Gigantic trilobite larvae
The life cycle of today’s marine invertebrates includes various larval stages that may be very different compared to adults in terms of their way of life. This early post-embryonic stage can be simply divided into two categories based on feeding. The first category consists of tiny larvae that hatch from small eggs and after hatching live off of plankton or detritus and are therefore known as feeding larvae (or also as planktotrophic or detritotrophic larvae). Others emerge from large eggs rich in yolk, are larger, and during their early development live off of this yolk supply (non-feeding or lecithotrophic larvae). While we have a relatively good understanding of the biology and ecology of early post-embryonic stages of today’s invertebrates, our knowledge of long-extinct organisms is still very limited.
Palaeontologists from the Institute of Geology and Palaeontology of the Charles University Faculty of Science and the Complutense University in Madrid were able to discover gigantic larvae of Cambrian trilobites. Larvae belonging to the species Hydrocephalus carens and Eccaparadoxides pusillus (fig. 1) come from several locations around the villages of Skryje and Týřovice in the Křivoklát region. Compared to the larvae of other trilobites, these are two or even three times as large. While for most Cambrian trilobites the smallest stage measures around 0.2 to 0.6 mm, for E. pusillus it is 0.9 mm and for H. carens 1.9 mm. Both of these species have a very unusual morphology characterized in particular by a large protruding glabella (part of the head shield on top of the front part of the digestive tract). In addition, H. carens has a significantly shortened development and incremental changes between individual larval stages. Similar characteristics are typical of the lecithotrophic larvae of present-day crustaceans. In their study, the authors also ponder what could have led to the emergence of lecithotrophic development in Cambrian trilobites.
Today, lecithotrophic larvae are much more plentiful in seas where plankton production is very low or seasonally unstable. Such a situation applies, for example, in high latitudes, deeper seas or freshwater. An analysis of the size of early stages of Cambrian trilobites shows that in tropical seas near the Cambrian equator larvae were small, while large larvae (including species H. carens and E. pusillus) come from high latitudes of the southern hemisphere along the continent of Gondwana (fig. 2). We find a similar distribution of the larval stages of marine invertebrates today, where along the equator there are numerous species of small, planktotrophic larvae, while in the polar areas we find primarily large lecithotrophic larvae. For certain trilobites unstable plankton production in the waters of high latitudes at that time could have been one of the causes leading to the emergence of lecithotrophy.
This article was published in Palaeogeography, Palaeoclimatology, Palaeoecology:
Laibl, L., Esteve, J., Fatka, O. 2017. Giant postembryonic stages of Hydrocephalus and Eccaparadoxides and the origin of lecithotrophy in Cambrian trilobites. Palaeogeography, Palaeoclimatology, Palaeoecology 470, 109–115.