THE SLOW RACE OF TIME: LIVING FOSSILS

Horseshoe crabs are marine and brackish water arthropods of the order Xiphosura — a slowly evolving, conservative taxa.

Much like (slow) Water Striders (Aquarius remigis), (relatively sluggish) Coelacanth (Latimeria chalumnae) and (the current winner on really slow evolution) Elephant Sharks (Callorhinchus milii), these fellows have a long history in the fossil record with very few anatomical changes. 

But slow change provides loads of great information. It makes our new friend, Yunnanolimulus luoingensis, an especially interesting and excellent reference point for how this group evolved. 

We can examine their genome today and make comparisons all the way back to the Middle Triassic (with this new find) and other specimens from further back in the Ordovician — 445 million years ago. 

These living fossils have survived all five mass extinction events. They are generalists who can live in shallow or deep water and will eat pretty much anything they can find on the seafloor.

The oldest horseshoe crab fossil, Lunataspis aurora, is found in outcrops in Manitoba, Canada. Charmingly, the name means crescent moon shield of the dawn. It was palaeontologist Dave Rudkin and team who chose that romantic name. Finding them as fossils is quite remarkable as their shells are made of protein which does not mineralized like typical fossils.

Even so, the evolution of their exoskeleton is well-documented by fossils, but appendage and soft-tissue preservation are extremely rare. 

A new study analyzes details of the appendage and soft-tissue preservation in Yunnanolimulus luoingensis, a Middle Triassic (ca. 244 million years old) horseshoe crab from Yunnan Province, SW China. The remarkable anatomical preservation includes the chelicerae, five pairs of walking appendages, opisthosomal appendages with book gills, muscles, and fine setae permits comparison with extant horseshoe crabs.

The close anatomical similarity between the Middle Triassic horseshoe crabs and their recent analogues documents anatomical conservatism for over 240 million years, suggesting persistence of lifestyle.

The occurrence of Carcinoscorpius-type claspers on the first and second walking legs in male individuals of Y. luoingensis tells us that simple chelate claspers in males are plesiomorphic for horseshoe crabs, and the bulbous claspers in Tachypleus and Limulus are derived.

As an aside, if you hadn't seen an elephant shark before and were shown a photo, you would likely say, "that's no freaking shark." You would be wrong, of course, but it would be a very clever observation.

Callorhinchus milii look nothing like our Great White friends and they are not true sharks at all. Rather, they are ghost sharks that belong to the subclass Holocephali (chimaera), a group lovingly known as ratfish. They diverged from the shark lineage about 400 million years ago.

If you have a moment, do a search for Callorhinchus milii. The odd-looking fellow with the ironic name, kallos, which means beautiful in Greek, sports black blotches on a pale silver elongate body. And their special feature? It is the fishy equivalent of business in the front, party in the back, with a dangling trunk-like projection at the tip of their snout and well-developed rectal glands near the tail.

As another small point of interest with regards to horseshoe crabs, John McAllister collected several of these while working on his MSc to see if they had microstructures similar to trilobites (they do) and whether their cuticles were likewise calcified. He found no real calcification in their cuticles, in fact, he had a rather frustrating time getting anything measurable to dissolve in acid in his hunt for trace elements. 

Likewise, when looking at oxygen isotopes (16/18) to get a handle on water salinity and temperature, his contacts at the University of Waterloo had tons of fun getting anything at all to analyze. It made for some interesting findings. Sadly, for a number of reasons, he abandoned the work, but you can read his very interesting thesis here: https://dr.library.brocku.ca/handle/10464/1959

Ref: Hu, Shixue & Zhang, Qiyue & Feldmann, Rodney & Benton, Michael & Schweitzer, Carrie & Huang, Jinyuan & Wen, Wen & Zhou, Changyong & Xie, Tao & Lü, Tao & Hong, Shuigen. (2017). Exceptional appendage and soft-tissue preservation in a Middle Triassic horseshoe crab from SW China. Scientific Reports. 7. 10.1038/s41598-017-13319-x.

