Arthropoda

Reconstruction of Isoxys, from Cambrian Cafe.

Belongs within: Ecdysozoa.
Contains: Leanchoiliida, Euarthropoda.

The Arthropoda are a major clade of animals characterised by the presence of sclerotised, jointed limbs. The arthropods have been one of the most successful of all animal groups and dominate many modern environments. As defined herein, this taxon includes the Anomalocarida, a Cambrian to Devonian group of nektonic animals with large grasping appendages at the front of the head and a mouth surrounded by a radial array of sclerotised, overlapping plates (Vinther et al. 2014). A number of Cambrian forms may also represent lineages outside the arthropod crown group (Euarthropoda). Examples of these forms include the genus Isoxys, members of which had a large carapace covering most of the body, with a more or less well-developed spine at the anterior end of the carapace.

The Cheliceriformes are here recognised as a group uniting the Chelicerata with related Palaeozoic fossil forms. Members of this group have a postabdomen lacking appendages, and commonly have dorsal, sessile (unstalked) eyes. Living members of this clade belong to the Chelicerata, which ancestrally have the first pair of head appendages formed as pincered chelicerae with the terminal segment opposed to a spinose projection of the penultimate segment (Cotton & Braddy 2004). The sister group of the Chelicerata may be the Vicissicaudata, a group of early Palaeozoic arthropods with a differentiated postabdomen, commonly with a long telsonic spine or appendicular derivatives on the pretelson.

A quick primer on arthropod growth
Published 23 April 2009
Successive instars of a generalised bug (Heteroptera). Image from here.

One of the trickiest things to wrap one’s head around about insects and other arthropods* is also one of the most basic—how they grow. We tend to forget just how different arthropod growth is from our own—I’ve even known people who work with arthropods regularly to have it slip their mind.

*Other than that a scorpion’s anus is at the very end of its tail next to the sting, not under the base of the tail as we chordates might tend to imagine.

For us as vertebrates, growth to maturity is fairly continuous. We start out small, we get steadily bigger. Take a balloon, blow it up, and you’ve got a fairly good representation of how we grow (yes, I’m massively simplifying things, but bear with me for a moment). Arthropod growth, on the other hand, is more like a series of balloons of different sizes all one inside the other, with the smallest balloon on the outside and the largest balloon at the centre. Start blowing up the balloons, and you’ll only be able to blow it up to the size of the smallest balloon. If that smallest balloon breaks open (like an insect moulting its skin), then the balloons can inflate to the size of the second-smallest balloon. And so on and so forth, until you reach the largest size. Instead of growing in size continuously like we do, arthropods grow in steps—an extended period of no obvious increase in size, then a moult followed by a near-instantaneous increase as the animal swells up to fill its new skin, then another period without obvious growth. The change between moults can be drastic, as most obviously shown by the holometabolous insects with their radically different larval and adult stages. Even if the differences in morphology are not so drastic, separate instars (life cycle stages) may occupy distinctly different size ranges, with little or no overlap, and may have very distinct ecologies.

Internal pupal development of Rhagoletis pomonella (apple maggot fly), from a newly developed pupa on the left, to a pharate adult (fully developed adult still enclosed within the pupa) on the right. Photos by John Fuller, via here.

It’s not that the arthropod is not growing at all between moults. A new layer of cuticle is being grown inside the old layer, albeit sort of crinkled up so that it can fit. Once the new cuticle has finished growing, the animal enters the pharate (“cloaked”) state until the old cuticle is shed to reveal the new. Sometimes, the pharate period will be minimal, and the old cuticle will be shed pretty much as soon as the new one is ready. At other times, though, the pharate period will last for a considerable time. If conditions aren’t right for the arthropod to move on to the next stage in its life, its growth may be effectively put on hold. Desert spiders may remain as subadults almost indefinitely, waiting for the rains to come before they moult into mature adults (and if the rains don’t come one year, they can wait as subadults until the next). Caterpillars may moult into pupae at the beginning of autumn, but not emerge as butterflies until some time in the next spring when the flowers they feed on are beginning to bloom. If environmental conditions suddenly deteriorate, vertebrates are forced into the awkward position of having to maintain growth despite their reduced food supply. Arthropods, on the other hand, can afford to wait things out.

