Crinoid success

May 25th, 2011

Will Cary collecting crinoid pieces at a site we creatively call "GPS 055". In the upper left you can see a triangular exposure of marl where Jeff Bowen did his Independent Study work in 2005.

MITZPE RAMON, ISRAEL–One of our missions on this expedition to Israel is to find more and better examples of a distinctive crinoid in the Middle Jurassic Matmor Formation. Crinoids are stemmed echinoderms with a very long geological history, dating back to the Ordovician (or Cambrian, depending on who you believe). They are still alive today so we know much about their biology. They usually have long stems with a holdfast on one end (attaching it to the substrate) and a calyx on the other containing most of the body. The calyx has feathery arms attached at the top that filter the water to catch fine-grained organic particles and pass them down to a central mouth.

Parts of the Matmor Formation have abundant crinoid fragments, all belonging to at least two types of Apiocrinites (a crinoid genus). Two years ago I collected some beautiful specimens, but still lacked some critical pieces. Today Will and I revisited my earlier localities (thank you, GPS technology) and found beautiful specimens.

Our prize is the holdfast pictured above. This is a mass of skeletal calcite the crinoid used to glue itself to the bottom of a coral. The shallow pits apparently represent additional “roots” it used to brace itself in a cavity under the coral. The stem then horizontally protrudes to the right so that the calyx and feeding arms could eventually reach the open seawater. I’ve never seen a holdfast this elaborate in the Jurassic.

Above are typical other parts of this Jurassic crinoid (imaged with all my hotel room photographic skills). At the top are two calyx side pieces showing the interior (left) and exterior (right). The star-shaped object in the middle is the calyx base, seen from the inside. It is flanked by stem fragments, the one on the far right encrusted by an oyster. At the bottom is a crinoid stem with a branching holdfast of another crinoid attached to it.

Mission accomplished as far as the crinoids go!

First field day: Makhtesh Gadol (A large bowl of geological delights)

May 23rd, 2011

MITZPE RAMON, ISRAEL–Today Will Cary, Yoav Avni (our friend from the Geological Survey of Israel) and I worked in the northern end of Makhtesh Gadol (“the large crater”). This geomorphic feature looks a bit like an oblong impact crater, but it is actually a kind of breached anticline known as a makhtesh.

Makhtesh Gadol from Google Maps.

We are interested in the Matmor Formation, a series of Middle Jurassic marls and limestones in the center of the structure. Our special interest is a fossiliferous unit in the Matmor Formation that is found throughout the exposure. It is very rich in crinoids, echinoids, corals and sponges, with a few brachiopods, ammonites and bivalves as well. We want to understand the distribution of this unit and its fossils.

Yoav Avni and Will Cary marching through the Matmor Formation.

If we saw this formation in only two dimensions, as in a typical roadcut, it would be easy to interpret. However, we have it exposed in 3-D because it is heavily dissected by small wadis. More data this way, and far more complications. We learned today that there are distinct facies (rock types characterized by fossils and/or sediments indicating a particular depositional environment) found in very close relationships. The rock units are patchy and the fossils patchy within the lithological patchiness. The number of variables used to predict fossil occurrences is now very large!

All these facies are laterally equivalent in a very small space.

One of the many scleractinian corals in the Matmor Formation. These corals were originally aragonitic and are now replaced by calcite. The replacement process was unusually fine-grained here.

Wooster’s Fossil of the Week: A scleractinian coral (Middle Jurassic of Israel)

May 15th, 2011

In advance of my next field trip to Israel (watch this space!), our highlighted fossil this week is the scleractinian coral Microsolena, a genus named by the French naturalist Jean Vincent Félix Lamouroux in 1821. The specimen above was collected from the Matmor Formation in Hamakhtesh Hagadol in the Negev Desert. It is Callovian in age, specifically the athleta Zone. (I know a lot of details about this area!) This coral is thus roughly 160-165 million years old.

Scleractinian corals appeared first in the Triassic and are the primary coral in today’s oceans. Unlike their extinct Paleozoic cousins, scleractinians have skeletons made of aragonite rather than calcite. Aragonite is relatively unstable and easily dissolves over geological time. Our specimen above has been replaced with the more stable calcite. This means that the exterior is preserved well enough to identify to the genus level, but details in the interior necessary for species determination have been recrystallized beyond recognition.

