Ecdysozoa

Etymology

The name Ecdysozoa is scientific Greek, derived from ἔκδυσις () "shedding" + ζῷον () "animal".

Characteristics

The most notable characteristic shared by ecdysozoans is a three-layered cuticle (four in Tardigrada) composed of organic material, which is periodically molted as the animal grows. This process of molting is called ecdysis, and gives the group its name. The ecdysozoans lack locomotory cilia and produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other protostomes. Ancestrally, the group exhibited sclerotized teeth within the foregut, and a ring of spines around the mouth opening, though these features have been secondarily lost in certain groups. An unpaired ventral nerve cord, present in Priapulida and Nematoida, appear to be the ancestral condition, making the paired ventral nerve cord found in Panarthropoda, Kinorhyncha and Loricifera a derived trait. A respiratory and circulatory system is only present in onychophorans and arthropods (often absent in smaller arthropods like mites); in the rest of the groups, both systems are missing. Ecdysozoans rely exclusively on the arginine phosphate/arginine kinase (AP/AK) system, a high-energy phosphate system used to regenerate ATP from ADP. In contrast, vertebrates use only the creatine phosphate/creatine kinase (CP/CK) system, while some other invertebrates may employ both systems.

Phylogeny

The Ecdysozoa include the following phyla: Arthropoda, Onychophora, Tardigrada, Kinorhyncha, Priapulida, Loricifera, Nematoda, and Nematomorpha. A few extinct taxa have been classified as stem group ecdysozoans, such as Uncus dzaugisi and Acosmia. Other groups such as the gastrotrichs, have been considered possible members but lack the main characters of the group, and are now placed elsewhere. The Arthropoda, Onychophora, and Tardigrada have been grouped together as the Panarthropoda because they are distinguished by segmented body plans. Dunn et al. in 2008 suggested that the tardigrada could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods. The non-panarthropod members of Ecdysozoa have been grouped as Cycloneuralia but they are more usually considered paraphyletic in representing the primitive condition from which the Panarthropoda evolved.

A modern consensus phylogenetic tree for the protostomes is shown below. It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya); dashed lines show especially uncertain placements.

The phylogenetic tree is based on Nielsen et al. and Howard et al..

Older alternative groupings

Articulata hypothesis

The grouping proposed by Aguinaldo et al. is almost universally accepted, replacing an older hypothesis that Panarthropoda should be classified with Annelida in a group called the Articulata, and that Ecdysozoa are polyphyletic. Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group of Annelida, though later considered them unrelated. Inclusion of the roundworms within the Ecdysozoa was initially contested but since 2003, a broad consensus has formed supporting the Ecdysozoa and in 2011 the Darwin–Wallace Medal was awarded to James Lake for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia.

Coelomata hypothesis

Before Aguinaldo's Ecdysozoa proposal, one of the prevailing theories for the evolution of the bilateral animals was based on the morphology of their body cavities. There were three types, or grades of organization: the Acoelomata (no coelom), the Pseudocoelomata (partial coelom), and the Eucoelomata (true coelom). Adoutte and coworkers were among the first to strongly support the Ecdysozoa. With the introduction of molecular phylogenetics, the coelomate hypothesis was abandoned, although some molecular, phylogenetic support for the Coelomata continued until as late as 2005.

