Evolutionary Trends – Daragh O Boyle 112502307

 

            The first graptolites appeared in the Upper Cambrian period in the form of the order Dendroidia. The Dendroids are the oldest Graptolites in the fossil record.(McNamara 2014) The Dendroids lived in colonies that were comprised of several individuals, zooids, which were connected to each other by a common nervous system.(www2) The Dendroids were mostly comprised of sessile benthic filter feeders that were attached to the sea floor by a root-like holdfast system. During the Upper Cambrian and the Lower Ordovician periods species from the order Dendroidia evolved into new species of free flowing planktonic filter feeders. These pelagic species of Dendroids then continued without much further development until their extinction in the Upper Carboniferous period. The pelagic Dendroids are believed to have been a key evolutionary step in the formation of the most diverse and successful Graptolites, the order Graptoloidia.(Rickards 1978)

            The Graptoloids first appeared during the Lower Ordovician period and were by far the most successful, diverse and widespread order of Graptolites. The Graptoloids used the ocean currents to spread to new areas around the world and today their fossils can be found on every continent except Antarctica.(McNamara 2014) The Graptoloids went through a period of rapid evolution which started in the Lower Ordovician period and continued until the Upper Silurian period. This time of rapid evolution allowed the Graptoloids to quickly adapt to new niches and habitats and was a key factor in their global success.(Rickards 1978)

            Each graptolite colony, known as a rhabdosome has a variable number of branches called stripes. The colony originates from an initial conical cup called a sicula from which the whole colony develops. Each individual zooid was housed within a cup like structure called a theca. The number of branches and the arrangement of the thecae are important features in the identification of the graptolite fossils. The orders Graptoloidea and Dendroidea are generally distinguished by the number of branches on the colony with the Dendroids having more branches than the Graptoloids. The most notable changes seen as the Graptoloids evolved were seen in the number of stripes per colony along with a change in the attitude of the stripes. These changes coincided with an increase in the complexity and change in the positioning of the thecae.(Rickards 1978)

            Graptolite fossils are most commonly found in deep water black shales but are also found in shallower conditions. Most of the Graptolite fossils found are carbonised. The process of carbonisation along with the easily compressed shales has made most graptolite fossils flat and therefore very difficult to study. This has made studying the evolution of the graptolites very difficult but much information has still been pieced together using the help of three dimensional fossils that have been infilled with iron pyrite. (www1)

References:

1. R.B Rickards , Major aspects of evolution of the Graptolites. Acta Palaeontologica Polonica, Vol. 23 Pg. 585.
2. (www1) http://www.graptolite.eu/about.htm
3. (www2)http://www.oldearth.org/curriculum/history/earth_history_c5_silurian_graptolites.htm
4. UCC Blackboard. Maria McNamara, 2014. Graptolites Lecture notes.

      

Fossil Localities of Graptolites – Ruairi Wiseman 112546067

Introduction:
An important area to consider when studying graptolites is their locality and diversity throughout the world. Graptolite fossils are common as they have a worldwide distribution, they are often found within mud rock and shale where sea bed fossils would not commonly be found.

Graptolite localities in Europe: Devonian graptolites have been found in diverse localities such as the French Armorican Massif, the southern Hesperian Massif of Spain and from southern Sardinia. The first records of Devonian graptolites in south-western Europe were collected from France and Spain, they were identified as common Silurian species. The only descriptions and illustrations of the Devonian graptolites in this region were by Bourrouilh (1963, 1983), geologists are able to divide rocks of the Ordovician and Silurian periods into graptolite bio zones.
These zones are less than one million years in duration. A world-wide ice age at the end of the Ordovician killed off the majority of the then-living graptolites. The species present during the Silurian period were the result of diversification from only one or two species that survived the Ordovician glaciations. (Journal,1996)

Another important graptolite fossil locality is Heartfeld Score in the southern Uplands of Scotland, here graptolite fossils can be observed in the Upper Ordovician to Silurian mudstones Charles Lapworth was a leading expert on this group of fossils. Lapworth recognised the stratigraphic zonation value of the graptolites, and used this to help him unravel the geological structure of the Southern Uplands of Scotland. Apart from Scotland, Wales is also another well known location for the study of the graptolite fossil record. Wales, along with England was part of Avalonia, a high latitude paleo-continent during the late Ordovician.(William berry,2010)

Graptolite from heartfeld Score

Apart from the multitude of European countries, the fossil locality of graptolites also extends to Australia and its surrounding region. The region of Australia lay in low latitudes during the Ordovician, leading its graptolite fossil locality to be one of the most complete in the world.

