Minterichnus shieldi, a new type of trace fossil (resting trace) of a phyllocarid crustacean from Texas shows animals trapped in a tidal pool, just before animals first emerged onto land.
Minterichnus shieldi, a new 500-million-year-old fossil resting trace of a phyllocarid crustacean from Texas, 55 mm long.
Trace fossils represent the remains of an ancient organism’s activity. They include animal tracks, burrows and resting traces.
A new type of resting trace from the middle Cambrian period, around 500 million years ago, slightly younger than the famous Burgess Shale fauna in Canada, reveals what life was like in tidal settings at this time.
The ancient animal that produced it was about 6 cm long and belonged to a group of crustaceans called phyllocarids (the distant ancestor of crabs and lobsters), probably like their earliest example called Arenosicaris found in Wisconsin around this time.
The fossil resting trace was discovered in the Riley Formation of central Texas. It consists of small isolated traces on the underside of a sandstone slab, comprising imprints of the arthropod’s antennae, five walking legs, at least five gill plates, an abdomen and tail spines.
It is named Minterichnus shieldi, after Dr. Nicholas Minter from Portsmouth University, in recognition of his research on trace fossils and Mr. Elgean Shield who discovered the fossil in 1937 and later received a 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 stepping stone in the evolution of life.
Worms, lingulid brachiopods and stranded dying phyllocarids occupied the tidal pools, while trilobites and dendroid graptolites lived in the deeper tidal channels.
It was previously attributed to an obscure group of chelicerate arthropods called chasmataspidids, and was the oldest record of a euchelicerate (horseshoe crabs, eurypterids [sea scorpions] and arachnids); the oldest unequivocal euchelicerate is now Ordovician in age.
“This makes sense by shifting the oldest record of euchelicerates into the Ordovician when the first horseshoe crabs appear,” explained Dr. Jason Dunlop from the Leibniz Institute for Evolution and Biodiversity in Berlin, who reviewed the study.
“The fact that the apparently more derived-looking chasmataspidids predated horseshoe crabs was always something of an anomaly in the fossil record.”
The oldest tracks on dry land were made by the ancestors of millipedes (called euthycarcinoids) on ancient coastal dunes (in New York State and Ontario) and tidal flats (in Quebec and Wisconsin) around the same time.
Phyllocarids, euthycarcinoids and mollusks all left traces on ancient tidal flats 500 million years ago, representing the precursors of terrestrial crustaceans, millipedes and molluscs, respectively.
They were shortly followed by the eurypterids, the precursors of arachnids, leading to the widespread invasion of the land by animals, perhaps via neotony (the retention of juvenile traits in descendant species).
This paper appears in the Neues Jahrbuch für Geologie und Paläontologie.
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S.J. Braddy. 2023. A new arthropod resting trace from the middle Cambrian of Texas. Neues Jahrbuch für Geologie und Paläontologie 309/3: 291-300; doi: 10.1127/njgpa/2023/1163
