#hyolith

My 25 years of palaeoart chronology...

Cambrian Cuties

Apologies for the delays! This was meant to be a December post, but it got pushed to early January, so Happy New Year! Cave-Clicker (artwork by Yuujinner)

Kingdom: Animalia

Phylum: incertae sedis

Class: Hyolitha

Order: Anomalohyolithida

Family: Anomalohyolithidae

Genus: Anomalohyolithes

Species: A. troglodytus (“cave-dwelling unusual hyoid stone”)

Temporal range: unknown to recent (??? - present)

Information:

Relatively new to science, very little is known about this roughly cat-sized creature. The one thing which scientists know definitively is that it is a member of the hyolith clade, albeit one so heavily derived from the ancestral body plan, that it is hardly recognizable as one. 

Typically covered in striated black and gray bands with a fleshy pink head, these creatures are seemingly adept at navigating through the canopies of giant fungi found deep below in the caverns, swinging and hopping to get across the difficult terrain. Their diet is not fully understood, but tissue sampling would suggest that they consume small animals in addition to the fungi found in their habitat, possibly filling a generalist omnivore niche, and observed behavior would indicate that they frequently get into scuffles over food, the horn-like projection on their “face” being used to jab at other individuals. Exceptionally social for a non-arthropod invertebrate, these creatures sometimes gather in groups of up to 50 individuals. Its social nature can be dually inferred by its broad set of vocalizations: chittering, squeaking, and clicking (the latter of which gave the creature its name) have all been recorded, though the actual functions of these calls remains unknown. The total population size is unknown, and based on any number of estimates given for the size of the cave system it inhabits, it could be anywhere from simply a few thousand to over half a million. It appears this animal is capable of seeing light in the UV and infrared spectrums, as wild specimens have reacted to such lights in the presence of researchers, whom they appear to be naturally inquisitive towards. 

Their reproductive biology and life cycle is unknown, though as they are inferred to be poikilothermic, it is possible that they may live for several decades, maturing at a gruelingly slow pace and mating once or twice throughout their lifetime. Likewise, they are believed to be oviparous, though as no eggs have been found, much less even female specimens (it is known that they have separate sexes, but every specimen captured thus far has been male), there is no way to definitively prove this. The means by which these animals do sexually reproduce also leaves open more questions than it answers: there is no obvious orifice through which they could theoretically deposit sperm, and considering that this creature descends from an aquatic clade, it cannot be ruled out that they do not return to subterranean lakes and rivers to spawn. To further compound this issue, no young of this species have ever been identified, suggesting that they may have simply not been found yet, are indistinguishable from the adult form, or (most likely) that they look so radically different from the adult form, that they may be perceived as a different species entirely. Unfortunately, this is difficult to study in a laboratory setting, in part due to the fact that all captured specimens have been male and in part due to the fact that only 3 specimens have ever been brought to the surface, and within 2 days of being brought the surface, all 3 of them died of unknown causes.

Cambrian Explosion Month #30: Phylum(?) Hyolitha

Hyoliths were a group of small shelled animals that first appeared in the fossil record just after the start of the Cambrian, about 536 million years ago. They had conical calcareous shells with a lid-like operculum, and some species also featured long curling spines that made them look like ice-cream cones with mammoth tusks.

They were so odd that for a long time their evolutionary relationships were unknown. They were generally accepted to be lophotrochozoans, but some studies considered them to be part of their own unique phylum while others tended to place them as being closely related to molluscs.

It wasn't until 2017 that well-preserved soft tissue fossils revealed a tentacled feeding structure that resembled a lophophore – and hyoliths finally found their place in the lophotrochozoan family tree as close relatives of brachiopods and horseshoe worms, possibly even being a stem lineage within the brachiopod phylum.

However, this isn't universally accepted and some recent studies continue to dispute it. The feeding organ of a different hyolith fossil has been interpreted as not being a lophophore, classifying the group as an early lophotrochozoan stem lineage, while an analysis of shell microstructure has instead suggested realigning them with molluscs. I'm grouping them with brachiopods here, but future discoveries might still make this obsolete.

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Lingulosacculus nuda might represent another possible link between brachiopods and hyoliths.

Discovered in the Mural Formation in Alberta, Canada (~524-522 million years ago), it was about 4cm long (1.6") and had a long conical shell that was either poorly-mineralized or completely unmineralized.

Its position within brachiozoan evolution is uncertain. It was originally proposed as a stem-phoronid, but other analyses place it as a stem-brachiopod, an early linguliform brachiopod related to lingulellotretids, or possibly a stem-hyolith.