Avalonia in map of world during Late Ordovician

The graptolite biodiversity in Victoria was extensively recorded by Vandenberg and Cooper(1992). In the Ouachita Mountains in Oklahoma, Upper Ordovician strata are exposed along a ridge at the western end of the mountains. The strata are composed of graptolite rich shale’s which are associated with deep marine limestone’s and cherts. There is a great diversity of graptolite fossils in this particular area. These locations have had a significant impact on improving the fossil record and study of graptolites for more then a century and are a key component in unravelling the biostratigraphy of these surrounding areas.

References: 1.The Ordovician Earth System edited by Stanley Charles Finney, William B. N. Berry 2010
2.GEOLOGICAL JOURNAL, VOL. 31. 349-358 (1996) Devonian graptolites from southwestern Europe: a review with new data
3.http://www.birmingham.ac.uk/facilities/lapworthmuseum/collections/palaeontology/graptolite.aspx

Biology of graptolites – James Quirk 111559423

Introduction:

Graptolites are an extremely important organism in the fossil record, there high abundance during the middle Cambrian to the carboniferous means there are a large amount of good fossils preserved. This is highly appealing to me as someone studying geology as these fossil can give a great insight into understanding the order in which beds of rocks were deposited. In this report I will be studying the biology of graptolites, I will discuss it under the following headings, morphology and structure, movement and nutrition, distribution and interrelationships. 

Morphology & Structure: Graptolites are marine colonial invertebrate organisms consisted of 3 dimensional twig like or net like colonies composed of straight or curved, single, double or many branches. There structure in ways resembles seaweed. Each colony was made up of large amounts of tiny microscopic organisms that lived in tiny cup-like structures arranged along the branches of the colony. Each individual organism (zooids) was joined together by a nerve cord. The exoskeleton of graptolites are made of organic materials secreted by organism.

The organism originates as a conical cup sicula and from this the colony develops which is referred to as a rhabdosome. It consists of hollow tubes (stipes) single or multiple. Thecae are small cups joined together, these thecae play host to zooids. Thecae form as overlapping cups along the length of the skeleton. These may be present on one two or four sides. (1)(2)

Image 1- Morphology of graptolites (3)

 

Movement and nutrition: Firstly it is widely believed that graptolites were planktonic, there are many possibilities for the way in which graptolites moved. The first one being they were free floating adaptations such as vanes and floats would have been useful in maintain the organism position in the water. Another method of movement may have been certain spiral shaped graptolites may have been passively moved by water currents and would have undergone a spiralling motion due to their shape. This theory was put forward by Bulman (1964) and he based this on the observation that the umbrella sponge Axoniderma spiralled upwards from the sea floor in response to slight currents. Also some graptolites were supported by a holdfast which was attached to the seabed floor.

Graptolites obtained nutrition through ciliary-feeding, this was a method of feeding were cilia causes a current of water to be drawn towards and through the animal and micro-organisms in the water are filtered out by the cilia. (2)

Image 2 – Graptolites free floating supported by floats. (4)

 

Distribution: Graptolites are found almost all over the world in every continent excluding Antarctica they are found in limestones, sandstones and shale’s which were deposited in a wide range of environment’s.  The earliest graptolites are found in the middle Cambrian. These were the dendroid graptolites, filter feeders which lived attached to the sea floor. Graptolites can be potentially found wherever Cambrian to carboniferous rocks crop out. However, they are relatively uncommon at both ends of their range in the Cambrian and carboniferous, and most common, often very abundant, in Ordovician and Silurian rocks.  In certain regions they occur frequently in Devonian rocks also. (2)

 

