The genus Thrissops belongs to the order Ichthyodectiformes, one of the earliest orders of true teleosts, the group that today makes up over ninety-six percent of all fish species with around thirty thousand living representatives. The oldest known teleosts come from the Triassic of China, approximately 250 million years old, but the Upper Jurassic Solnhofen Archipelago of Germany and Cerin in France represent the world’s oldest window into Earth history for studying the evolution of teleosts with a higher diversity of species. Ichthyodectiformes are distinguished from all other teleosts by the construction of the caudal fin skeleton and a special bone in the cranium called the ethmopalatine. All genera in the order possess an anterior-posteriorly elongated anal fin with twenty-four to thirty-seven fin rays, and a small dorsal fin located far back on the trunk, in most genera even posterior to the anal fin origin.

These were predatory fish of considerable consequence. In the Cretaceous, the ichthyodectiform Xiphactinus could reach over five meters in length, but in the Jurassic the group already dominated as by far the largest teleosts, reaching up to ninety centimeters. Crucially, they were the only predatory teleosts of the Jurassic exceeding thirty centimeters in length, making Thrissops the genus with the first larger predatory fish among the teleosts. In the Jurassic, three genera of Ichthyodectiformes are accepted so far: Occithrissops from the Middle Jurassic of Wyoming, USA, and Allothrissops and Thrissops from the Upper Jurassic of England, France, and Germany. The group contains at least twenty genera and about twice that number of species, distributed across marine and to a lesser extent freshwater deposits of all continents from the Middle Jurassic to the Upper Cretaceous.

Thrissops ettlingensis is a small species reaching a maximum total length of only 15.5 centimeters, with a standard length to body depth ratio averaging thirty-one percent. The species is defined by a unique combination of characters including thirty to thirty-one supraneurals, forty-nine to fifty vertebrae without ural centra, twenty-four to twenty-five ribs anterior of the anal fin, and twenty-three to twenty-five anal pterygiophores. The dentition consists of small teeth with a curved maxilla and regular dentition in the lower jaw. The holotype, a complete specimen in lateral view measuring 11.6 centimeters standard length, is housed in the Jura-Museum Eichstätt. A juvenile specimen of just 3.5 centimeters standard length is also known from Ettling, in which most vertebral centra are not yet developed and only embryonal ring centra are visible, appearing in black color in both normal light and UV light while all other bones are brown.

The cranial morphology of Thrissops ettlingensis is remarkably well preserved, allowing detailed examination of the skull roof, circumorbital series, and jaw apparatus. The elongated frontals are the largest bones of the skull roof, with the posterior part twice as wide as the anterior part. Four thin infraorbitals are present ventral and posterior of the orbit, and the preopercle is large with a long slender dorsal part and a broad ventral part bearing a well-developed posteroventral process. The upper jaw consists of maxilla, premaxilla, and two supramaxillae, with eight to ten small teeth on the convexly rounded anteroventral border of the premaxilla that are about twice as long and wide as the maxillary teeth. The dentary carries twenty-two small pointed teeth in a single row of approximately equal size along its entire length.

Perhaps most striking is the evidence of feeding behavior preserved in the Ettling specimens. Two specimens of Thrissops ettlingensis each contain a single, quite large prey fish of Orthogonikleithrus hoelli in their stomachs. In one specimen the prey fish is well visible, while in the holotype the prey is already half digested, with the intestines in both fishes filled with almost completely digested fish remains. This contrasts with the related Thrissops formosus, which has been found with an estimated thirty to fifty small prey fish in its stomach, suggesting that the smaller Thrissops ettlingensis took relatively larger individual prey in relation to its own body size. The vertebrae of prey fish found in Thrissops formosus specimens remain connected, indicating that all prey were eaten in a very short period of a few hours, a feeding strategy comparable to mackerel and herring, which continue to eat until the stomach is full before resting to digest.

Thrissops kimmeridgensis from the Kimmeridge Clay of Dorset is a very different animal. With a maximum standard length of approximately 75.5 centimeters, corresponding to about ninety centimeters total length, it is the largest known specimen in the genus Thrissops, substantially revising the previous maximum of 390 millimeters reported for the genus. The species is characterized by an irregular dentition in the lower jaw, with the most anterior tooth and the teeth in the middle of the dentition being twice the size of the other dentary teeth, and a hook at the anterior tip of the dentary armed with one large fang-like, posteriorly directed tooth. The species possesses approximately fifty-five to fifty-seven vertebrae and twenty-nine to thirty anal pterygiophores. Of the more than ninety known specimens, only a few are preserved as complete fish, with most represented by isolated crania or caudal fins that appear to be the remains left by larger predators such as crocodiles, ichthyosaurs, plesiosaurs, pliosaurs, and sharks known from the same sites.

