Despite looking more like some sort of scaly tubeworm, Rhenopyrgus viviani here was actually an echinoderm, distantly related to modern starfish, brittle stars, sea urchins, crinoids, and sea cucumbers.
It was part of an extinct Paleozoic echinoderm lineage known as edrioasteroids, which lived attached to the seabed or on hard surfaces like the shells of other marine animals, using the tube feet on their five arms to catch food particles from the water around them.
Living during the Silurian, about 435 million years ago, in what is now Quebec, Canada, it stood around 3-4cm tall (1.2-1.6″), firmly anchored into the seafloor sediment by a bulbous sac-shaped base. Its long stalked body was somewhat flexible, and it was able to partially contract the top feeding region down under a “collar” of large scale-like armor plates.
As we come to the end of the month, here’s another #Spectember concept from an anonymous submission:
Continue reading “Spectember 2021 – Dimorphic Tunicates”
Nesonektris aldridgei here was one of the bizarre vetulicolians, a group of Cambrian animals that lived between about 520 and 505 million years ago.
Known from the Emu Bay Shale fossil deposits in Kangaroo Island, South Australia (~514 million years ago), Nesonektris was one of the larger known vetulicolians, growing to at least 17cm long (~6.5″). Like most of its relatives it had a large streamlined forebody with a mouth opening at the front, and no obvious appendages or sensory structures. A groove down each side may have housed gill openings, and a segmented flexible tail provided propulsion for swimming.
Very little is known about the ecology of these animals. They were clearly adapted for active swimming in the water column, and may have filter-fed on plankton – but some other vetulicolians have been found preserved with their guts full of seafloor sediment, suggesting some sort of detritivorous lifestyle instead.
Their evolutionary relationships are also still uncertain, but preservation of what appears to be a notochord in Nesonektris suggests that vetulicolians may have been part of the chordate lineage, possibly close relatives of tunicates.
Vetulicolians were a group of odd Cambrian animals known from between about 520 and 505 million years ago. The front half of their bodies were large and streamlined, with a prominent mouth, no eyes, and five pairs of openings that seem to have been gills, with some species having a rigid exoskeleton-like carapace. Their back half was slender, segmented, and flexible, and functioned as a tail for swimming, giving them an overall appearance like alien tadpoles.
Their evolutionary affinities have been problematic for a long time, but evidence of a notochord in some specimens suggest they were probably related to the chordates in some way. Sometimes they’re considered to represent their own phylum, but they might also be stem-chordates or stem-tunicates.
Continue reading “Cambrian Explosion Month #17: Phylum(?) Vetulicolia & Other Early Deuterostome Weirdos”
Vertebrates are by far the most numerous and diverse group of chordates today, with over 65,000 known species including fish, amphibians, reptiles, birds, and mammals. Genetic studies show that they’re closely related to the weird bag-like tunicates, and their shared common ancestor was probably something lancelet-like.
And the earliest true vertebrates would have looked something like Haikouichthys ercaicunensis.
Continue reading “Cambrian Explosion Month #16: Phylum Chordata – Vertebrata”
Chordates are one of the most diverse animal phyla, ranging from tiny lancelets to sac-like tunicates to all fish and tetrapods. They share a common deuterostome ancestor with echinoderms and hemichordates, probably diverging from them sometime in the Ediacaran Period, and are characterized by having specific anatomical features at some point during their life cycle – a notochord, a dorsal nerve cord, pharyngeal slits, a post-anal tail, and an endostyle.
The earliest chordates were all small soft-bodied animals with no mineralized tissues, so their fossil record is poor aside from rare locations with exceptional preservation. But one of the best known examples is Pikaia gracilens from the Canadian Burgess Shale fossil deposits (~508 million years ago).
Continue reading “Cambrian Explosion Month #15: Phylum Chordata – Early Forms & Tunicates”
Modern hemichordates and echinoderms are the closest living relatives of each other, part of a larger lineage of deuterostome animals known as ambulacrarians – but they also seem to have had some other strange cousins during the Cambrian.
Cambroernids were a bizarre group with branching feeding tentacles and a gut enclosed in a coiled sac. They came in a range of forms from worm-like to cup-like to disc-shaped, and despite their fossils being known since the early 1900s their evolutionary affinities were a longstanding problem. Various species had been interpreted in the past as sea cucumbers, jellyfish, tunicates, gnathiferans, or lophophorates, but in recent years they’ve been recognized as all being related, and linked to the ambulacrarians.
And it’s still not entirely clear where in that group they actually belong. They were probably a weird early stem lineage, but they might also be early stem-hemichordates or stem-echinoderms.
Continue reading “Cambrian Explosion Month #14: Phylum(?) Cambroernida”
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.
Continue reading “Cambrian Explosion Month #13: Phylum Echinodermata – Sticking Around”
While many of the earliest echinoderms had bizarre asymmetrical forms, at some point members of their lineage adopted radial symmetry instead – a development that would eventually lead to the familiar five-way symmetry of most modern species.
And they may have transitioned to that via three-way symmetry.
Continue reading “Cambrian Explosion Month #12: Phylum Echinodermata – Radial Revolution”
During their early evolution, echinoderms started developing unusual asymmetric body plans – and some of them were so strange-looking that for a while it wasn’t clear if they even were echinoderms.
Continue reading “Cambrian Explosion Month #11: Phylum Echinodermata – Increasing Asymmetry”