References

References

1. (November 2024). "An Ediacaran bilaterian with an ecdysozoan affinity from South Australia". Current Biology.
2. (October 2020). "New macrobenthic cycloneuralians from the Fortunian (lowermost Cambrian) of South China". Precambrian Research.
3. (31 December 2022). "Careful amendment of morphological data sets improves phylogenetic frameworks: re-evaluating placement of the fossil Amiskwia sagittiformis". Journal of Systematic Palaeontology.
4. (May 2019). "New armoured scalidophorans (Ecdysozoa, Cycloneuralia) from the Cambrian Fortunian Zhangjiagou Lagerstätte, South China". Papers in Palaeontology.
5. (September 2022). "The evolutionary relationships of the earliest known cycloneuralians and a new record from the Cambrian Fortunian of South China". Palaeoworld.
6. (October 2020). "A new scalidophoran animal from the Cambrian Fortunian Stage of South China and its implications for the origin and early evolution of Kinorhyncha". Precambrian Research.
7. (2022). "The Ediacaran origin of ecdysozoa: Integrating fossil and phylogenomic data". [[Journal of the Geological Society]].
8. (2020-11-23). "Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan". [[BMC Evolutionary Biology]].
9. (March 2025). "Origin and evolution of bodyplans of ecdysozoans during the Cambrian explosion". Chinese Journal of Nature.
10. (2022-08-17). "Saccorhytus is an early ecdysozoan, and not the earliest deuterostome". [[Nature (journal).
11. (2008). "The evolution of the Ecdysozoa". Philosophical Transactions of the Royal Society B: Biological Sciences.
12. (1992). "Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralian metazoan morphology". [[Systematic Biology (journal).
13. (29 May 1997). "Evidence for a clade of nematodes, arthropods, and other moulting animals". [[Nature (journal).
14. (10 April 2008). "Broad phylogenomic sampling improves resolution of the animal tree of life". [[Nature (journal).
15. Ax, P.. (1985). "The Origins and Relationships of Lower Invertebrates". Clarendon / Oxford University Press.
16. Nielsen, Claus. (1995). "Animal Evolution: Interrelationships of the living phyla". Oxford University Press.
17. (8 April 2002). "The evolutionary position of nematodes". [[BMC Evolutionary Biology]].
18. (December 1999). "The Ecdysozoa: Artifact or monophylum?". [[Journal of Zoological Systematics and Evolutionary Research]].
19. Barnes, Robert D.. (1982). "Invertebrate Zoology". Holt-Saunders International.
20. (2 July 2015). "Hallucigenia's head and the pharyngeal armature of early ecdysozoans". [[Nature (journal).
21. (2016). "The mouth apparatus of the Cambrian gilled lobopodian ''Pambdelurion whittingtoni''". Palaeontology.
22. (2025). "Preservation and early evolution of scalidophoran ventral nerve cord". Science Advances.
23. (2011). "ATP distribution and localization of mitochondria in ''Suberites domuncula'' (Olivi 1792) tissue". Journal of Experimental Biology.
24. (2006). "The capacity for the de novo biosynthesis of creatine is present in the tunicate Ciona intestinalis and is likely widespread in other protochordate and invertebrate groups". Comparative Biochemistry and Physiology Part D: Genomics and Proteomics.
25. "Panarthropoda".
26. (November 2006). "Mitogenomics and phylogenomics reveal priapulid worms as extant models of the ancestral Ecdysozoan". [[Evolution & Development]].
27. (June 2011). "Higher-level metazoan relationships: Recent progress and remaining questions". [[Organisms, Diversity & Evolution]].
28. (2017-04-04). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". [[Nature Communications]].
29. (2014). "Hallucigenia's onychophoran-like claws and the case for Tactopoda". [[Nature (journal).
30. "Palaeos Metazoa: Ecdysozoa".
31. (June 2015). "Phylogenetic position of Loricifera inferred from nearly complete 18S and 28S rRNA gene sequences". [[Zoological Letters]].
32. Nielsen, C.. (2002). "Animal Evolution: Interrelationships of the Living Phyla". Oxford University Press.
33. (2008-04-27). "The Ediacaran emergence of bilaterians: Congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences.
34. (2018-08-22). "Evolution of the bilaterian mouth and anus". [[Nature Ecology & Evolution]].
35. (2022-03-10). "The Ediacaran origin of Ecdysozoa: Integrating fossil and phylogenomic data". [[Journal of the Geological Society]].
36. (September 2003). "Proposing a solution to the Articulata–Ecdysozoa controversy". [[Zoologica Scripta]].
37. Nielsen, Claus. (2012). "Animal Evolution: Interrelationships of the living phyla". Oxford University Press.
38. Zrzavý, J.. (12 January 2002). "Ecdysozoa versus Articulata: Clades, artifacts, prejudices". [[Journal of Zoological Systematics and Evolutionary Research]].
39. (September 2001). "On quality of evidence in phylogeny reconstruction: A reply to Zrzavý's defence of the 'Ecdysozoa' hypothesis". [[Journal of Zoological Systematics and Evolutionary Research]].
40. (27 April 2008). "The evolution of the animals: Introduction to a Linnean tercentenary celebration". Philosophical Transactions of the Royal Society B: Biological Sciences.
41. "The Darwin-Wallace Medal". The Linnean Society of London.
42. (25 April 2000). "The new animal phylogeny: Reliability and implications". [[Proceedings of the National Academy of Sciences of the USA]].
43. (9 February 2005). "The Opisthokonta and the Ecdysozoa may not be clades: Stronger support for the grouping of plant and animal than for animal and fungi, and stronger support for the Coelomata than Ecdysozoa". Molecular Biology and Evolution.