Tadpole Shrimps: Living, Not Fossils

The concept of a 'living fossil' has a fraught history. It tends to appear a lot in popular publications where it conveys a sense of drama and mystery to the organisms so described. In the actual scientific literature, however, it tends to be heavily criticised, due to being poorly defined and of uncertain significance. It commonly gets used to refer to the modern representatives of relictual lineages, often glossing over ways the modern taxa differ notably from their forebears. A reference to 'living fossils' may say more about the writer making it that it does about the intended subject. Consider, for instance, what some have described as the ultimate living fossils: the tadpole shrimps of the Notostraca.

Triops longicaudatus, dorsal and ventral views, copyright Micha L. Rieser.

The tadpole shrimps are a cosmopolitan group of freshwater crustaceans that grow up to ten centimetres in length. They feed on detritus on the bottom of marginal habitats such as temporary pools, brackish lagoons or marshes. Their vernacular name refers to their characteristic body shape, with a flattened oval carapace covering the front of the body (beneath which are concealed the limbs) followed by an elongate, legless abdomen. A pair of compound eyes is visible dorsally near the front of the midline. The body ends in a pair of long caudal rami. Notostracans are a subgroup of the branchiopods, the class of crustaceans that also includes brine shrimps and water fleas. Like other branchiopods, tadpole shrimps have legs with numerous leaf-like branches, adapted for swimming rather than walking. However, whereas many branchiopods have a long pair of second antennae that is used for swimming, tadpole shrimps have both pairs of antennae quite short, presumably in connection with their more benthic lifestyle. The thorax bears eleven pairs of legs, the first of which is elongate and serves to replace some of the sensory function of the rudimentary antennae. The last pair of legs sits on the same segment as the reproductive organs, and in females is modified to form a basket for carrying egs. Like brine shrimps, tadpole shrimps can form a resistent cyst to survive the drying out of their habitat. Also like brine shrimps, this has lead to them being cultured commercially, either as a food supply for fish or as a curiosity in their own right.

Modern tadpole shrimp species are divided between two genera, Lepidurus and Triops, distinguished by the shape of the end of the abdomen. Only about a dozen species are distinguished all up though molecular studies have suggested that there should be more. Notostracans have a sporadic but extensive fossil record, going back as far as the Upper Devonian (Lagebro et al. 2015). And this is where the 'living fossil' concept comes in. The overall form of the tadpole shrimps has been established for a very long time. Indeed, fossils from the Triassic and Permian periods have been described as subspecies of the living Triops cancriformis, which would make it the oldest known species on Earth. Even older fossils from the Carboniferous have been assigned to the genus Triops.

Lepidurus arcticus, copyright Per Harald Olsen.

The problem with these grandiose claims is that their basis is fairly weak. Tadpole shrimps are not heavily sclerotised so their finer features tend to be preserved only rarely. Notostracan fossils often will not preserve much more than the overall proportions of the carapace. And if one wishes to describe Triops cancriformis as a living fossil simply because it has an oval carapace and narrow abdomen, one might as well describe lizards as living fossils because they still have four legs and a tail. A study of the ontogeny of the Triassic 'Triops cancriformis minor', originally described as indistinguishable from the modern species except for its overall smaller size, by Wagner et al. (2017) found notable differences from its modern relative. Both forms have a carapace that becomes longer and narrower over time but, whereas that of T. cancriformis is an oval shape from birth, 'T.' minor begins life rounder and becomes oval with time.

Reconstruction of Almatium gusevi, from Olesen (2009).

One fossil group of crustaceans closely related to the Notostraca is the Kazacharthra, known from the Triassic and Mesozoic of Asia; the two groups have been united in a clade dubbed the Calmanostraca. Kazacharthrans share a number of features with modern tadpole shrimps such as the broad, flattened carapace (albeit one that is proportionately broader than that of Notostraca) and reduced antennae. However, whereas the legs of tadpole shrimps differ from front to back, those of the kazacharthran Almatium gusevi are all similar in structure. In particular, kazacharthrans lack the antenniform first legs of modern notostracans. As it happens, the first legs are also not modified in one Recent species: the little-known Lepidurus batesoni, so far collected once from a location in Kazakhstan (suggesting that the genus Lepidurus is not monophyletic). A phylogenetic analysis of calmanostracans by Lagebro et al. (2015) placed both Almatium gusevi and 'Triops' minor outside the notostracan crown group. It also left open the possibility that 'notostracans', with their much earlier fossil record, are paraphyletic to the Mesozoic kazacharthrans. Lepidurus batesoni was placed closer to other crown notostracans than Almatium or minor, owing to its relatively narrow carapace compared to those taxa and the presence of a rounded anal plate.

So all up, there are problems with describing tadpole shrimps as 'living fossils'. The label focuses on superficial habitus while ignoring the possibility of noteworthy changes in less commonly preserved features. In particular, the antenniform first legs of most modern tadpole shrimps, never yet identified in any fossil species, may be a quite recent innovation. In his 2012 thesis on branchiopod phylogeny, Thomas Hegna ended up concluding that "it seems that Triops cancriformis has no fossil record at all—a dramatic twist of fate for the 'oldest living species'". Hard to qualify as a living fossil when you're not even a fossil!

REFERENCES

Lagebro, L., P. Gueriau, T. A. Hegna, N. Rabet, A. D. Butler & G. E. Budd. 2015. The oldest notostracan (Upper Devonian Strud locality, Belgium). Palaeontology 58 (3): 497–509.

Olesen, J. 2009. Phylogeny of Branchiopoda (Crustacea)—character evolution and contribution of uniquely preserved fossils. Arthropod Systematics and Phylogeny 67 (1): 3–39.

Wagner, P., J. T. Haug, J. Sell & C. Haug. 2017. Ontogenetic sequence comparison of extant and fossil tadpole shrimps: no support for the "living fossil" concept. Paläontologische Zeitschrift 91: 463–472.