CHELICERATA: EURYPTERIDS, SPIDERS AND HORSESHOE CRABS

Sanctacaris uncata

This lovely is Sanctacaris uncata — a wonderful example of Chelicerata.

We first see them emerge in our ancient oceans in the Middle Cambrian, some 508 million years ago, as the arthropod Sanctacaris uncata (Briggs & Collins, 1988) known from the Glossopleura Zone, Stephen Formation of Mount Stephen in the Burgess Shale, British Columbia, Canada. 

Sanctacaris is proof positive that chelicerates, although rare, were present in the Middle Cambrian sea. Even at this early stage of evolution, Sanctacaris had the number and type of head appendages found in modified form in the eurypterids and xiphosurids, the major Palaeozoic groups that succeeded it. Even more interesting is that Sanctacaris had all the characteristics of later chelicerates except chelicerae — placing this early arthropod in a primitive sister group of all other chelicerates.

An extinct marine creature half a billion years old may sound otherworldly, but you know some of their more well-known marine brethren — sea spiders, the sexy eurypterids, chasmataspidids and horseshoe crabs — and some of their terrestrial cousins — spiders, scorpions, harvestmen, mites and ticks. 

They are grouped together because, like all arthropods, they have a segmented body and segmented limbs and a thick chitinous cuticle called an exoskeleton. Add those characteristics to a body system with two body segments — a cephalothorax and an abdomen. 

Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle made mainly of chitin and chemically hardened proteins. 

Since this cannot stretch, the animals must moult to grow. In other words, they grow new but still soft cuticles, then cast off the old one and wait for the new one to harden. 

Until the new cuticle hardens the animals are defenceless and almost immobilized.  This also helps to explain why you find so many cephalons or moulted head shields — or whatever else our good arthropod friends shed and regrow — in the field and far fewer body fossils of the whole animal.

Some chelicerate are predatory animals that patrol the warm waters near thermal vents. They can be found feeding upon other predators and fish. Although the group were originally solely predatory, they have diversified to use all sorts of feeding strategies: predation, parasitism, herbivory, scavenging and dining on bits of decaying organic matter. 

Although harvestmen can digest solid food it is more akin to a mashed pulp by the time they do. The guts of most modern chelicerates are too narrow to digest solid food, instead, they generally liquidize their chosen meal by grinding it with their chelicerae and pedipalps then flooding it with digestive enzymes. 

To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood by Malpighian tubules, structures that also evolved independently in insects — another case of convergent evolution.

The evolutionary origins of chelicerates from the early arthropods have been debated for decades. And although there is considerable agreement about the relationships between most chelicerate sub-groups, the inclusion of the Pycnogonida in this taxon has recently been questioned and the exact position of scorpions is still controversial, though they were long considered the most primitive or basal of the arachnids. 

We still have much to explore to sort out their evolutionary origins and placement within the various lineages but we will get there with time.

Image One: Reconstruction of Sanctacaris uncata, a Cambrian Habeliidan arthropod (stem-Chelicerata: Habeliida). by Junnn11 @ni075; Image Two: Chelicerata by Fossil Huntress

Aria C, Caron JB (December 2017). "Mandibulate convergence in an armoured Cambrian stem chelicerate". BMC Evolutionary Biology. 17 (1): 261. doi:10.1186/s12862-017-1088-7. PMC 5738823. PMID 29262772.

Legg DA (December 2014). "Sanctacaris uncata: the oldest chelicerate (Arthropoda)". Die Naturwissenschaften. 101 (12): 1065–73. doi:10.1007/s00114-014-1245-4. PMID 25296691.

Briggs DE, Collins D (August 1988). "A Middle Cambrian chelicerate from Mount Stephen, British Columbia" (PDF). Palaeontology. 31 (3): 779–798. Archived from the original (PDF) on July 16, 2011. Retrieved April 4, 2010.

Briggs DE, Erwin DH, Collier FJ (1995). Fossils of the Burgess Shale. Washington: Smithsonian Institution Press. ISBN 1-56098-659-X. OCLC 231793738.

