Paleoecology of the Liberty Formation (Cincinnatian, Upper Ordovician) of Southeastern Indiana

College of Wooster Invertebrate Paleontology Class Investigation, Fall 2000

The Liberty Formation exposed at Stop #2 of the September 2000 Invertebrate Paleontology field trip. It looks very much like the Waynesville Formation in outcrop because of its high claystone content. See the project pages for the underlying Waynesville and overlying Whitewater Formations.

The Liberty Formation Student Team

Josh Peters is a junior geology major from Mansfield, Ohio. His responsibilities include the Liberty trilobites and trace fossils.

Sarah Gaudio is a sophomore from Chagrin Falls, Ohio. She is working on the Liberty orthid and rhynchonellid brachiopods.

Brian Sites is a sophomore from Columbus, Ohio. His duties include the Liberty bryozoans.

Tom Johnson is a senior archaeology major from Wilmerding, Pennsylvania. He is assessing the Liberty strophomenid brachiopods.

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The Liberty Strophomenid Brachiopods
(by Tom Johnson)

This is an interior view of Strophomena planumbona. This an articulate brachiopod that shows the characteristic shape of the muscle impressions on the interior surface of the pedicle valve.
 In Ordovician remains, strophomenid brachiopods are the most abundant, especially in the Liberty formation. Though there are many strophomenid fossils in the Liberty formation, they are also found in the Whitewater and Waynesville formations where other more diverse brachiopods are found. Brachiopods have survived from the Early Cambrian to the present and live in exclusively marine to brackish waters, but due to the Permian extinction, strophomenids slowly declined until their extinction in the Triassic. They are solitary animals that secrete a two-valved shell around their bodies and lophophore. A lophophore is a feather-like structure bearing tentacles for filter feeding, found also in bryozoans.

The Phylum Brachiopoda is subdivided into two classes, Inarticulata and Articulata. Inarticulate brachiopod shells consist of two valves, a pedicle and brachial valve, which are held together by adductor and diductor muscles. The valves usually exhibit growth lines, which are concentric around the beak, the initial point of origin of the valve. Articulate brachiopods also have two valves, a pedicle and brachial valve, which are composed of calcite. But unlike the Inarticulates, they have a hinge along one side of the paired valves. The hinge line can be very short or the widest part of the shell. Along this hinging mechanism, postlike teeth occur on the pedicle valve that fit securely into sockets on the brachial valve. This allows the valves to be opened around the hinge but prevents the valves from becoming misaligned. The pedicle of the brachiopod extends out of the pedicle opening, which is in the center of the hinge. With the pedicle, the animal attaches to a substrate and is able to rotate. Brachiopod shell shape is variable and ranges from almost circular to long hinge shield-shaped, flattened forms. They can be biconvex, plano-convex, concavo-convex, convexo-plane, or convexo-concave, all of which normally have one valve larger than the other. The interior of the shells consist of a wide variety of features, such as adductor and diductor scars and pits for attatching the adductor and diductor muscles, which are used for opening and closing the valves. Also inside the shell is the lophophore, which is used for obtaining food, and a complex set of structures known as the brachidium.

Interior view of Hebertella occidentalis. Also an articulate brachiopod showing the interior of the pedicle valve and the central muscle opening.

Exterior view of Leptaena richmondensis. This is an articulate brachiopod that is distinguished by the irregular concentric wrinkles and by the sharp flexture in the shell near the anterior margin.
 All brachiopod samples in the Liberty formation are preserved as original hard parts that show much detail. Abundantly found in the Liberty are Leptaena richmondensis and Rafinesquina ponderosa, whereas Strophomena planumbona, Hebertella occidentalis, and Strophomena vetusta are commonly found. Rarely found in the collection are Zygospira modesta, Glyptorthis insculpta, and Plaesiomys subquadrata. Strophomenids were sessile benthic epifaunal filter feeders, that rested on top of the sea floor. They obtained their food by filtering the water, which was clear and full of nutrients, as it went by, and rested on the substrate with their dorsal valve facing up.

