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904 tocNew Species of Fish from the Genus Eosemionotus

Jaleigh Pier

 

Although some fossil fish can be found in great abundance within mass mortality layers, others like the Middle Triassic genus Eosemionotus are extremely rare. For more than 80 years, Eosemionotus was known only through a single species discovered in Germany. It wasn’t until 2004 that a second species was officially published and now fifteen years after that we finally can include several new species into this once lonely genus. Dr. Adriana López-Arbarello and her team have identified three brand new species (E. diskosomus, E. sceltrichensis, and E. minutus) and have also further solidified the differences between the now five species based on their morphological variability.

fig6Eosemionotus diskosomus. Complete specimens. 1, holotype MCSN 8082 (45 mm SL); 2, MCSN 8006 (60.5 mm SL); 3, PIMUZ T 2924 (22.5 mm SL); 4, MCSN 5617 (46.7 mm SL). Scale bars equal 5 mm.

These fish are amongst the oldest relatives of the modern gar fish, whose ancient lineage is depicted below as the order Lepisosteiformes (shaded in green). Their sister group, of which Eosemionotus belongs (branches depicted in light blue), is known as the Semionotiformes (shaded in blue). These two groups diverged from a common ancestor in the Early Triassic. Semionotiformes were very diverse through most of the Mesozoic and became extinct just before the K-T boundary during the Late Cretaceous. Their sister group, Lepisosteiformes, was able to persist and there are seven known species of gar today.

fig24Strict consensus tree of 16 most parsimonious trees. Tree length = 968 steps, consistency index = 0.380 and retention index = 0.645. Bremer indexes and bootstrap values larger than 50 are indicated with red and black numbers at the corresponding nodes, respectively. Macrosemiid branches are indicated in blue, with the branches corresponding to the five species of Eosemionotus in light blue.

For palaeontologists, determining species can be tricky since rarely a fossilized specimen will be completely preserved. Morphological differences provide some of the best clues for fish taxonomy. Morphology can also sometimes infer habitat or diet preferences. “When species are very closely related, morphological differences are usually minor. Features like the relative position of the fins and the number of rays in one or more fins usually vary between species, but morphology might also vary within the same species,” explains López-Arbarello. Analyzing the distribution of morphological features is key, since different populations or individuals of the same species may have slight variations of physical traits. The cladistic analyses within this paper are based on morphological differences between species and helps solidify the inclusion of macrosemiids within the order Semionotiformes.

fig19Eosemionotus sceltrichensis. Anal fin. 1, specimen MCSN 8491; specimen MCSN 8418; 3, specimen MCSN 8497. Scale bars equal 2 mm.

A Dive into Eosemionotus Ecology

Dr. López-Arbarello explains “the Monte San Giorgio at Lake Lugano of Switzerland is the most intensively studied and best-known record of marine life during the Middle Triassic Period. This represents one of the most important windows to study the process of life recovery after the largest episode of mass extinction at the Permian-Triassic boundary about 250 million years ago.”

In the Middle Triassic, these fish species likely would have inhabited shallow marine environments along the coast of the Tethys sea. Small islands would have dotted the shoreline surrounded by carbonate platforms with volcanoes looming just off in the distance. Eosemionotus would have flourished along the sandy coasts and warm lagoons “preying on worms, soft-shelled invertebrates, and possibly algae too,” explains Toni Bürgin. This picturesque landscape may have resembled modern day Bahamas or Maldives, except modern coral reefs would have been exchanged for other primary reef building organisms such as members of the calcareous algae genus Diplopora.

“The presence of these species indicates well established ecosystems and indicates organisms were capable of developing complex ecological networks. This evidence suggests that the biosphere recovered astonishingly fast after the most dramatic phase of extinctions in the history of our planet,” says López-Arbarello.

Through analyzing their phylogenetic relationships, they were able to determine that the oldest named species of Eosemionotus, E. vogeli, originated from the Tethys sea and migrated into the Germanic Basin. It is likely that the migration of E. vogeli led to further speciation of the Eosemionotus genus, but the complete evolutionary history of this group will remain a mystery until older occurrences of the species and other records of the genus are fully studied.

fig25Simplified palaeogeography of the Alpine and the Germanic Triassic at the Anisian-Ladinian boundary with the Triassic position of the localities with Eosemionotus occurrences: 1 Alcover, Spain; 2 Winterswijk, The Netherlands; 3 Grebenberg, Germany; 4 Förderstedt, Germany; 5 Rüdersdorf, Germany; 6 Ducan and Landwasser areas, Switzerland; 7 Monte San Giorgio, Switzerland/Italy; 8 Velika Planina, Slovenia. BG Burgundy Gate; SMG Silesian-Moravian Gate; ECG eastern Carpathian Gate (modified from Brack et al. 1999). Small letters indicate location of major cities: Ba, Barcelona; Be, Berlin; Bs, Basel; Bu, Budapest; Fr, Frankfurt am Main; Ha, Hamburg; Kr, Cracow; Ly, Lyon; Mr, Marseilles; Mü, Munich; Pr, Prague; Wa, Warsaw.

The discoveries summarized here were made possible through a collaborative team effort of combined expertise and, thus, different viewpoints of the research. Dr. Adriana López-Arbarello and Dr. Toni Bürgin are fish experts and work out the taxonomy, while Dr. Rudolf Stockar and Dr. Heinz Furrer are able to add geological and paleoenvironmental context for each species. Together these efforts provide the tools allowing them to shape a possible scenario in which this fish evolved.

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