Supermajor worker of the ant Pheidologeton affinis, surrounded by minor workers. Both sizes are fully adult—the small ants will not grow into the big ones. Photo by Alex Wild.

On the other hand, vertebrates have some liberties that arthropods do not. Most arthropods (including insects) have a set number of moults in their life cycle and do not reach maturity until the very last moult. The flipside, of course, is that once an individual of these species does reach maturity, that’s it. They are unable to resume growth should the occasion arise (this is not necessarily a problem because many, if not most, arthropods do not live long as mature adults; those arthropod groups which do moult after maturity are long-lived species such as lobsters and trapdoor spiders). Contrary to what the cartoons may suggest, little ants do not grow into big ants. Both are fully adult, both are as big as they’re going to get. In those ant species that have different sized castes, it’s easy to imagine otherwise, but that’s simply not the case. Big ants hatched out from their pupa as big ants, little ants hatched out as little ants. Similarly, if the queen of an ant colony were to die, it would not be possible for one of the workers to develop a functional reproductive system and take her place—sterility is a one-way trip.

A persistent bandit
Published 6 February 2009

This little beastie (just under ten centimetres long) is called Schinderhannes bartelsi*, and its fossil remains are described in a paper by Kühl et al. (2009) (from whence comes the above reconstruction). Some of you may immediately recognise the similarity to the famed larger animals Anomalocaris and Laggania of the Cambrian Burgess Shale. However, Schinderhannes bears a few significant differences from those taxa: (1) it has that bizarre pair of ‘wings’ attached to the back of the head; (2) certain details of its anatomy suggest that it is more closely related to living arthropods than is Anomalocaris, showing that arthropods are descended from an ‘anomalocarid’ grade; and (3) it doesn’t come from the Burgess Shale, but the German Hunsrück Slate, which is from the Lower Devonian, and shows that ‘anomalocarid’-type animals were around for some 100 million years longer than we previously knew. I hate to repeat the old cliché about it being like discovering a Tyrannosaurus alive today, and in fact it’s not like that, because the amount of time separating Tyrannosaurus from the present is considerably less than 100 million years.

*The name Schinderhannes is apparently derived from that of an 18th century bandit in the area from which it was found. Neat name, but it hints frustratingly at a back story that we are sadly denied in the paper.

Schinderhannes resembles anomalocarids in its radial mouth, and the large pair of spiny pre-oral appendages. However, certain of its features are more like true arthropods – it has a dorsum divided into distinct, sclerotised tergite plates, and it has biramous (two-branched) appendages like crustaceans. The combination of the large ‘wings’ and ‘flukes’ on either side of the tail spine suggest that it was an active swimmer.

Large raptorial pre-oral appendages (dubbed ‘great appendages’) have also been found in a number of Cambrian arthropods such as Leanchoilia and Yohoia. The phylogenetic position of such ‘great-appendage’ arthropods has been hotly debated. Budd (2002) suggested that they were a stem grade to the arthropod crown clade, but Cotton & Brady (2004) placed them within the crown clade, in the stem group for chelicerates. Researchers have also debated whether the great appendages of these arthropods are homologous to those of anomalocarids, and whether the great appendages are homologous to the chelicerae of modern chelicerates. The (admittedly pretty rudimentary) phylogenetic analysis of Schinderhannes by Kühl et al. (2009), the results of which are shown above, supports a position of great-appendage arthropods as stem chelicerates (despite the great appendages of these arthropods being a priori coded as homologous to those of anomalocarid-grade animals), which supports the comparison between great appendages and chelicerae. It also suggests that trilobites are closer to crustaceans than chelicerates, contrary to the idea of a trilobite + chelicerate “Arachnomorpha” clade. In some regards, this would make sense—trilobites, like crustaceans and insects, have lost the plesiomorphic state of grasping pre-oral appendages as found in chelicerates and have filamentous antennae instead. However, the position of trilobites in the tree above seems to be primarily due to the presence of antennae, so I don’t know if it can be considered well-supported.

