#halkieria

[yee-qi]

Perspectives paleoart five

"We Do Not Like This, Slugs Who Bug" Australohalkieria, Archaeocyathan sponges, Hyoliths (Turcutheca), Yochelcionella Early Cambrian, 520 mya, Australian sponge reef Out! the Beasts said. You'll ruin our reef! Go take your shell elsewhere, you Cambrian thief! But the mollusks stood steady and kept at their crawl. The Creatures just hoped they would not eat it all. Archaeocyathans were sponges that came in a variety of outlandish shapes and formed reefs much like the coral reefs of today. They may have sustained populations of halkieriids like Australohalkeria above, unfortunately going extinct around the mid-Cambrian. Hyoliths such as Turcutheca might be weird relatives of brachiopods, while Yochelcionella is repping that distinctive shell as one of the first shelled mollusks. This is a part of my Perspectives series! You can find previous entries in #perspectives on my blog.

[alphynix]

Cambrian Explosion Month #24: Phylum Mollusca – Coats of Mail

Much like Odontogriphus and Wiwaxia, the evolutionary relationships of a group called the halkieriids have been debated for a long time. These animals looked like "slugs in chain mail", covered in thousands of tiny overlapping mineralized armor plates along with a larger shell plate at each end.

In the past they've been assigned to different parts of the lophotrochozoan family tree, sometimes being placed closer to annelids or brachiopods, but at this point they're generally accepted to be molluscs. The spiny species Orthrozanclus may link halkieriids with wiwaxiids in a larger "halwaxiid" lineage of early molluscs – or they might instead be early members of a group called aculiferans. 

Aculiferans are represented in modern times by chitons and aplacophorans, and they're distinguished from all other molluscs by having either eight shell valves (chitons) or no shell at all and a worm-like body covered with tiny calcareous spines (aplacophorans).

(Also chitons are especially weird, with magnetite teeth and thousands of eyes in their armor plates.)

A related fossil species called Calvapilosa kroegeri from the early Ordovician of Morocco (~480 million years ago) seems to link halkieriids with aculiferans, placing the chain-mail-slugs as a stem lineage close to the common ancestor of modern forms.

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Halkieria evangelista is the best known species of halkieriid, found in the Sirius Passet fossil deposits in Greenland (~518 million years ago). Up to 8cm long (~3"), it was preserved with all of its armor fully articulated, making it vital in our understanding of the appearance of other halkieriid species that are generally only found as scattered separate pieces.

But those little isolated pieces are incredibly common and widespread, known from Early Cambrian deposits all around the world between about 530 and 513 million years ago – and if halkieriids were aculiferans this suggests their ancestors diverged very early in mollusc evolution, possibly in the late Ediacaran.

While halkieriids largely disappeared after 513 million years ago, fossils from the Canadian Burgess Shale (~508 million years ago) and Australia (~507 million years ago) indicate that the group survived for a little while longer, at least into the mid-Cambrian.

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One of the earliest definite aculiferans is Qaleruaqia sodermanorum from the Aftenstjernesø Formation in Greenland (~516-513 million years ago).

Only known from a few isolated shell plates, its full appearance and size is uncertain. But based on other fossil aculiferans it was probably a worm-like animal around 1cm long (0.4") with a total of eight shells running along its back. Numerous microscopic holes in its shells may represent sensory pores similar to those in modern chitons.

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[palaeopedia]

The Halkieria (1967)

Phylum : Mollusca Subphylum : Halwaxiida Genus : Halkieria Species : H. alata, H. amorpha, H. bisulcata, H. costulata, H. curvativa, H. deplanatiformis, H. desquamata, H. directa, H. elonga, H. equilateralis, H. folliformis, H. fordi, H. hexagona, H. lata, H. long, H. longispinosa, H. maidipingensis, H. mina, H. mira, H. obliqua, H. operculus, H. pennata, H. phylloidea, H. praeinguis, H. projecta, H. sacciformis, H. solida, H. sthenobasis, H. stonei, H. symmetrica, H. terastios, H. uncostata, H. undulata, H. ventricosa, H. wangi, H. zapfei

Cambrian

8 cm long (size)

Armor plates called "sclerites" had long been known as elements of the small shelly fossils, and detailed analysis showed that some of these belonged to the same animal and how they fitted together. The first articulated specimens of Halkieria evangelista, with all their hard parts together, were collected in 1989 from the Sirius Passet lagerstätte in Greenland, and were described in 1990 by Simon Conway Morris and John S. Peel. H. evangelista is used as a model for identifying and reconstructing as halkieriids other similar shells and sclerites; its epithet evangelista reflects its power to explain the Lower Cambrian fossil record

The animals looked like slugs in chain mail - 1.5 centimetres to 8 centimetres long, bilaterally symmetric, flattened from top to bottom and unarmored on the bottom. Very near each end there is a shell plate with prominent growth lines rather like the growth rings of trees. The rest of the upper surface was covered with about 2,000 sclerites that overlapped each other like tiles and formed three zones with sclerites of different shapes: "palmates", shaped rather like maple leaves, ran along the center of the back between the shell plates; blade-shaped "cultrates" lay on either side of the palmates and pointing towards the middle of the upper surface; and slim, sickle-shaped "siculates" covered the outer edges. The sclerites bore a wide central cavity, and (at least in some specimens) finer lateral canals. As the animals grew, the shell plates grew by adding material to the outer edges. Individual sclerites stayed the same size; since the cultrate sclerites form a pattern that is constant in all fairly complete specimens, the old ones that were too small may have been shed and replaced by larger ones as the animals grew. The sclerites seem to have grown by basal secretion There are traces of thin ribs between the sclerites and the skin.

The shellplates and the sclerites were probably made of calcium carbonate originally; it has been suggested on the basis of how they were preserved that they may have been wholly organic, but this is less likely since fossils of non-calcified organisms are usually thin films while Halkeieria fossils are three-dimensional like those of trilobites and hyoliths - in fact several specimens show curvature in the horizontal plane, which suggests that the muscles associated with the sclerites were still present at the time of burial

The sole was soft and probably muscular. Since Halkieria was unsuited to swimming and had no obvious adaptations for burrowing, it must have lived on the sea-floor, "walking" by making its muscular sole ripple. The backward-projecting siculate sclerites may have improved its grip by preventing it from slipping backwards. Some specimens have been found partially rolled up, rather like a pillbug, and in this position the cultrate sclerites projected outwards, which probably deterred predators. It is difficult to determine the functions of the cap-shaped shells at either end of the animal, as the sclerites appear to have offered adequate protection. Scars on the inner surface of the front shell may indicate that it provided an attachment for internal organs. In one specimen the rear shell appears to have rotated by about 45° before fossilization, which suggests there was a cavity underneath, which may have housed gills.

Traces of a gut have been found in the rear halves of some fossils. Parts of one specimen have been interpreted as a radula, the toothed chitinous tongue that is the signature feature of molluscs, but in this specimen the edge of the "scleritome", i.e. coat of sclerites, is folded and the putative radula could be a group of dislocated siculate sclerites.