Echidnacaris

Before formal naming or identification, this organism was known as the following on this wiki:
**Emu Bay tamisiocaridid**

## Statistics

### Length

33.2-49 centimeters (undersized?; based on *Anomalocaris*)[*citation needed*]

### Diet

Planktivorous

*Echidnacaris* is an extinct genus of tamnisiocaridid radiodont that lived in Australia during the Lower/Middle Cambrian.

## History

Nedin (1995) first named material from the Emu Bay Shale as *Anomalocaris briggsi*. At this time, the tamisiocaridids, filter-feeding radiodonts, remained undiscovered and the genus *Anomalocaris* had been used as a wastebasket for various unidentified and vague radiodont remains, which spawned many false or wrongfully-placed species. However, this designation has been called into question since description, as many suspected the unique morphology of the appendages. This was confirmed upon the discovery and study of *Tamisiocaris*, where phylogenetic analyses cemented the species as firmly nestled into Tamisiocarididae as a sister to the latter genus. The specific name, *briggsi*, is after Derek Briggs, a major radiodont researcher[*citation needed*].

## Description

The first article has a single pair of ventral spines near to the distal margin, who are stout and facing backwards and the ventral spines curve to the posterior; these traits are shared with *Tamisiocaris*. A specimen from Emu Bay Shale described in 2020 (but announced in 2011) were so well preserved, the eye structure was intact. This found that this taxon has about 13,000 lenses, which equates to excellent eyesight and the judgement that eyesight was likely important in the Emu Bay Shale, and cones that may have exceeded 335 μm. The large lenses were acute in perceiving the dim light in larger depths. Additionally, the eyes grew similar to modern arthropods: along the edge of the eyes so that they grew as the animal did. It was hypothesized that these eyes made obtaining prey easier, and that the evolution of these structures may have triggered an "evolutionary arms race" between it and it's prey. The 2020 description states that *Echidnacaris* had eyes adapted for sensing plankton, further confirming that it is not a member of *Anomalocaris*. The study also found that it bore sclerites atop the eyes, with the eyes being attached to the body rather than on stalks, was sessile and has a cuticular marginal rim. About 68 frontal appendage specimens of this taxon are known. The lens diameters of *Echidnacaris* are beaten by several marine euarthropods. The frontal appendages are 87-175 millimeters (3.4-6.9 inches) with 14 segments and podomeres; the first few being much longer than they are wide and 2-12 bearing-long podomeres with endites/ventral spines that are slender and posteriorly-curving. The endites/ventral spines have many small auxiliary spines, and this frontal structure was capable of suspension feeding. An oral cone is also described, triradial with small tubercle studding on each of the three plates.

## Classification

*Echidnacaris* was once referred to Anomalocarididae as *"Anomalocaris" briggsi*, but was later recovered as the sister taxon to *Tamisiocaris* in a family dubbed "Cetiocaridae" (after an artwork by John Meszaros that depicted a hypothetical filter-feeding radiodont for the book All Your Yesterdays). However, since the speculative taxon, "Ceticaris", is fictitious, it may not be used as a valid name. Instead, Tamisiocarididae was used to fit the Emu Bay Shale taxon and *Tamisiocaris*:

## Paleoecology

*Echidnacaris* may have made vertical migrations daily to retain constant ambient light levels at different times of the day, which may have also coincided with feeding schedules. It lived with *Anomalocaris* *daleyae*, which unlike it, was a predatory anomalocaridid. Emu Bay Shale dates to Cambrian Stage 4, which is also known as the Lower Cambrian. Original remains were found in a trilobite bed alongside trilobites, brachiopods, emucaridids, worms and problematic taxa *Myoscolex* and *Vetustovermis,* and others. Parts of these waters may have been shallow and tropical[*citation needed*].

## References

1. ↑ 1.0 1.1 https://en.wikipedia.org/wiki/Emu\_Bay\_Shale
2. ↑ 2.0 2.1 https://en.wikipedia.org/wiki/Echidnacaris
3. ↑ https://en.wikipedia.org/wiki/Anomalocaris
4. ↑ https://www.science.org/doi/10.1126/sciadv.abc6721
5. ↑ https://english.netmassimo.com/2020/12/09/anomalocaris-briggsi-had-excellent-eyesight-over-500-million-years-ago/
6. ↑ https://dro.dur.ac.uk/21270/1/21270.pdf