Systematics of Arthropoda
<==Arthropoda (see below for synonymy)LSE13
    |--+--‘Anomalocaris’ pennsylvanicaVS14
    |  `--+--Paranomalocaris multisegmentalisVS14
    |     `--+--Anomalocaris Whiteaves 1892AC19, B95 [Anomalocarididae]
    |        |    `--*A. canadensis Whiteaves 1892WB85
    |        `--AmplectobeluidaeVS14
    |             |--‘Anomalocaris’ saronVS14
    |             `--AmplectobeluaVS14
    |                  |--A. kunmingensisVS14
    |                  `--+--A. stephenensisVS14
    |                     `--A. symbrachiataB14
    `--+--+--+--+--Jiangfengia Hou 1987AC19, B95 [Jianfengiida]
       |  |  |  |    `--J. multisegmentalis Hou 1987CB04
       |  |  |  `--+--Parapeytoia yunnanensisAC19, KBR09
       |  |  |     `--+--Fortiforceps foliosa Hou & Bergström 1997AC19, CB04
       |  |  |        `--OelandocarididaeLSE13
       |  |  |             |--OelandicarisAC17
       |  |  |             `--+--SandtorpiaLSE13
       |  |  |                `--Henningsmoenicaris scutula (Walossek & Müller 1990)LSE13, WD02
       |  |  `--+--+--Yohoia Walcott 1912AC19, B95 [Yohoiida]
       |  |     |  |    `--Y. tenuis Walcott 1912CB04
       |  |     |  `--+--Haikoucaris ercaiensisAC19, C12
       |  |     |     `--LeanchoiliidaAC19
       |  |     `--EuarthropodaAC17
       |  `--+--SurusicarisAC17
       |     `--IsoxysLSE13
       |          |--I. auritusLSE13
       |          `--+--I. acutangulusLSE13
       |             |--I. curvirostrusLSE13
       |             `--I. volucrisLSE13
       `--+--AegirocassisAC19
          |--CetiocaridaeVS14
          |    |--‘Anomalocaris’ briggsiVS14
          |    `--Tamisiocaris borealisVS14
          `--HurdiidaeVS14
               |--+--Peytoia Walcott 1911VS14, WB85
               |  |    `--*P. nathorsti Walcott 1911 [=Anomalocaris nathorsti]WB85
               |  `--Laggania Walcott 1911KBR09, WB85
               |       `--*L. cambria Walcott 1911WB85
               `--+--Schinderhannes Kühl, Briggs & Rust 2009VS14, KBR09
                  |    `--*S. bartelsi Kühl, Briggs & Rust 2009KBR09
                  `--+--Stanleycaris hirpexVS14
                     `--Hurdia Walcott 1912AC19, B95
                          `--*H. victoria Walcott 1912 (see below for synonymy)DB09
Arthropoda incertae sedis:
  Dictyocaris Salter 1860LS02
  Tontoia Walcott 1912B95
  Serracaris Briggs 1978B95
    `--S. lineata [=Anomalocaris lineata]WB85
  Achanarraspis Anderson et al. 2000N02
  Magnoculocaris blindiCB04
  ‘Leuconoe’ Bogachev 1930 non Boie 1830 (n. d.)H75
    `--*L. paradoxa Bogachev 1930 (n. d.)H75
  Velancorina Pflug 1966G79
    `--*V. martina Pflug 1966G79
  Occacaris oviformisAC17
  Caryosyntrips serratusVS14
  Combinivalvula chengjiangensis Hou 1987AH07
  KootenichelidaeLSE13
    |--KootenichelaLSE13
    `--Worthenella Walcott 1911LSE13, H62
         `--*W. cambria Walcott 1911H62
  Tanglangia longicaudataC12
Inorganic: Hurdia davidi Chapman 1926H75

