Wooster Geologists

Our fossil this week is one I don’t share with my Invertebrate Paleontology classes until they’re ready for it. Those of us who grew up with Paleozoic fossils think we recognize it right away. Surely this is a solitary rugose coral? It has the right shape and the fine growth lines we call rugae (think “wrinkles”). This view below of the oral surface is not surprising either, unless you’re an enthusiast of septal arrangements.
Instead of a rugose coral, though, this is a scleractinian coral from the Matmor Formation (Middle Jurassic, Callovian) of Hamakhtesh Hagadol, Israel. It is part of the collection of Matmor corals Annette Hilton (’17) and I are working through. This coral belongs to the genus Axosmilia Milne Edwards, 1848.
These corals are excellent examples of evolutionary convergence. The scleractinians are only very distantly related to the rugosans. They do not share a common ancestor with a calcareous skeleton, let alone a cone-shaped one like this. Instead the scleractinians like Axosmilia developed a skeleton very similar to that of the solitary rugosans, probably because they had similar life modes in similar environments, and thus similar selective forces. The rugosans, though, built their skeletons out of the mineral calcite, whereas the scleractinians use aragonite. (This specimens are calcite-replaced, like our specimen last week.) The vertical septa inside the cone are also arranged in different manners. Rugosans insert them in cycles of four (more or less), giving them a common name “tetracorals”; scleractinians have septal insertions in cycles of six, hence they are “hexacorals”. Rugose corals went extinct in the Permian; scleractinians are still with us today. Our friend Axosmillia appeared in the Jurassic and went extinct in the Cretaceous.

Rugose coral skeletons in the Paleozoic are commonly encrusted with a variety of skeletal organisms, and many are bored to some degree. I expected to see the same sclerobionts with these Jurassic equivalents, but they are clean and unbored. I suspect this means they lived semi-infaunally (meaning partially buried in the sediment).
Axosmilia was named by the English-French zoologist Henri Milne-Edwards (1800-1885) in the politically complex year of 1848. Henri was the twenty-seventh (!) child of an English planter from Jamaica and a Frenchwoman. He was born in Bruges, which is now part of Belgium but was then under the control of revolutionary France. Like many early 19th century scientists, Milne Edwards earned an MD degree but was seduced away from medicine by the wonders of natural history. He was a student of the most accomplished scientist of his time, Georges Cuvier, and quickly became a published expert on an amazing range of organisms, from crustaceans to lizards. The bulk of his career was spent at the Muséum National d’Histoire Naturelle in Paris. When he was 42 he was elected a foreign member of the Royal Society, receiving from them the prestigious Copley Medal in 1856. He died in Paris at the age of 85.

References:

Fürsich, F.T. and Werner, W. 1991. Palaeoecology of coralline sponge-coral meadows from the Upper Jurassic of Portugal. Paläontologische Zeitschrift 65: 35-69.

Martin-Garin, B., Lathuilière, B. and Geister, J. 2012. The shifting biogeography of reef corals during the Oxfordian (Late Jurassic). A climatic control?. Palaeogeography, Palaeoclimatology, Palaeoecology 365: 136-153.

Pandey, D.K., Ahmad, F. and Fürsich, F.T. 2000. Middle Jurassic scleractinian corals from northwestern Jordan. Beringeria 27: 3-29.

Pandey, D.K. and Fürsich, F.T. 2005. Jurassic corals from southern Tunisia. Zitteliana 45: 3-34.

This week’s fossil is in honor of Annette Hilton (’17), who is my Sophomore Research Assistant this year. She has been diligently working through a large and difficult collection of scleractinian corals from the Matmor Formation (Middle Jurassic, Callovian) of Hamakhtesh Hagadol, Israel. These specimens were collected as parts of many paleoecological studies in our Wooster paleontology lab, so I thought it was time they received some systematic attention on their own. I knew it would be difficult, but Annette was up to the task and has done a splendid job.