HORSESHOE CRABS: WINNING THE SLOW RACE OF TIME

Horseshoe crabs are marine and brackish water arthropods of the order Xiphosura — a slowly evolving, conservative taxa.

Much like (slow) Water Striders (Aquarius remigis), (relatively sluggish) Coelacanth (Latimeria chalumnae) and (the current winner on really slow evolution) Elephant Sharks (Callorhinchus milii), these fellows have a long history in the fossil record with very few anatomical changes. 

But slow change provides loads of great information. It makes our new friend, Yunnanolimulus luoingensis, an especially interesting and excellent reference point for how this group evolved. 

We can examine their genome today and make comparisons all the way back to the Middle Triassic (with this new find) and other specimens from further back in the Ordovician — 445 million years ago. 

These living fossils have survived all five mass extinction events. They are generalists who can live in shallow or deep water and will eat pretty much anything they can find on the seafloor.

The oldest horseshoe crab fossil, Lunataspis aurora, is found in outcrops in Manitoba, Canada. Charmingly, the name means crescent moon shield of the dawn. It was palaeontologist Dave Rudkin and team who chose that romantic name. Finding them as fossils is quite remarkable as their shells are made of protein which does not mineralized like typical fossils.

Even so, the evolution of their exoskeleton is well-documented by fossils, but appendage and soft-tissue preservation are extremely rare. 

A new study analyzes details of the appendage and soft-tissue preservation in Yunnanolimulus luoingensis, a Middle Triassic (ca. 244 million years old) horseshoe crab from Yunnan Province, SW China. The remarkable anatomical preservation includes the chelicerae, five pairs of walking appendages, opisthosomal appendages with book gills, muscles, and fine setae permits comparison with extant horseshoe crabs.

The close anatomical similarity between the Middle Triassic horseshoe crabs and their recent analogues documents anatomical conservatism for over 240 million years, suggesting persistence of lifestyle.

The occurrence of Carcinoscorpius-type claspers on the first and second walking legs in male individuals of Y. luoingensis tells us that simple chelate claspers in males are plesiomorphic for horseshoe crabs, and the bulbous claspers in Tachypleus and Limulus are derived.

As an aside, if you hadn't seen an elephant shark before and were shown a photo, you would likely say, "that's no freaking shark." You would be wrong, of course, but it would be a very clever observation.

Callorhinchus milii look nothing like our Great White friends and they are not true sharks at all. Rather, they are ghost sharks that belong to the subclass Holocephali (chimaera), a group lovingly known as ratfish. They diverged from the shark lineage about 400 million years ago.

If you have a moment, do a search for Callorhinchus milii. The odd-looking fellow with the ironic name, kallos, which means beautiful in Greek, sports black blotches on a pale silver elongate body. And their special feature? It is the fishy equivalent of business in the front, party in the back, with a dangling trunk-like projection at the tip of their snout and well-developed rectal glands near the tail.

As another small point of interest with regards to horseshoe crabs, John McAllister collected several of these while working on his MSc to see if they had microstructures similar to trilobites (they do) and whether their cuticles were likewise calcified. He found no real calcification in their cuticles, in fact, he had a rather frustrating time getting anything measurable to dissolve in acid in his hunt for trace elements. 

Likewise, when looking at oxygen isotopes (16/18) to get a handle on water salinity and temperature, his contacts at the University of Waterloo had tons of fun getting anything at all to analyze. It made for some interesting findings. Sadly, for a number of reasons, he abandoned the work, but you can read his very interesting thesis here: https://dr.library.brocku.ca/handle/10464/1959

Ref: Hu, Shixue & Zhang, Qiyue & Feldmann, Rodney & Benton, Michael & Schweitzer, Carrie & Huang, Jinyuan & Wen, Wen & Zhou, Changyong & Xie, Tao & Lü, Tao & Hong, Shuigen. (2017). Exceptional appendage and soft-tissue preservation in a Middle Triassic horseshoe crab from SW China. Scientific Reports. 7. 10.1038/s41598-017-13319-x.