A nice oyster is still attached to the coral surface. Oyster shells are made of calcite and so are usually preserved very well. You can also see holes in the coral made by boring bivalves and given the name Gastrochaenolites. One of the bivalve borings is in a raised lump of the coral (center top of the image). This is reaction tissue built by the coral in response to the invading bivalve, a clear indication that some of the boring took place while the coral was alive. Most of the corals in the Matmor Formation are heavily bored by bivalves.

Field view of cross-sections of bivalve borings (some with bivalve shells still in them) in a scleractinian coral in the Matmor Formation.

The Matmor Formation is exposed only in the cavity of Hamakhtesh Hagadol. Here it is about 100 meters thick and consists mostly fossiliferous marls and sponge-coral patch reefs. (One of the previous Fossils of the Week is a thecideide brachiopod attached to corals like the one above.) The Matmor sediments were deposited on a shallow marine ramp near the Middle Jurassic equator. It is this equatorial deposition that makes the Matmor such an interesting subject for paleoecological analysis. Most other described Jurassic faunas are in Europe and North America, and they were all formed under more temperate conditions.

Fossil patch reef exposed in the Matmor Formation.

References:

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

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.

Wooster’s Fossil of the Week: Oyster balls! (Middle Jurassic of Utah)

April 17th, 2011

The technical term is ostreolith, but “oyster ball” is much more descriptive. These fossils are found by the thousands in the Carmel Formation (Middle Jurassic) in southwestern Utah. As far as I know, this is the only place they’ve ever been found. Colin Ozanne (’96) worked on these ostreoliths as part of his Independent Study project, and the results of our work were published in a 1998 issue of Palaios. Colin now, by the way, is an Engineer Trial Attorney for the US Army Corps of Engineers in Buffalo, New York.

Ostreoliths are “circumrotatory accumulations” of the little oyster Liostrea strigilecula. The most common form began with a clam shell fragment as a nucleus. Oyster larvae recruited on the top shell surface and grew in the normal way. A current, though, flipped the shell over, exposing the underside that was in turn encrusted by more oyster larvae. These grew into larger oysters until, again, the shell flipped back over. A new generation of oysters then encrusted the older layer. The shell then overturned again and … you get the idea. Some ostreoliths grew this way to almost a quarter meter in diameter. The cup-shaped left valve of Liostrea was an essential feature for ostreolith development. A typical flat oyster would not build the necessary depth with each layer.

Polished section through the middle of an ostreolith showing the curved nucleus shell and calcite-filled bivalve borings.

Closer view of oysters on the surface of an ostreolith. Note how juvenile oysters are clustered within the left valves of an older generation.

Several sclerobionts (hard substrate dwellers) grew with the oysters on the ostreoliths, including the bivalve Plicatula, disciniscid brachiopods and cyclostome bryozoans. Mytilid bivalves also drilled holes (called Gastrochaenolites) in the oyster skeletons to form cavities for their filter feeding.

Ostreoliths, strange and unique as they are, tell us a lot about the depositional environment of the Carmel Formation. The sediments accumulated in these horizons under fairly high energy with periodic storm disturbances. The mytilid borings trapped ooids during formation of the ostreoliths, showing that this characteristic carbonate sediment was more common in the environment than indicated by the rocks alone.

Carmel Formation exposed at Gunlock Reservoir near St. George, Utah.

Regardless of their scientific value, though, oyster balls certainly start good conversations!

Wooster’s Fossil of the Week: A little brachiopod gets a name (Middle Jurassic of southern Israel)

April 3rd, 2011

Moorellina negevensis Krawczyński & Wilson 2011; 1a – general view of the dorsal valve interior; 1b – oblique view showing brachial cavities and cardinalia.

This week our fossil star is a new brachiopod species a Polish colleague (Cezary Krawczyński — a brachiopod expert) and I described in this March 2011 paper:

The first Jurassic thecideide brachiopods from the Middle East: A new species of Moorellina from the Upper Callovian of Hamakhtesh Hagadol, southern Israel. Acta Geologica Polonica, Vol. 61, No. 1, p. 71-77. [Free pdf available on that site.]

These tiny shells of Moorellina negevensis encrust corals and sponges in the Matmor Formation (Lamberti Zone, Upper Callovian, Middle Jurassic) in the Negev Desert of southern Israel.  (Our species name means “from the Negev”.) They are prominent members of a diverse sclerobiont assemblage including tubeworms, oysters, bryozoans and various borings. Several specimens were collected over the past few years of our Wooster work in Israel. Wooster student Will Cary and I will return to these outcrops in Israel this summer for further Jurassic work.