Arthropoda [Annulosa, Anomalocarida, Anomalocaridata, Brachiocaridea, Burgessidea, Canadaspididea, Cheliceriformes, Cheliceromorpha, Emeraldellidea, Euthycarcinomorpha, Megacheira, Merostomoidea, Protochelicerata, Pseudonotostraca, Radiodonta, Schizoramia, Trilobitoidea, Vicissicaudata, Yohoiidacea, Yohoiidea]LSE13

*Hurdia victoria Walcott 1912 [incl. Proboscicaris agnosta Rolfe 1962, H. dentata Simonetta & Della Cave 1975, P. hospes Chlupáč & Kordule 2002, Liantuoia inflata Cui & Huo 1990, P. ingens Rolfe 1962, P. obtusa Simonetta & Della Cave 1975, Amiella ornata Walcott 1911, Sidneyia ornata, Hurdia triangulata Walcott 1912, Huangshandongia yichangensis Cui & Huo 1990]DB09

*Type species of generic name indicated

References

[AH07] Aldridge, R. J., Hou X.-G., D. J. Siveter, D. J. Siveter & S. E. Gabbott. 2007. The systematics and phylogenetic relationships of vetulicolians. Palaeontology 50 (1): 131–168.

[AC17] Aria, C., & J.-B. Caron. 2017. Burgess Shale fossils illustrate the origin of the mandibulate body plan. Nature 545: 89–92.

[AC19] Aria, C., & J.-B. Caron. 2019. A middle Cambrian arthropod with chelicerae and proto-book gills. Nature 573: 586–589.

[B95] Bousfield, E. L. 1995. A contribution to the natural classification of Lower and Middle Cambrian arthropods: food-gathering and feeding mechanisms. Amphipacifica 2: 3–34.

[C12] Chen, J.-Y. 2012. Evolutionary scenario of the early history of the animal kingdom: evidence from Precambrian (Ediacaran) Weng’an and Early Cambrian Maotianshan biotas, China. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 239–379. Springer.

[CB04] Cotton, T. J., & S. J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 169–193.

[DB09] Daley, A. C., G. E. Budd, J.-B. Caron, G. D. Edgecombe & D. Collins. 2009. The Burgess Shale anomalocaridid Hurdia and its significance for early euarthropod evolution. Science 323: 1597–1600.

[G79] Glaessner, M. F. 1979. Precambrian. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A79–A118. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[H75] Häntzschel, W. 1975. Treatise on Invertebrate Paleontology pt W. Miscellanea Suppl. 1. Trace Fossils and Problematica 2nd ed. The Geological Society of America: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[H62] Howell, B. F. 1962. Worms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W144–W177. Geological Society of America, and University of Kansas Press.

[KBR09] Kühl, G., D. E. G. Briggs & J. Rust. 2009. A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsrück Slate, Germany. Science 323: 771–773.

[LS02] Lange, S., & F. R. Schram. 2002. Possible lattice organs in Cretaceous Thylacocephala. Contributions to Zoology 71 (4): 159–169.

[LSE13] Legg, D. A., M. D. Sutton & G. D. Edgecombe. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4: 2485.

[N02] Newman, M. J. 2002. A new naked jawless vertebrate from the Middle Devonian of Scotland. Palaeontology 45 (5): 933–941.

[VS14] Vinther, J., M. Stein, N. R. Longrich & D. A. T. Harper. 2014. A suspension-feeding anomalocarid from the Early Cambrian. Nature 507: 496–499.

[WD02] Waloszek, D., & J. A. Dunlop. 2002. A larval sea spider (Arthropoda: Pycnogonida) from the Upper Cambrian ‘Orsten’ of Sweden, and the phylogenetic position of pycnogonids. Palaeontology 45 (3): 421-446.

[WB85] Whittington, H. B., & D. E. G. Briggs. 1985. The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London 309: 569–609.

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