 

The Liberty Bryozoans
(by Brian Sites)

Parvohallopora ramosa as viewed with the camera perpendicular to the organisms actual vertical axis. The distinguishing characteristic to note here is the sharply raised mounds called maculae.
 Phylum Bryozoa are as old as the Paleozoic and have been successful enough through the paths of evolution to exist today. They consist of individual organisms called zooids that live in massive colonies called zoarium. Each zooid serves an individual purpose that may be unique to the colony or may be what several other zooids in the same colony do. There are several types of zooids: autozooids, gonozooids, avicularia, vibracularia and kenozooids. The autozooids are responsible for feeding, the gonozooids are responsible for reproduction, the avicularium defends the zoarium by biting predators, the vibraculum protects the zoarium from mud and silt deposits that would hamper feeding and reproductive processes by spinning and generating current and the kenozooid allows the zoarium to expand and form additional skeleton. Because they contain several individual housings of zooids (called zooecia) they appear to be perforated and sometimes look coral-like.

Bryozoans are relatives of Brachiopods as they too have lophophores (used for feeding). They are, except for one freshwater family, entirely marine and only exist in colonies. They prefer clear water i.e. low energy environments and are filter feeding organisms.

All Bryozoans represented in this collection are in the form of unaltered hard parts. The most common members of the Phylum Bryozoa that are represented here are Batostomella gracilis and Parvohallopora ramosa. Batostomella forms large, branching colonies and have irregularly shaped apertures. They have monticules, which are spaced unevenly and are sparse when present. Parvohallopora also form branching colonies. Their apertures are oval-shaped and smooth as well as evenly spaced. They too have monticules which are more prominent than those of Batostomella which aids in identification.

This is perhaps the most visually interesting of the bryozoans, Constellaria. The name refers to the star-shaped markings that clearly give its identification away. This is an external view showing the top of the organism.

This is the most common of the bryozoans found at the Liberty formation, Batostomella gracilis; this species is characterized by branches with small angles of separation. The camera is perpendicular to the organism's vertical axis.
 A relatively rare group is Constellaria of the Suborder Fistuliporina. They are very distinct with star-shaped monticules making them easy to identify. They, like the previous two, form branching colonies. Another characteristic of this genus that aids in identification is their lack of lunarium (a rim on one side of an aperture). The last group represented in this collection is Homotrypella. Homotrypella differ from the other three in that they form round, tear-shaped colonies. There apertures are circular. They are present but neither abundant nor rare. All Bryozoans, as mentioned, are epifaunal filter feeders.

 

The Liberty Trilobites and Trace Fossils
(by Josh Peters)

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The Liberty Orthid and Rhynchonellid Brachiopods
(by Sarah Gaudio)

The articulate brachiopod specimen to the left of the photograph shows the dorsal view of Rhynchonellid, Hiscobeccus capax. Note the more conspicuous pedicle opening and the astrophic hinge line. The Hiscobeccus capax specimen to the right of the photograph shows a zigzag commisure and the identifying three strong plications in the sulcus.
 The Phylum Brachiopoda, or "arm-foot" is common in the marine or brackish waters of the Ordovician. Articulate brachiopods are defined as sessile benthic epifaunal filter-feeders. The Orders Orthida, Rhynchonellida, and Spiriferida are members of the class Articulata. Articulate brachiopods have simple musculature, and developed hinge socket that holds the brachiopod's two calcitic valves together. The valves in articulates are operated by diductor and adductor muscles. Diductor muscles pull at the hinge of the brachiopod that allows the valves to open, while the adductor muscles close the valves with their contraction. The soft part morphology of articulates contains a digestive system that surprisingly lacks an anus. The lophophore is the brachiopod's organ for feeding. It is a fringed fleshy organ that creates a current bringing nutrient rich water to the mouth of the brachiopod. The lophophore is attached to its dorsal valve, and is supported by a calcitic structure called a brachidium.

The Order Orthida is the most primitive of all brachiopod orders with a fossil record that extends back into the Cambrian. The warm shallow seas of the Ordovician teamed with most of the 340 know species of orthids. Their numbers were drastically reduced by an extinction at the end of the Ordovician yet they lingered, in greatly decreased abundance, until their final extinction at the end of the Permian. Orthids are characterized by their highly strophic hinge lines, an unequally biconvex shell that can be costate or plicate. They also have a strong pedicle for gripping onto a hard substrate. Once fixed onto the substrate the orthids remain there for the duration of their life unless involuntarily moved. As sessile benthic filter-feeders utilizing currents to direct the best possible flow of sediment-rich water to their lophophore, the orthids flourished in the Liberty formation, located just before the wave base. The orthids identified at the Liberty formation are, Platystrophia cypha, Platystrophia clarksvillensis, and Platystrophia moritura, and Platystrophia acutilirata.

