invertebrate – Nix Illustration

Mosura

Mosura fentoni was a small radiodont living during the mid-Cambrian, about 508 million years ago, in near-equatorial shallow marine waters covering what is now western Canada.

Sixty specimens have been discovered in the Burgess Shale fossil deposits, ranging from 1.5cm long juveniles (~0.6″) to 6cm long adults (~2.4″), giving us a detailed look at Mosura’s anatomy and life history. It had three eyes – two on the sides of its head on short stalks and one in the middle of its face – and a pair of grasping frontal appendages each with six long sickle-shaped spines.

Unusually for a radiodont its body was divided into distinct regions: a four-segment neck, a six-segment mesotrunk with large swimming flaps, and an abdomen-like posterotrunk with up to at least sixteen segments (fewer in juveniles), all bearing gills along their undersides.

Its vaguely moth-like shape led to it being nicknamed “sea-moth” by field collectors, and inspired its genus name – “Mosura” is the Japanese name of the fictional giant kaiju moth-monster Mothra.

With a very high proportion of respiratory surface area for its size, Mosura was probably an active and agile fast-swimming predator, possibly living in low-oxygen waters around the outer continental shelf. Its wide oval central eye may have helped it stay orientated during rapid maneuvers, keeping track of the horizon line similar to the median eyes of modern dragonflies.

Shishania

Shishania aculeata lived during the mid-Cambrian, around 512 million years ago, in shallow tropical waters covering what is now southwestern China.

Up to about 6cm in length (~2.4″), its spine-covered body was initially thought to be an early mollusc, but the discovery of more specimens has resulted in a new interpretation: instead of a slug-like creature, the fossils of Shishania might instead represent a flattened and ruptured chancelloriid.

Chancelloriids were an enigmatic group of Cambrian animals that had hollow bag-like bodies armored with numerous sharp star-shaped spines. They were historically classified as sponges due to their similar body plan and immobile filter-feeding lifestyle, and they’ve also been proposed to be relatives of halkieriid molluscs due to similarities in the microscopic structure of their spines – but currently it seems most likely that chancelloriids were actually their own separate lineage of early animals, closer related to eumetazoans than to sponges.

Shishania had much simpler spines than other chancelloriids, so it may represent an early stage of the evolution of these animals’ armor, showing that these structures were developed from scratch rather than adapted from a pre-existing ancestral feature.

Solenochilus

The distinctive pinhole eyes, leathery hood, and numerous tentacles of modern nautiluses were traditionally thought to represent the “primitive” ancestral state of early shelled cephalopods – but genetic studies have found that that nautiluses actually secondarily lost the genes for building lensed eyes, and their embryological development shows the initial formation of ten arm buds (similar to those of coeloids) with their hood appearing to be created via fusing some of the many tentacles that form later.

There’s a Cretaceous nautilidan fossil that preserves soft tissue impressions of what appear to be pinhole eyes and possibly a remnant of a hood, so we know these modern-style nautilus features were well-established by the late Mesozoic. But for much more ancient Paleozoic members of the lineage… we can potentially get more speculative.

So, here’s an example reconstructed with un-nautilus-like soft parts.

Solenochilus springeri was a nautilidan that lived during the Late Carboniferous, around 320 million years ago, in shallow tropical marine waters covering what is now Arkansas, USA.

Up to about 20cm in diameter, (~8″), its shell featured long sideways spines which may have served as a defense against predators – or possibly as a display feature since they only developed upon reaching maturity.

Rhyniella

Rhyniella praecursor was an early springtail that lived during the early Devonian, about 410-400 million years ago, in what is now Scotland. Discovered in the exceptionally well-preserved Rhynie chert fossil site, it’s one of the earliest known hexapods.

It was around 2mm long (~0.08″) and closely resembled some of its modern relatives – with distinctive anatomical features like a collophore and a furca – showing that springtails were already well-established in such an early terrestrial ecosystem.

It probably had a similar sort of ecological role to modern springtails, too, being involved in the breaking down of organic matter and the formation of soils.

Greenwaltarachne

Greenwaltarachne pamelae was an orb-weaver spider that lived in what is now Montana, USA, during the mid-Eocene, around 46 million years ago.

Known from a single fossil of an adult female, it had a body length of about 2mm (~0.08″) and a legspan of around twice that. The specimen is even well-preserved enough to show banded markings on the legs resembling those of some modern orb-weaver species.

It would have lived in what was then a rift valley with a tropical climate, along the shoreline of the ancient 160km long (~100 miles) Lake Kishenehn. It was part of a highly diverse ecosystem full of numerous other invertebrates – including miniscule fairyflies, and even mosquitoes with evidence of blood preserved inside their bodies – and a wide variety of mammals ranging from tiny rodents to large brontotheres.