Moorellina negevensis is among the smallest of adult brachiopods, averaging only about two millimeters in width. It is the first species of the Order Thecideida found in the Jurassic of the Middle East. No doubt it escaped previous notice because it is so tiny!

One of our specimens has a gall-like structure that we believe was likely made by a ascothoracid parasite in the shell. The ascothoracids are tiny crustaceans usually found as parasites in echinoderms and cnidarians.

Parasitic (ascothoracid?) infestation in the dorsal valve interior of Moorellina negevensis; A – interior of the dorsal valve of Moorellina negevensis with parasitic (ascothoracid?) infestation marked in red; B – enlargement of parasitic infestation, posterior-lateral view; C – Synagoga paucisetosa Grygier, 1990, a recent ascothoracid parasite (redrawn from Grygier 1990, slightly modified); D – recent ophiuroid Ophiocten sericeum (Forbes, 1852) with the genital bursae infested by Ascothorax ophioctenis Djakonov, 1914 (redrawn from Wagin 1946, slightly modified).

So a little fossil with the surprise of an even smaller fossil inside!

Matmor Formation exposed in the Matmor Hills, Hamakhtesh Hagadol, Negev Desert, southern Israel. Type locality for Moorellina negevensis. This kind of outcrop is heaven for paleontologists and sedimentary geologists. It is a beautiful desert setting.

Back to granite on Cima Dome

March 17th, 2011

A granite exposure near Teutonia Peak on Cima Dome. Note our jackets and hands in pockets!

ZZYZX, CALIFORNIA–Our last stop of the rapidly-cooling day was on the huge Cima Dome east of Zzyzx in the Mojave National Preserve. The dome is so large (about 70 square miles) that it is impossible to detect when you are actually on it, but easily visible from miles away. It apparently is the eroded root of a granitic intrusion formed during subduction in the Jurassic to Cretaceous. The alkali granite exposed here is very similar to that of the Granite Mountains we saw yesterday.

Potassium feldspar crystals in the coarse alkali granite of Cima Dome.

The soil of Cima Dome is derived almost entirely from the underlying alkali granite.

Wooster Geologists return to the Mojave Desert

March 13th, 2011

ZZYZX, CALIFORNIA–All four geology faculty members, our administrative coordinator Patrice Reeder, Jesse Wiles and eight students have safely arrived at the Desert Studies Center in the delightful Zzyzx.  We spent a few hours exploring the Jurassic sandstones exposed in the Red Rocks Conservation Area outside Las Vegas (with marvelous dune cross-bedding) and then slogged through horrible Los Angeles-bound traffic from Las Vegas to Baker, California.  (All too typical for a Sunday night here.)

Unfortunately bandwidth is highly restricted, so our posts will be infrequent until we return to Wooster.

Lichen and bits of desert varnish on the cross-bedded sandstone at the Red Rocks National Conservation Area.

Lindsey Bowman apparently saves Becky Alcorn with a dramatic backdrop of cross-bedding.

Wooster’s Fossil of the Week: A tiny sclerobiont community (Middle Jurassic of Poland)

March 6th, 2011

T=thecideide brachiopod; B=cyclostome bryozoans; S=sabellid worm tubes; g=gonozooids on one of the cyclostome bryozoans.

This delightful little community is the subject of a current research project that developed from Independent Study fieldwork in 2006. Elyse Zavar (’07) and I traveled to southern Poland to work on Jurassic fossils associated with a carbonate hardground at the Callovian-Oxfordian boundary near the village of Zalas (the link goes to Elyse’s photographs). With our Polish colleagues we found complex stratigraphy and an even more complex set of fossils. Now Elyse (a graduate student in the great state of Texas) and I have joined with Michał Zatoń of the University of Silesia in Sosnowiec, Poland, and submitted a manuscript describing and interpreting the sclerobionts (hard substrate dwellers) on the large limid bivalve Ctenostreon proboscideum. Michał is the lead author because of his long experience with these fossils and the complicated stratigraphy.

The hardground complex in the Zalas Quarry, southern Poland. It is Callovian below the blade of the well-traveled hammer and Oxfordian above (both stages of the Jurassic).