The above specimen is a scleractinian coral of the genus Amphiastrea Étallon, 1859. It was collected from locality C/W-227 in the makhtesh. Considering the original was aragonite, it is remarkably preserved in a calcitized version. The large disks stuck to it are encrusting bivalves, probably of the genus Atreta.
Here we see the reverse with more encrusters. It is apparent that this cylindrical specimen was encrusted on all sides while it was in its erect living position, or this piece rolled around loose on the seafloor for an extended interval.
One of the encrusting bivalves was itself encrusted by a serpulid worm, which left part of its twisty calcitic tube behind.
This thin, ghostly encruster is probably the bivalve Plicatula.
A close view of the corallites shows how well preserved they are on the surface of the coral. Each of these pits shows the vertical septa (walls of a sort) that were underneath the coral polyps in life. Despite this beautiful outer preservation, the interior of the specimen is mostly occupied by blocky calcite crystals.

This coral was found in a marly sediment, which explains why it is not locked into a solid piece of limestone as many Jurassic corals are. Amphiastrea apparently preferred environments with a significant amount of siliciclastic sediment (see Pandey and Fürsich, 2001, and other references below). I hope my students and I can further study this diverse and abundant coral fauna in the Matmor Formation. Annette Hilton has prepared the way.

Claude Auguste Étallon (1826-1862) named the genus Amphiastrea in 1859. He was a prominent paleontologist and geologist in his time. He was only 35 years old when he died, though, and has almost completely dropped out of the literature in English, except for the numerous invertebrate taxa he named. (There is a kind of immortality in our system of adding author’s names to taxa.) Using my Google Translator skills, I can read in the French literature that he was born to “an honest merchant family” in Luxeuil, France. He was a mathematics teacher first at collège Paul Féval à Dol-de-Bretagne and then later several other institutions. He developed a specialty in the rocks and fossils of the local Jurassic. Étallon created and published a geological map (“Carte géologique des Environs de St. Claude”), which was quite advanced for the time. The Late Jurassic turtle Plesiochelys etalloni was named after him in 1857. Auguste Étallon died suddenly of “the rupture of an aneurysm after two days of a slight indisposition” in February 1862.

Here’s to the memory of the energetic, productive and too short-lived Auguste Étallon.

References:

d’Amat, R. 1975. Étallon, Claude Auguste. Dictionnaire de Biographie Française 13: 163-164.

Löser, H. 2012. Revision of the Amphiastraeidae from the Monti D’Ocre area (Scleractinia; Early Cretaceous). Rivista Italiana di Paleontologia e Stratigrafia 118: 461-469.

Pandey, D.K., Ahmad, F. and Fürsich, F.T. 2000. Middle Jurassic scleractinian corals from northwestern Jordan. Beringeria 27: 3-29.

Pandey, D.K. and Fürsich, F.T. 2001. Environmental distribution of scleractinian corals in the Jurassic of Kachchh, western India. Journal Geological Society of India 57: 479-495.

Pandey, D.K. and Fürsich, F.T. 2005. Jurassic corals from southern Tunisia. Zitteliana 45: 3-34.

Vinn, O. and Wilson, M.A. 2010. Sabellid-dominated shallow water calcareous polychaete tubeworm association from the equatorial Tethys Ocean (Matmor Formation, Middle Jurassic, Israel). Neues Jahrbuch für Geologie und Paläontologie 258: 31-38.

Wilson, M.A., Feldman, H.R., Bowen, J.C. and Avni, Y. 2008. A new equatorial, very shallow marine sclerozoan fauna from the Middle Jurassic (late Callovian) of southern Israel. Palaeogeography, Palaeoclimatology, Palaeoecology 263: 24-29.

Wilson, M.A., Feldman, H.R. and Krivicich, E.B. 2010. Bioerosion in an equatorial Middle Jurassic coral-sponge reef community (Callovian, Matmor Formation, southern Israel). Palaeogeography, Palaeoclimatology, Palaeoecology 289: 93-101.