional resources. “And we have now discovered an example of the realization of this principle from a very early period in evolutionary history.”