Wooster Geologists

Add this to the list of fossils that have confused me. This summer, during a Wooster expedition, Lizzie Reinthal and Steph Bosch collected the above specimen from the Matmor Formation (Middle Jurassic, Callovian) of southern Israel. I simply assumed it was an ammonite, especially because we were anxious to find ammonites to further reinforce our biostratigraphic framework (how we tell in which geological time interval our fossils belong). When I later tried to identify it by searching through the Jurassic ammonite literature, though, I could find nothing like it. I then sent a photograph to my friend Zeev Lewy, a prominent ammonite expert recently retired from the Geological Survey of Israel. His answer was a surprise: this fossil is the gastropod Discohelix tunisiensis Cox 1969.
How could this be a snail when it looks so much like a cool, multi-whorled planispiral ammonite, complete with ribs? Well, it is not planispiral, now that I look at it again. Above you see the other side of the specimen, with its slightly depressed center. Most ammonites don’t show such asymmetry. This actually is a gastropod, and it represents an ancient group (the clade Vetigastropoda — don’t get me started on the complications of gastropod systematics!) with primitive features reminiscent of Paleozoic marine snails (from a group I learned to call “archaeogastropods“). It is not as much that the snail has converged on an ammonite style of shell, it’s that the ammonites developed a similar shell much later for entirely different reasons (swimming, for example). Discohelix was likely an herbivore grazing in patchy coral reefs like we have represented in the Matmor Formation. It has become a useful index fossil for the Jurassic of the Tethyan Realm, although this is the first time I’ve found it in Israel.
The above is the marine snail Pseudotorinia (Architae-group) retifera. It used to be called Discohelix retifera, and you can see why. It may not be in the same genus, but you can see that this modern group and Discohelix are closely related. Discohelix itself is now known only from the fossil record.
Discohelix was named as a genus in 1847 by Wilhelm Bernhard Rudolph Hadrian Dunker (1809-1885), a German natural scientist with interests in geology, paleontology and marine zoology. (I love that middle name of “Hadrian”.) Like so many 19th Century paleontologists, Dunker started with a practical training in mining engineering and then followed a passion for fossils and modern shells. He had a huge collection of materials that eventually ended up in the Museum für Naturkunde in Berlin. He traded and corresponded with many top scientists of his day, including Charles Darwin. He also published many monographs on modern and fossil molluscan taxa. In 1846, he and Hermann von Meyer established the journal Palaeontographica. This journal survives to this day in two descendants: Palaeontographica A (Paleozoology, Stratigraphy) and Palaeontographica B (Paleobotany).

References:

Cox, L.R. 1969. Gasteropodes Jurassiques du Sud-Est Tunisien [Jurassic gastropods from SE Tunisia]. Annales de Paleontologie, Invertebres 55: 241-268.

Grundel, J. 2005. The genus Discohelix Dunker, 1847 (Gastropoda) and on the content of the Discohelicidae Schroder, 1995. Neues Jahrbuch fur Geologie und Palaontologie-Monatshefte 12: 729-748.

Tëmkin, I., Glaubrecht, M. and Köhler, F. 2009. Wilhelm Dunker, his collection, and pteriid systematics. Malacologia 51: 39-79.

Wendt, J.1968. Discohelix (Archaeogastropoda, Euomphalacea) as an index fossil in the Tethyan Jurassic. Palaeontology 11: 554-575.

From the view above, this fossil from the Matmor Formation (Middle Jurassic, Callovian) of southern Israel looks like your standard echinoid (a group that contains sea urchins and sand dollars), but turn it on its side (see below) and you see it is unusual. Echinoids have two large categories: those that are globular in shape (like sea urchins) are called “regular“, and those that are flattened (like sand dollars) are “irregular“. (I know, oddly value-laden terms, these.) This specimen belongs to a group that is rounded on its top portion and flattened on its bottom (oral) surface. This between-ness makes it a fun little specimen.
I could not identify this echinoid, which we collected on our Israel expedition this summer, because I could not find the most important diagnostic features. Fortunately my colleague Andrew Smith, recently retired from the Natural History Museum in London, quickly knew what it was. (This is not surprising — he’s the world’s expert on fossil echinoids. Check out his incredible Echinoid Directory.) Andrew identified this specimen as belonging to the genus Holectypus Desor, 1842, and probably the species Holectypus depressus (Leske, 1778).
The first feature Andrew noticed was the apical disk on the very top of the test (the term for an echinoid skeleton). In the above image (where the black scale bar = 200 microns) I’ve labelled the four gonopores (where gametes exit, as you might have guessed) and the madreporite (a sieve plate at the opening of the water vascular system). This arrangement is characteristic of the genus.
Most surprising to me was Andrew’s identification of the most obvious defining feature of Holectypus, the periproct (the anal opening). I couldn’t find it, but Andrew knew where to look. In this view of the oral surface, it is the gap labeled “P”. Looks just like a place where the test is broken, right?
Here is the oral surface of an unbroken Holectypus specimen from the Callovian of France. The large periproct is immediately visible as the whole at the bottom. Now the broken margin of the periproct on our specimen makes sense.

Holectypus belongs to a group of irregular echinoids still around today. They are sometimes characterized as having “conservative” evolution, meaning they have not changed much over long periods. The irregular echinoids appeared earlier in the Jurassic as a modification of their regular ancestors. They became flattened and bilateral, the periproct moved out of the apical disk, their ambulacra (rows of tube feet visible as tiny holes radiating from the apical disk on the top image) pulled up away from the mouth, and their spines were reduced in size and increased in number. These were primarily adaptations for burrowing into the sediment. Holectypus has retained its inflated upper portion, has relatively large spines (some still cling to our specimen), and still is circular in outline. It was a deposit feeder but not specialized for burrowing.

We wouldn’t want to call this a “transitional fossil”, but it is a nice example of the gradient of adaptations present when there is a major outbreak of innovation as during the rise of the irregular echinoids in the Jurassic.

References:

Kroh, A. and Smith, A.B. 2010. The phylogeny and classification of post-Palaeozoic echinoids. Journal of Systematic Palaeontology 8: 147-212.

Rose, E.P.F. and Olver, J.B.S. 1985. Slow evolution in the Holectypidae, a family of primitive irregular echinoids, p. 81-89. In: Keegan, B.F. and O’Connor, B.D.S. (eds.), Proceedings of the Fifth International Echinoderm Conference, Galway, 24-29 September, 1984.

Saucède, T., Mooi, R. and David, B. 2007. Phylogeny and origin of Jurassic irregular echinoids (Echinodermata: Echinoidea). Geological Magazine 144: 333-359.