Tardisia

Tardisia broedeae was a small arthropod recently discovered in the 308-million-year-old Late Carboniferous Mazon Creek fossil beds in Illinois, USA.

Its anatomy has distinctive features of vicissicaudatans (close relatives of the trilobites), but its presence in Mazon Creek makes it by far the youngest known member of this group – indicating a previously-unknown ghost lineage of around 100 million years, and inspiring its name based on the time-travelling TARDIS in Doctor Who.

About 1.5cm long (~0.6″), Tardisia had an oval segmented body ending in a pair of pointed “tail” appendages. It also appears to have been eyeless, although some vicissicaudatans had eyes on their undersides so this might just be an artifact of preservation.

Falcatacaris

The enigmatic thylacocephalans were a group of bizarre little arthropods, found in marine deposits all over the world from the late Ordovician (~435 million years ago) to the late Cretaceous (~85 million years ago). They had shield-like bivalved carapaces, large compound eyes, three pairs of spiny grasping limbs, and multiple pairs of small paddle-like swimming limbs, but details of their internal anatomy are poorly known and their evolutionary relationships to other arthropods are still very uncertain.

Traditionally they’ve been classified as crustaceans, possibly as close relatives of remipedes or malacostracans – but they’ve also recently been proposed as instead being part of a much more ancient branch of arthropods, potentially related to stem–mandibulates like Acheronauta.

Falcatacaris bastelbergeri was a thylacocephalan living during the late Jurassic, about 150 million years ago, in what is now Germany. Around 2.5cm long (~1″), its carapace had tiny interlocking square “teeth” resembling a zipper along the hinge line between the two valves, a ridge along each side, and a long pointed knife-shaped spine at the front.

Like other thylacocephalans it was probably a swimming predator, likely nocturnal or hunting in murky conditions based on its enlarged eyes, and would have captured smaller aquatic prey using its raptorial limbs.

Tachypleus syriacus

Tachypleus syriacus was a horseshoe crab from the late Cretaceous (~100-95 million years ago) of what is now Lebanon.

Closely related to modern tri-spine horseshoe crabs, it displayed a similar level of sexual dimorphism. Females grew to at least 25cm long (~10″), with rounded front edges to their carapaces and shorter rear spines, while males were around 30% smaller with a scalloped shape to the front of their carapaces.

One recently described female specimen also preserves distinctive nodules around the rim of its carapace, which may represent some sort of sensory structure.

This particular specimen is also unique for preserving a coprolite in the process of being expelled from the horseshoe crab’s body – that’s right, it died while pooping.

Hexameroceras

Hexameroceras panderi was a nautiloid cephalopod that lived during the late Silurian, about 425-423 million years ago, in what is now Czechia.

Around 5cm long (2″), it had a downwards-curving egg-shaped shell that preserved the original color pattern on one fossil specimen, showing closely-packed crisscrossing vertical and horizontal bands.

Like several of its close oncocerid and discosorid relatives, its shell also developed a highly constricted opening as it reached maturity. This eventually formed into a narrow visor-like shape with several lobes that probably correlated to the life positions of the eyes and arms, with a “spout” at the bottom for the siphon.

Diagram showing how the lobed "visor" formed in Octameroceras — Development of the “visor” in the related *Octameroceras sinuosum*
From fig 6 in Stridsberg (1981)

The function of this structure is still unclear. It may have been a defensive measure against predators – but it would have also severely limited the range of motion of the arms and the size of food that could be eaten through the mouth, suggesting that Hexameroceras may have specialized in very small prey, perhaps even filter-feeding.

Another possibility is that these visored nautiloids might represent brooding females, walling themselves into their shells to protect their eggs and dying after releasing the hatchlings through the tiny remaining gap.

Panacanthocaris

Panacanthocaris ketmenica* here was a member of an extinct group of crustaceans known as kazacharthrans – close relatives of modern tadpole shrimp known mainly from Central Asia during the mid-to-late Triassic (but with possible German relatives from both the late Triassic and further back in the late Paleozoic).

Fossils of Panacanthocaris have been found in Kazakhstan and northwest China, dating to about 235-221 million years ago. It was fairly big compared to most of its modern cousins, reaching at least 10cm in length (~4″), and had distinctive spines around the edges of its carapace and its telson.

It’s not clear if it had eyes – there’s a single opening near the front of its carapace that may have housed some, and so I’ve depicted it here with just one naupliar eye similar to the “third eye” of tadpole shrimp.

It probably had a fairly similar lifestyle to its modern relatives, living in shallow freshwater and temporary pools and opportunistically feeding on everything from algae to smaller aquatic animals.

(* Sometimes also called P. ketmenia. May also be the same thing as Iliella spinosa, but until that paper is officially published the current name still stands.)