The assignment of both new species to the order Ichthyodectiformes is strongly supported by the characters of the skull, the positions and shape of the dorsal and anal fins, and features of the caudal skeleton. Both species belong to the genus Thrissops by morphological and skeletal features such as high body shape and a short but high cranium. Thrissops ettlingensis still possesses some primitive features, including a comparably low number of abdominal vertebrae and a low number of anal pterygiophores. Indeed, with forty-nine to fifty vertebrae, Thrissops ettlingensis has the lowest vertebral count of any known ichthyodectiform, while all other Thrissops and Allothrissops species have fifty-two to sixty-one vertebrae, the highest number among Upper Jurassic teleosts. In the Cretaceous, ichthyodectiforms evolved with more than one hundred vertebrae, and the number of anal pterygiophores increased from a Jurassic maximum of thirty-two to a Cretaceous maximum of forty-two, making the new Ettling species with twenty-three to twenty-five anal pterygiophores the second lowest in the order after Occithrissops willsoni with eighteen to twenty.

The two new species are compared in the study to all other known Upper Jurassic Thrissops species of the Solnhofen Archipelago in Germany, Cerin in France, and Dorset in England, as well as all other known Jurassic Ichthyodectiformes. Seven valid species of the genus are now accepted, including Thrissops formosus from the Solnhofen Archipelago and Cerin, Thrissops subovatus from the Solnhofen Archipelago, Thrissops cirinensis from Cerin, Thrissops portlandicus and Thrissops curtus from Dorset, and the two new species. Previously, the specimens of Thrissops ettlingensis were considered to be juveniles of Thrissops formosus, but the detailed examination of vertebral counts, rib numbers, and anal pterygiophore counts now clearly separates them. The genus Thrissops from the Upper Jurassic, spanning the Kimmeridgian and Tithonian stages approximately 152 million years ago, is among the first true teleosts and belongs to the stratigraphically oldest ichthyodectiforms.

The discovery also highlights the geographic distribution of these early predators across the opening Tethys. Thrissops is quite common in the Kimmeridge Clay of Dorset, in Cerin in France, and in Ettling and Kelheim in the Solnhofen Archipelago of Bavaria, but very rare at other localities of the archipelago, particularly in the Eichstätt and Solnhofen Basins, the Zandt Basin, Brunn, Painten, and Wattendorf. In Nusplingen in Baden-Württemberg, Thrissops appears to be absent entirely. Thrissops kimmeridgensis, with ninety-four known specimens, is one of the most common fish species in the Kimmeridge Clay, though smaller teleost species are often overlooked there due to the poor splitting of the clay and the difficulty of preparing small fishes in this bituminous material. Thrissops ettlingensis, by contrast, is known only from Ettling and with seven specimens remains rather rare. The study also points out persistent problems with the related genus Allothrissops, whose literature has probably caused more confusion than clarity because important authors described fish from different species under the name of the type species Allothrissops salmoneus. Together, the two new species represent a further step in comparing fish fauna diversity in the Upper Jurassic of the opening Tethys across Southern England, Southern France, and Southern Germany, illuminating how the first large predatory teleosts diversified across the carbonate platforms of Central Europe roughly 150 million years ago.

Subject of Research: Paleontology

Article Title: New species of the genus Thrissops ( Teleostei , Ichthyodectiformes ) in the Upper Jurassic of the Solnhofen-Archipelago (Germany) and Kimmeridge Clay (England)

Article References: New species of the genus Thrissops ( Teleostei , Ichthyodectiformes ) in the Upper Jurassic of the Solnhofen-Archipelago (Germany) and Kimmeridge Clay (England). (n.d.). https://doi.org/10.3897/zitteliana.99.159055

Image Credits: AI Generated

DOI: 10.3897/zitteliana.99.159055

Keywords: paleontology, Jurassic, Thrissops, Ichthyodectiformes, Teleostei, Solnhofen, Kimmeridge Clay, new species, species, genus, Upper, Solnhofen-Archipelago