I have a soft spot for thecideide brachiopods, so they’re the stars of this show for me. I’ve come across them on Jurassic hard substrates many times, so they are old friends. They are filter-feeders like their larger brachiopod cousins, but they cemented one valve to a shell or rock for stability. The bryozoans in this community are also fun, especially when they have gonozooids as imaged above. A gonozooid is a specialized zooecium for brooding eggs and larvae. They are sometimes the only diagnostic features on sheet-like cyclostome bryozoans. There are plenty of serpulid and sabellid worm tubes as well, along with occasional oysters, calcareous sponges, and borings.

A sabellid worm tube and other encrusters.

We are comparing this Polish Jurassic community to others of the same time interval around the world. Curiously and probably not by chance, a similar sclerobiont assemblage is found in the Middle Jurassic of southern Israel — the subject of another Wooster study.

An encrusting oyster.

Wooster’s Fossil of the Week: The ‘osses ‘ed clam (Upper Jurassic of southwestern England)

February 27th, 2011

Katherine Nicholson Marenco (’03) and I did delightful fieldwork on the Isle of Portland in Dorset, England, during the summer of 2002. We collected fossils from the famous Portland Limestone (Upper Jurassic) in a series of working quarries. Katherine completed her Independent Study thesis on the topic (“Paleoecology of a cryptic shell-encrusting community: observations from ‘upside-down’ encrusters in internal molds (Upper Jurassic of southern England)”) and the next fall gave her first GSA talk about her findings. Now Katherine is a paleontologist at Bryn Mawr College.

One of the common fossils in the Portland Limestone is the trigonid bivalve Laevitrigonia gibbosa (J. Sowerby, 1819) pictured above. It is usually preserved as an internal mold, meaning that the aragonitic shell dissolved away, leaving the sediment fill. The Dorset quarrymen thought these fossils resembled horses’ heads (in their dialect this comes out as “‘osses ‘eds”) and the name stuck among collectors. If you hold the internal mold upside down, there is a vague resemblance.

Trigonid bivalve in an outcrop of the Portland Limestone. It is "butterflied" meaning that the valves were still attached to each other by the ligament.

Our scientific fascination with this fossil went beyond its preservation and paleobiology (it was a mobile infaunal suspension feeder). After the clam died and the soft parts rotted away, a variety of organisms settled on the inside of its shell. These sclerobionts are preserved upside-down on the internal mold.

Close-ups of the outside of the L. gibbosa internal mold showing "upside-down encrusters": an encrusting oyster Liostrea expansa (o), a colony of the cyclostome bryozoan Hyporosopora portlandica (b), and a serpulid worm (s).

Katherine was able to sort out the identity of these encrusters (not an easy task from the bottom!), their succession and their growth and development. Entire communities lived in these half-closed shells lying on a shallow seafloor 150 million years ago. An ordinary ‘osses ‘ed yields extraordinary paleontological detail.

Wooster’s Fossil of the Week: A brittle star trace fossil from the Jurassic of Utah

February 13th, 2011

This week we have a trace fossil that looks almost exactly like the animal that made it. A trace fossil is evidence of organism activity recorded in the rock record. The photograph above shows one of my favorite specimens: Asteriacites lumbricalis von Schlotheim 1820 from the Middle Jurassic (Bathonian) Carmel Formation in southwestern Utah. I collected it while doing fieldwork with Wooster student Steve Smail too long ago for either of us to mention.

This fossil was made when a brittle star (ophiuroid) burrowed into carbonate sediment to either hide from predators or to look for a bit of food. Brittle stars are echinoderms that appeared in the Ordovician and are still very much alive today (see below). This Jurassic trace was formed when a brittle star essentially vibrated its way down into the loose sediment in a manner many of their descendants do today. The result is what appears to be an impression of the body (an external mold) but is actually formed by action of the animal.

Green Brittle Star (Ophiarachna incrassata) courtesy of Neil at en.wikipedia.

The trace fossil Asteriacites is far more common in the rock record than the brittle stars and seastars that made it. These traces thus often indicate the occurrence of organisms in critical intervals where they would otherwise be unknown. For example, Asteriacites lumbricalis is found in Lower Triassic rocks showing that brittle stars were part of the recovery fauna after the Permo-Triassic Mass Extinction (see, for a Wooster example, Wilson & Rigby, 2000).

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