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The earliest true hyoliths were the conical orthothecids, which mostly lived resting on top of the seafloor feeding on organic detritus around themselves. A few have also been found vertically oriented in the sediment, suggesting they may have been filter-feeders collecting particles from surrounding water currents.

Later members of the lineage, the hyolithids, developed distinctive long tusk-like spines on each side of their opercula. These moveable structures have been termed "helens" and were probably used as stilts, holding up the front end of the animal slightly above the seafloor.

There's also some evidence that helens gave hyoliths the ability to move themselves around, albeit probably rather awkwardly. They may have dug out shallow scrape-like "burrows" in the surface of the seafloor to shelter from stronger currents that could overturn them, and clusters of individuals found around carcasses of larger animals indicate they might have been opportunistic scavengers.

Haplophrentis reesei was a typical hyolithid, known from Utah and Idaho, USA, about 509-504 million years ago. A closely related species, Haplophrentis carinatus, is also known from Canada (~508 million years ago) and southwest China (~516-513 million years ago).

Up to about 6cm long (2'4"), this hyolith was one of the first found to preserve soft tissue feeding organs, showing up to 16 tentacles in a lophophore-like arrangement.

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Whatever they actually were, the hyoliths were highly successful animals for a time, found abundantly worldwide for most of the Cambrian. They later began to decline, but still hung on throughout the rest of the Paleozoic and only finally went completely extinct during the catastrophic "Great Dying" mass extinction at the end of the Permian, about 252 million years ago.

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Cambrian Explosion Month #13: Phylum Echinodermata – Sticking Around

It seems like echinoderms became five-way symmetric incredibly quickly following the group's first appearances in the early Cambrian. We don't really know why this secondary radial symmetry evolved in the group – but we do know that the common ancestors of all modern pentaradial echinoderms were suspension-feeding animals that lived attached to the sea floor.

And those ancestors were probably a group called the edrioasteroids.

Edrioasteroids looked like discs or pillows with a starfish stuck on the top (although a few later forms resembled turrets), and they lived permanently attached to either the seafloor sediment or to hard surfaces like rocks and shells. Their five arms could grow straight, curved, or spiralling over the top surface of their bodies, and they captured food particles from the water using tube feet.

They first appeared in the fossil record at around the same time as helicoplacoids, about 525 million years ago, and were either ancestral to all modern echinoderms or to everything except crinoids (depending on whether eocrinoids and blastozoans were related to true crinoids or not).

Camptostroma roddyi was an unusual early edrioasteroid that resembled a cupcake, known from Pennsylvania, USA, around 516-513 million years ago. Growing up to about 6cm in diameter (2.4"), its wide thick base helped it suction onto soft sediment and algal mats.

Most other early edrioasteroids were also soft-surface suction-stickers, but later in the Cambrian microbial mats became less common and members of the group began to specialize for attaching to hard surfaces instead.

Totiglobus spencensis was one of the first species to use hard surfaces, living in Idaho, USA, about 509-504 million years ago. The only known specimen was about 2cm in diameter (0.8"), and was found attached onto a hyolith shell – possibly with the aid of a bioglue – with its position and the completeness of the hyolith suggesting it had been growing on a living host until they both got buried in sediment and preserved.

A very closely related species, Totiglobus nimius, was still a soft-sediment-sticker, and the two had differently shaped basal regions of their body – T. nimius had a wide flat attachment surface, while T. spencensis had a conical tapering base that ended in a smaller attachment disc.

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While edrioasteroids were never particularly common echinoderms, they were still widespread around the world and surprisingly long-lived as a group – they actually continued on through most of the Paleozoic, with their last known fossils coming from the Mid-Permian about 270 million years ago.

And, of course, if early edrioasteroids were the ancestors of modern echinoderms then their descendants are still going strong today. Even now, half a billion years later, some modern echinoderm larvae still may show some traces of that ancestry, attaching themselves onto the seafloor before metamorphosing.

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Hyoliths (Class Hyolitha)

Hyoliths were a group of of shelled animals that lived during the Lower Cambrian and Upper Permian era. Their Phylogenetic position is not yet confirmed as it is unclear what they were most closely related to, some say the were molluscs and others say they constitute their own phylum. Most hyoliths had a large calcareous shell, an operculum and two support structures which were called helens. Hyoliths were probably bottom-dwellers like modern gastropods.

Phylogeny

Animalia-Mollusca?-Hyolitha