Two new Thrissops species provide insights into early teleost evolution

A recent study by Dr. Martin Ebert in Zitelliana described two new species from the poorly known genus Thrissops. Thrissops ettlingensis sp. nov. was recently discovered in the lower marine Tithonian Plattenkalk of Ettling, Germany. Meanwhile, Thrissops kimmeridgensis sp. nov. fossils were recovered from the Kimmeridge Clay of Dorset, England.

The work is published in the journal Zitteliana.

The two species are members of the Teleostei. Today, the Teleostei make up over 96% of all fish, including both marine and freshwater species. Some of the oldest Teleostei specimens come from the Triassic (ca. 250 million years old) of China.

Among them was one of the oldest orders, the Ichthyodectiformes, which have been discovered on every continent from the Mid-Jurassic (Bathonian) up to the Upper Cretaceous (Maastrichtian).

This order was unique among Teleostei due to their unique tail skeleton structure and distinctive skull bone called the ethmopalatine. Additionally, they had long anal fins and small dorsal fins positioned far back on the body, often behind the start of the anal fin.

The two new species described by Dr. Ebert both belong to this order and lived ca. 150 million years ago.

The first species, Thrissops ettlingensis sp. nov., is represented by seven specimens. Among them is the only known juvenile Thrissops, with all other known specimens belonging to probable adults.

“I realized that the juveniles of Thrissops lived somewhere else. From other Teleostei of the Solnhofen Archipelago, we have a lot of juveniles in the fossil record (up to 50%),” explained Dr. Ebert.

Meanwhile, the Thrissops kimmeridgensis sp. nov. specimen included over 80 fossils recovered in the Kimmeridge Clay by Steve Etches.

The two species share most of the typical features associated with the group, including a small head, single-rowed teeth, a deeply forked tail fin, rear-positioned opposing fins, and ethmopalatine bones in the nasal floor. However, what they lack is an elongated body shape and the broad/saber-like first rays in the pectoral and pelvic fins, which are present in most other Thrissops species.

Additionally, some of the Ettling specimens are so well-preserved that they still contain their last meals and color patterns. More specifically, two of the Th. ettlingensis contained Orthogonikleithrus hoelli fish in their stomach. The still-connected vertebrae indicated that the fish had not been fully digested at the time of death, indicating they had been eaten only shortly prior.

Similarly, the color patterns are preserved on some of the specimens, similar to Thrissops formosus. A dark pigment melanin was observed in the center of a scale.

Interestingly, the Kimmeridge Clay specimens were commonly represented by isolated crania and caudal fins, possibly a result of predation in deep waters. Dr. Ebert explains, “As I imagine, the fish at the time of the Kimmeridge Clay lived in deeper water, farther from the coast than the small, relatively shallow Plattenkalk basins of the Solnhofen Archipelago. There, in the open water, was probably enough food for larger predators, which also fed on the abundant Thrissops, as can be seen from the isolated Thrissops heads and caudal fins.”

This species had a similar body shape and proportions to Th. formosus. Additionally, they were characterized by having irregular lower jaw dentition and being the largest of the known Thrissops.

The new species provides important insights into the early evolution of the Teleostei, including their morphological variability, diets, and color patterns.

According to Dr. Ebert, future studies will hopefully propose a phylogeny which includes the new species. Otherwise, his future focus will be on the diverse ecology of the Upper Jurassic.

“When I work on Allothrissops, hopefully in a year or two, I will certainly think about a phylogeny that includes the new Thrissops specimens. But I’m even more interested in the diverse ecology of the Upper Jurassic sites,” says Dr. Ebert.

“There are sites where certain genera are common or rare, which also applies to the genus Thrissops, and that tells us a lot. For this comparison of fish faunas, I have now reviewed about 100 collections and have a database with more than 23,000 fish specimens. Most of them, I have updated the taxon names.”

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