Adam's Cambrian

[Tidgy's Dad]

I have a third specimen from the same location from the Chengjiang Biota! 

Thanks once more to the marvelous Salavador, @Paleorunner for this super surprise gift. 

This is a ventral valve of Heliomedusa orienta, once thought to be a jellyfish, hence the name, but now, with many new specimens, known to be a mickwitziid, possibly even a junior synonym of Mickwitzia itself. Mickwitziids are found only in the Early Cambrian, or possibly in the Middle Cambrian but not after that, seemingly a part of the Cambrian explosion that were only successful in the short term. They have a unique, phosphatic shell structure, may not have been able to fully close their valves and had no pedicle, so they have been classed as basal craniiforms, but are now seen by many as probably basal to the Liguliformea. There exact position therein is still uncertain, but they are pretty likely to be somewhere amongst the stem group Brachiopoda, so this is a new and important specimen for me and another one ticked off my wish list. 

The specimen is 2.3 cm wide. Notice the beak (pale yellow) near the top of the photo below and the visceral cavity (darker brownish) between the beak and the centre of the  fossil.

Posterior detail : 

Thanks a zillion, Salvador, it's beautiful. 

 

 

[Tidgy's Dad]

The Brachiopoda used to be split into two groups; the Inarticulata and the Articulata. 

Then some boffin noticed that in the inarticulates, there were two distinct groups, so the subphyla Linguliformea and Craniiformea were established, and this meant that the articulates were renamed the Rhynchonelliformea, as Articulata no longer had an Inarticulata to be differentiated from. The Lingulifmea have been divided into the two classes; Paterinata and Lingulata, but the former was an ancestral Cambrian group that didn't survive the Ordovician. 

The Lingulata are split into three orders; the Lingulida, the most successful group ( Cam - Rec), the minor taxon Acrotretida (Cam- Dev) and the smallest group of all, the Siphonotretida which were originally included in the Acrotretida but have now been placed in their own order. 

The Siphonotretida were thought to have originated in the Late Middle Cambrian and not survived the Ordovician extinction, but recent finds have shown they existed with described species until the Middle Silurian and there seem to have been fragments found in the Early Devonian. Some have claimed Permian bits, but I'm doubtful thus far. 

Anyway, the siphotretids are the fisrt spinose brachiopods and this is Schizambon typicalis, Schizambon being the earliest known genus of the order, not with the hollow spines of most of the species that came later, but extended pustules on the shell that later evolved into the hollow spines. 

This is my first example of this order, so an extra-special thank you to the Miraculous @Misha for sending me this wonderful specimen. 

I tidied it up a bit ; 

This is the ventral valve, though part of the dorsal valve is exposed at the top of the photo, the valves becoming disarticulated after death, which is hardly surprising considering they were only held together by a complex system of muscles during life, and had no teeth, sockets or other true articulation. 

I think you can see some evidence of pustules and a couple of signs of the muscle scars near the posterior even though this is the valve exterior, as it has been rather squashed. The species was dorsi-biconvex. 

You can see the pseuodointerarea and the dorsal umbo on the fragment of the other valve. 

 

A piece of another specimen on the same rock : 

Some things that look a bit like long spines aren't, but are actually cracks in the specmens with the matrix showing through. 

Edited June 21, 2023 by Tidgy's Dad

[Tidgy's Dad]

A fourth Chengjiang Biota specimen!

This one is a bradoriid, a kind of bivalved arthropod of uncertain affinities.

I think I could prep it out a little bit more but I think I will wait until I have my new glasses and a slightly steadier hand. 

Thanks to Tim @Fossildude19who provided me with the following information.

This goes nicely with my phosphatocopid, another Cambrian bivalved arthropod I posted on page 5 of this thread, as well as the later, more well-known ostracods from the Ordovician onwards.

I have examined other species of Chengjiang bradoriids, but believe Tim's ID to be the correct one.

The name Kunmingella is derived from the Kunming Prefecture, which contains Yunnan Province from where this species and my specimen come.

 

Edited October 28, 2023 by Tidgy's Dad