Aegirosaurus leptospondylus was a species of ichthyosaur which lived in the seas around Europe during the late Jurassic Period and early Cretaceous Period, roughly 147-135 million years ago.
Aegirosaurus belonged to a family of ichthyosaurs called the ophthalmosaurids (Bardet and Fernández 2000), named after the genus Ophthalmosaurus, which lived during the middle and late Jurassic Period. The ophthalmosaurids were distinctive for having unusually large eyes in proportion to body size, hence their name, literally “eye lizards”. Ichthyosaurs as a whole are frequently compared to dolphins, but the ophthalmosaurids bear more of a resemblance to swordfish or marlins. They had elongated thin snouts, compact round bodies, broad sideways-pointing pectoral fins, and often had large crescent-shaped tail fins. The whole shape indicates that they were very powerful swimmers capable of great bursts of speed, and they probably lived out in open pelagic water rather than the shallow water near the coast.
The first specimen of this animal was discovered during the middle 1800s within a layer of limestone near the town of Kelheim, Bavaria, Germany by a certain Dr. Oberndorfer, and as a result it became known as “the Oberndorfer specimen”. A second specimen was found near Solnhofen, a site famous for its exquisitely-preserved Late Jurassic fossils including those of the primitive bird Archaeopteryx. In 1853, it was officially named Ichthyosaurus leptospondylus by Dr. Johann Andreas Wagner (simply given as “A. Wagner” in multiple sources), a professor of paleontology at the Bavarian Academy of Science (Wagner 1853; Wagner 1861). Unfortunately, both specimens were destroyed during World War II (“Ichthyosaurus leptospondylus Wagner, 1853:26 (N#70944)”).
Lithographic print of the original holotype of Ichthyosaurus leptospondylus (later re-named as Aegirosaurus leptospondylus). Image from Wagner, A. (1861). “VIII. Ichthyosaurus leptospondylus”. Neue Beiträge zue Kenntniss der urweltlichen Fauna des lithographischen Schiefers. Table VI.
However, other skeletons have been found since then, and this led to a greater understanding of the creature’s anatomy. This, in turn, led paleontologists to conclude that it was not a species of Ichthyosaurus, but was a completely separate genus. It was named Aegirosaurus in 2000, named after Aegir, the ancient Germanic god of the sea. Fossils have been found within the Solnhofen Formation of Bavaria, Germany dated to the late Jurassic Period, and also in southeastern France within rocks dated to the Valanginian Stage of the early Cretaceous Period. This shows that this species was one of the few species to survive the mass extinction at the end of the Jurassic Period around 145 MYA (Bardet and Fernández 2000; Fischer et al 2011; Fischer et al 2012; Fischer et al 2014; Delsett et al 2022).
Neotype specimen of Aegirosaurus leptospondylus, found within the Solnhofen Formation of Bavaria, Germany. Note that the outline of the body is preserved, including the size and shape of the fins. Scale bar = 20 cm. Image from Bardet, Nathalie; Fernández, Marta S. (2000). “A new ichthyosaur from the Upper Jurassic lithographic limestones of Bavaria”. Journal of Paleontology, volume 74, issue 3. Page 505.
Aegirosaurus was a small marine reptile, reaching only 6 feet long. Soft tissue impressions have been found in association with the skeletons of this animal, which give us a very good idea about the outline of the body and the shape of the fins. It had a skull which measured one-third the total length of its body, with huge eyes and long thin gharial-like jaws lined with short conical teeth. Its body shape was like a rounded teardrop, and one well-preserved skeleton shows that it had a thick layer of blubber underneath the skin to provide insulation. Interestingly, most of its fins (with the exception of the caudal fin) were rather small, likely to reduce drag when sharply cutting through the water. The tail fin looked exactly like a cartoon picture of a crescent moon. Bardet and Fernández claimed in 2000 that the body may have been covered in extremely small scales due to the preserved presence of what looked like scales on the caudal fin. This is in contrast to the conventional scale-less morphology of other ichthyosaurs. However in 2022, another study was conducted on the specimens, and they were found to be scale-less (Bardet and Fernández 2000; Delsett et al 2022).