This articulate spiriferid, Zygospira modesta, is characterized by an astrophic hinge line, small size and more plications than its less common relative, Zygospira cinncinnatiensis. The spiriferid brachiopods are also defined by the presence of a spiral brachidium supporting their lophophore. This internal feature distinguishes spiriferids from all other brachiopod orders.

Anterior view of articulate brachiopod fossil, Platystrophia cypha, with dorsal shell facing upward. Note the presence of strong plicae, and prominent fold and sulcus. This specimen though partially hidden, also demonstrates a zigzag commisure and highly strophic hinge line.

Rhynchonellid brachiopods are one of two articulate orders that have survived into the present; however have demonstrated limited evolutionary changes to their morphology. The shell morphology of a the rhynchonellid is characterized by a highly astrophic hinge line(The name rhynchonellid originates from the nose-like appearance of the hinge. The root of Order Rhynchonellida, rhyno-- means nose in Greek), strong plications, and a zig-zag commisure with a deep fold and sulcus. They also have a pedicle indicating that this order inhabited environments with a hard substrate. On this substrate the rhynchonellids perform their life mode as sessile benthic epifaunal filter-feeders. The zig-zag commisure prevents larger grains of slit from entering the interior of the shell and inhibiting respiration. The rhynchonellids identified at the Liberty formation are Hiscobeccus capax, and Rynchotrema dentatum.

One species of articulate brachiopod Order Spiriferida, is also present and abundant in the Liberty formation. Zygospira modesta, demonstrates convergence with he orthids of the formation in exterior shell morphology. This species is a member of the Suborder Atrypidina whose external morphology is very different from another common spiriferid, Suborder Spiriferidina. Suborder Atrypidina is defined by a plano-convex shell shape with an astrophic hinge line. Internal morphology containing a spiral brachidium separates spiriferids from the orthids and rhynchonellids. This internal feature is the source of the great difficulty paleontologists have with identifying some brachiopods, particularly those only partially preserved. The brachidium is not always preserved, and sometimes the dorsal valve is the only valve avalible for identification. The spiriferids have demonstrated evolutionary convergence with nearly all the brachiopod orders and are easily mistaken as any a member of any order because of the similar external morphological features.

 

The Liberty Formation Paleoecology
(by the Liberty Team)

The paleoecology of the Liberty formation is defined by the relationship between the sessile benthic epifaunal filter-feeders and the vagrant benthic deposit-feeders. The formation is characterized as a shallow marine environment approximately at the wavebase. It is evident that the formation was contained within the photic zone because of the presence of cndarians who exist symbiotically with zooxanthellae. The Whitewater formation has abundant corals while in the Waynesville formation, corals are absent. The Liberty formation also has a hard substrate with a mud layer, thick enough to support strophomenid brachiopod populations. The presence of a thin sedimentary mud layer on the substrate indicates that the wave motion and currents of the formation produce an intermediate energy area. The energy of water motion is not high enough to produce a coarse substrate or cause the erosion of biogenic remnants, like Whitewater, but not low enough to allow the formation of thick muddy clays, like the Waynesville. The abundance of brachiopods and bryozoans supports this assertion because a moderate current is advantageous for their filter-feeding. The moderate energy levels and therefore sedimentary built up correlated to the abundance of orthid and rhynchonellid brachiopods that require a harder substrate for pedicle attachment. Although the Liberty formation sustains a diversity of brachiopod species, another group of sessile benthic epifaunal filter-feeders, the bivalves, was less successful. Because of predation by nautiloid cephalopods and invasion of the ecological niche by the brachiopods, only one genus of bivalves can be found in this formation; however, Ambonychia was absent form our collection. The ecosystem of the Liberty formation also supported gastropods (vagrant benthic epifaunal herbivores) and arthropods (vagrant benthic epifaunal deposit-feeders). In this formation the abundance of trilobites indicates that the arthropods have been more successful in developing within the ecosystem. The marine environment of the Liberty formation is best categorized as a carbonate ramp, located stratigraphically between the nearshore Whitewater formation, and the more offshore Waynesville formation.