They came from the sea: New 500-million-year-old crustacean trace fossil records ancient behavior

Mintererichnus shieldiA new type of footprint fossil (a resting footprint) of a phyllocariid crustacean from Texas shows the animal trapped in a tidal pool, shortly before the animal first appeared on land.

Mintererichnus shieldiA 500-million-year-old fossil rests 55 mm long from Texas.

Trace images Represents the remains of the activity of an ancient organism. They include animal tracks, burrows and resting tracks.

A new type of resting trace from the Cambrian period, about 500 million years ago, is slightly younger than popular Fauna of the Burgess Shale In Canada, it reveals what life was like in tidal systems during this time.

The ancient animal that produced it was about 6 cm long and belonged to a group of crustaceans called Phylocharids (distant ancestors of crabs and lobsters), probably the same as their previous example. Arinosicaris Found in Wisconsin at this time.

Trace fossils were found in the Riley Formation of central Texas. It consists of small isolated footprints at the bottom of a sandstone layer, including imprints of arthropod antennae, five walking legs, at least five gill plates, and an abdominal and tail spine.

It is named Mintererichnus shieldiAfter Dr. Nicholas Minter from the University of Portsmouth, in recognition of his research on trace fossils Mr. Elgian Shield 1937 and later who discovered the fossil received Presidential Citation for World War II Actions.

The fossil shows ancient behavior — animals trapped in a tidal pool before animals established themselves on land, an important step in the evolution of life.

Worms, lingulid brachiopods, and stranded dying phyllocarits occupied the tidal pools, while trilobites and dendroid graptolites lived in deep tidal channels.

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It was previously attributed to an obscure group of chelicerate arthropods called chasmataspidids, and is the oldest record of a euchelicerate (horseshoe crabs, eurypterids). [sea scorpions] and arachnids); The oldest undoubted euchelicerate is now of Ordovician age.

„This makes sense by shifting the oldest record of euchelicerates to the Ordovician, when the first horseshoe crabs appeared,” explained Dr. Jason Dunlop of the Leibniz Institute for Evolution and Biodiversity in Berlin, who reviewed the study.

„The apparent predation of horseshoe crabs by chasmataspidids has always been an anomaly in the fossil record.”

The oldest tracks on dry land were made by ancestors of millipedes (called Euthycarcinoids) on ancient coastal dunes (in New York State and Ontario). Waves (in Quebec and Wisconsin) at the same time.

Phyllocarids, euthycarcinoids and mollusks all left traces on ancient tidal flats 500 million years ago.

They were soon followed by the eurypterids, the forerunners of arachnids, leading to widespread invasion of land by animals, possibly through neodony (retention of juvenile characteristics in offspring species).

This is Paper appears in New Year Book of Geology and Paleontology.

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SJ Brady. 2023. A new arthropod resting trace from the Middle Cambrian of Texas. New Year Book of Geology and Paleontology 309/3: 291-300; doi: 10.1127/njgpa/2023/1163

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