A well-preserved specimen of Aegirosaurus (collection ID code: JME-SOS-08369) from Eichstätt-Blumenberg, Germany showing the rounded outline of the body, in contrast to the somewhat slimmer specimen shown earlier. Scale bar = 20 cm. Image from Delsett, Lene L.; Friis, Henrik; Kölbl-Ebert, Martina; Hurum, Jorn H. (2022). “The soft tissue and skeletal anatomy of two Late Jurassic ichthyosaur specimens from the Solnhofen archipelago”. PeerJ, volume 10: e13173.
Aegirosaurus was essentially a reptilian analog of a tuna, and likely would have been capable of great bursts of speed. Indeed, the ophthalmosaurid ichthyosaurs are often described as “thunniform”, meaning “tuna-shaped” within the scientific literature, although as I said earlier the swordfish analog seems to be more appropriate for the majority of species. It would be interesting if Aegirosaurus swam in schools like modern tuna, chasing after schools of mackerel-like Caturus or other smaller saltwater fish.
Aegirosaurus was not the only marine reptile within its ecosystem; it shared its underwater habitat with the 10 foot long leptocleidid plesiosaur Brancasaurus. It also possibly lived alongside the much larger ichthyosaur Arthropterygius and the pliosaur Hastanectes.
Below are a pair of illustrations which I made of Aegirosaurus in left lateral view and dorsal view. The illustrations are based upon the following fossil specimens: SM (neotype); BSPHGM 1954 I 608; JME-SOS-08369; RGHP LA 1. The drawings were made with No.2 pencil and Prismacolor colored pencils on printer paper, and with A LOT of touching up on the computer.
Aegirosaurus leptospondylus. © Jason R. Abdale (January 23, 2023).
Bardet, Nathalie; Fernández, Marta S. (2000). “A new ichthyosaur from the Upper Jurassic lithographic limestones of Bavaria”. Journal of Paleontology, volume 74, issue 3. Pages 503-511.
Delsett, Lene L.; Friis, Henrik; Kölbl-Ebert, Martina; Hurum, Jorn H. (2022). “The soft tissue and skeletal anatomy of two Late Jurassic ichthyosaur specimens from the Solnhofen archipelago”. PeerJ, volume 10: e13173.
Fischer, Valentin; Bardet, Nathalie; Guiomar, Myette; Godefroit, Pascal (2014). “High Diversity in Cretaceous Ichthyosaurs from Europe Prior to Their Extinction”. PLOS ONE, volume 9, issue 1: 10.1371.
Fischer, Valentin; Clement, Arnaud; Guiomar, Myette; Godefroit, Pascal (2011). “The first definite record of a Valanginian ichthyosaur and its implications on the evolution of post-Liassic Ichthyosauria”. Cretaceous Research, volume 32, issue 2. Pages 155-163.
Fischer, Valentin; Maisch, Michael W.; Naish, Darren; Kosma, Ralf; Liston, Jeff; Joger, Ulrich; Krüger, Fritz J.; Pérez, Judith Pardo; Tainsh, Jessica; Appleby, Robert M. (2012). “New Ophthalmosaurid Ichthyosaurs from the European Lower Cretaceous Demonstrate Extensive Ichthyosaur Survival across the Jurassic–Cretaceous Boundary”. PLOS ONE, volume 7, issue 1: e29234.
Wagner, A. (1853). “Die Charakteristik einer neuen Art von Ichthyosaurus aus den lithographischen Schiefern und eines Zahnes von Polyptychodon aus dem Grünsandsteine von Kelheim”. Gelehrte Anzeigen der königlich bayerischen Akademie der Wissenschaften, volume 36, issue 3. Pages 25-32.
Wagner, A. (1861). “VIII. Ichthyosaurus leptospondylus”. Neue Beiträge zue Kenntniss der urweltlichen Fauna des lithographischen Schiefers. Pages 119-123, Table VI.
Hesperomys. “Ichthyosaurus leptospondylus Wagner, 1853:26 (N#70944)”. http://hesperomys.com/n/70944. Accessed on January 22, 2023.