Calmanostraca

Triops longicaudatus, copyright Emilia Murray.

Belongs within: Branchiopoda.

Tadpole shrimps: living, not fossils
Published 20 March 2021

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 eggs. 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!

Systematics of Calmanostraca
<==CalmanostracaMD01
|--Ketmeniidae [Kazacharthra]BWW93
| |--Almatium gusevi Chernyshev 1940BWW93
| |--Jeanrogerium sornayi Novozhilov 1959BWW93
| |--Panacanthocaris ketmenia Novozhilov 1957BWW93
| |--Ketmenia schultzi Chernyshev 1940BWW93
| |--Iliella spinosa Chernyshev 1940BWW93
| |--Kungeja tchakabaevi Novozhilov 1957BWW93
| `--Kysyltamia tchiiliensis Novozhilov 1957BWW93
`--Notostraca [Apusidae]MD01
|--ProlepidurusS02
|--Lynceites [Lynceitidae]G88
|--Lepidurus Leach 1819 [incl. Bilobus Sidorov 1924; Lepiduridae]L55
| |--L. arcticus (Pallas 1793) (see below for synonymy)L55
| |--L. apus (Linnaeus 1758) (see below for synonymy)L55
| | |--L. a. apusL55
| | |--L. a. lubbocki Brauer 1873L55
| | |--L. a. packardi Simon 1886L55
| | |--L. a. patagonicus Berg 1900L55
| | `--L. a. viridis Baird 1850L55
| |--L. batesoni Longhurst 1955L55
| |--L. bilobatus Packard 1883L55
| |--L. lemmoni (Holmes 1894) (n. d.)L55
| |--L. lynchi Linder 1952 [incl. L. lynchi var. echinatus Linder 1952]L55
| `--L. stormbergensisL55
`--Triops Schrank 1803L55 (see below for synonymy)
|--T. australiensis (Spencer & Hall 1896) (see below for synonymy)L55
| |--T. a. australiensisL55
| `--T. a. sukalavus (Nobili 1905) [=Apus sukalavus]L55
|--T. cancriformis (Bosc 1801)L55 (see below for synonymy)
| |--T. c. cancriformisL55
| |--T. c. mauretanicus Ghigi 1921 [=Thriops (l. c.) mauretanicus]L55
| |--T. c. minorM79
| `--T. c. simplex Ghigi 1921 [=Thriops (l. c.) simplex]L55
|--T. granarius (Lucas 1864) (see below for synonymy)L55
|--T. longicaudatus (LeConte 1846) (see below for synonymy)L55
| |--T. l. longicaudatusL55
| `--T. l. intermedius Longhurst 1955L55
|--T. ornatus (Goldenberg 1873)BWW93
`--‘Limulus’ pennigerus [=Binoculus pennigerus]L02

Lepidurus apus (Linnaeus 1758)L55 [=Monoculus apusL55, Limulus apusL02; incl. L. angasi Baird 1866L55, L. barcaeus Ghigi 1921L55, L. compressus Thompson 1879L55, L. couessii Packard 1875L55, Apus extensus Braem 1893L55, L. hatcheri Ortmann 1911L55, L. kirki Thompson 1879L55, L. macrourus Lilljeborg 1877L55, Apus productus Bosc 1801L55, L. productusL55, L. viridulus Tate 1879L55]

Lepidurus arcticus (Pallas 1793) [=Monoculus arcticus; incl. L. glacialis Packard 1883, Apus productus var. glacialis, L. spitzbergensis Bernard 1893, L. ussuriensis Sidorov 1927]L55

Triops Schrank 1803L55 [=Thriops (l. c.)L55, Triopes (l. c.)L55; incl. Apodis Zaddach 1841 (non-binomial)L55, Apus Schaeffer 1756 (pre-Linnean) nec Scopoli 1777 nec Gray 1821 (ICBN)P08, Binoculus Geoffroy 1762 (non-binomial)L55, Proterothriops Ghigi 1921L55; Triopidae, Triopsidae]

Triops australiensis (Spencer & Hall 1896) [=Apus australiensis; incl. T. gracilis Wolf 1911, A. madagassicus Thiele 1907, A. strenuus Wolf 1911]L55

Triops cancriformis (Bosc 1801)L55 [=*Apus cancriformisP08; incl. Thriops (l. c.) apulius Ghigi 1921L55, A. cancriformis bidens Sidorov 1909L55, A. haliciensis Fiszera 1885L55, A. himalayensis Packard 1871L55, A. lublinensis Fiszera 1885L55, A. montagui Leach 1816L55, Triops palustris Schrank 1803L55, Limulus palustrisL02, Apus cancriformis transcaucasicus Sidorov 1909L55, A. varsovianus Fiszera 1885L55, A. viridis Bosc 1801L55]

Triops granarius (Lucas 1864) [=Apus granarius; incl. A. asiaticus Gurney 1921, A. bottegoi Prato 1886, A. sudanicus var. braueri Braem 1893, A. sudanicus var. chinensis Braem 1893, A. dispar Brauer 1877, A. dukeanus Day 1880, A. numidicus var. dybowskii Braem 1893, A. elongatus Thiele 1907, A. mavliensis Tiwari 1952, A. namaquensis Richters 1886, A. numidicus Grube 1865, A. orientalis Tiwari 1952, A. ovamboensis Barnard 1924, A. sculleyi Sars 1899, A. numidicus var. sinensis Uéno 1928, A. somalicus Wedenissow 1895, A. numidicus var. strauchii Braem 1893, A. sudanicus Brauer 1877, A. trachyaspis Sars 1899, Triops uebensis Colosi 1922, A. zanoni Colosi 1920]L55

Triops longicaudatus (LeConte 1846) [=Apus longicaudatus; incl. A. aequalis Packard 1871, A. biggsi Rosenberg 1947, A. domingensis Baird 1852, A. frenzeli Thiele 1907, A. guildingi Thompson 1834 (n. d.), A. lucasanus Packard 1871, A. newberryi Packard 1871, A. obtusus James 1823 (n. d.), A. oryzaphagus Rosenberg 1947, Triops pampaneus Ringuelet 1944]L55

*Type species of generic name indicated

References

[BWW93] Briggs, D. E. G., M. J. Weedon & M. A. Whyte. 1993. Arthropoda (Crustacea excluding Ostracoda). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 321–342. Chapman & Hall: London.

[G88] Gray, J. 1988. Evolution of the freshwater ecosystem: the fossil record. Palaeogeography, Palaeoclimatology, Palaeoecology 62: 1–214.

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.

[L02] Latreille, P. A. 1802. Histoire Naturelle, générale et particulière des crustacés et des insectes vol. 4. Familles naturelles des genres. F. Dufart: Paris.

[L55] Longhurst, A. R. 1955. A review of the Notostraca. Bulletin of the British Museum (Natural History): Zoology 3 (1): 1–57.

[MD01] Martin, J. W., & G. E. Davis. 2001. An updated classification of the Recent Crustacea. Natural History Museum Los Angeles County, Science Series 39: 1–124.

[M79] Müller, A. H. 1979. Fossilization (taphonomy). In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A2–A78. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

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

[P08] Paclt, J. 2008. On the triple usage of the family name Apodidae in zoology (Aves; Crustacea; Insecta). Senckenbergiana Biologica 88 (1): 49–52.

[S02] Sinitshenkova, N. D. 2002. Ecological history of the aquatic insects. In: Rasnitsyn, A. P., & D. L. J. Quicke (eds) History of Insects pp. 388–426. Kluwer Academic Publishers: Dordrecht.

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.

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