Interrelationships: There is surprisingly little interdependence or interrelationships between marine invertebrate’s outside reef communities, largely because the food chains are so short. It is very hard to understand the relationship habits of graptolites because the evidence simply is not there. Very few rigorous studies have been made of graptolites in relation to their associated faunas. However they are sometimes found among trilobites, brachiopods, hydroids, chitinozans and sponges so they may have shared some relationship with these organisms. (2)

Image 3- Fossilized graptolites.(5)

References:

1. http://www.jstor.org/stable/2400762?seq=1
2. Graptolites writing in the rocks, The Boydell press P. 11-15, P.23-26 & 41-48
3. http://www.earthsciences.hku.hk/shmuseum/earth_evo_05_01_8.php
4. http://coo.fieldofscience.com/2007_10_01_archive.html
5. http://en.wikipedia.org/wiki/File:DiplograptusCaneySprings.jpg

Fossil Record – Michael Whitelaw 112450278

Introduction

The fossil record is a very important tool to be used when studying graptolites. They are an extinct species and everything that is known on graptolites is derived from the study of their fossil record.

The fossil record is the collective accumulation of the remains of organisms that have been preserved….When viewed as a whole, it can provide interesting information about the life on earth (Wisegeek, 2014).

Lithology

Graptolites fossil record is of a global scale. They are exclusively found in marine sedimentary rocks of the upper Cambrian right through the Ordovician, Silurian and the Lower Carboniferous.

They are commonly found in black, organic and iron sulphide rich mudstones and shales, the so called ‘graptolite facies’(Jstor, 2014). In the black shale’s, they are flattened and resemble writings in the rocks. Although common, they are not exclusive to black shale’s, graptolites are now known from nearly all lithology’s (Jstor, 2014). Some of the best preserved samples of graptolites are recorded in the fossil record of limestones.

Graptolites are best preserved in fine grained sedimentary rocks indicating a low energy rate where depositional was slow, others have been found in coarse grained areas where there may have been strong currents.

Image 1: Dendroid Graptolite preserved in limestone (Louisville Fossils, 2011)

Distribution & Diversity

Graptolites have been found on every continent except Antarctica, they can be found where ever Cambrian to Carboniferous rocks crop out (Palmer 1992, p.16).
Some of the first graptolites to be found were the benthic species known as dendroid graptolites. They lived on the sea floor held down by holdfasts, these species where filter feeders and where first found in middle Cambrian rocks.

From here graptolites took the next step in their evolutionary journey and split to allow for the arrival of the free swimming planktonic graptoloids. They were recorded in the late Cambrian fossil record.

The first few planktonic species where still morphologically complex and were called planktonic dendroid.  They resembled their ancestral benthic group. They soon began to decline around the Tremdoc period of the early Ordovician rock. At the beginning of the Arenig, many different species began to occur with changes to their shape.

The fossil record towards the end of the Arenig of the early Ordovician shows that the morphology of graptolites began to become simpler by fusing two branches together. This lead the way for one of the most successful graptoloid fauna in the Llanvirn period. This species is known as the diplogratids.
Diplogratids survived right through into the Silurian period and achieved this without resorting to anything strenuous in the way of morphological evolution (Palmer 1992, p.19).

The fossil record even shows that the diplogratids survived a severe decline in graptolite species, where only two or three species survived until the early Silurian due to global environmental crisis of the end-Ordovician (Palmer 1992, p.19).
The diplogratid started to become scarce in the mid Llandovery of the Silurian period. The rest of the graptoloids suddenly became extinct in the mid-Devonian although the fossil record does not give an explanation.

The dendroid graptolite on the other hand remained unchanged on its benthic lifestyle, from the Cambrian right through to the Upper Carboniferous where it finally died out.

Image 2: Graptoloids preserved in black shales (Grandanatural 2008)

References;

Berry, William B.N. “Graptolite Occurrence and Ecology” Journal of Paleontology, Vol. 36, No. 2 (March, 1962) : 285-293. JSTOR. Web. 4 March 2014.

Dendroid Graptolite preserved in limestone 2011, digital image, Louisville Fossils, accessed 4 March 2014 <http://louisvillefossils.blogspot.ie/2011/03/silurian-graptolite.html&gt;

Graptoloids preserved in black shales 2008, digital image, Grandanatural, accessed 4 March 2014 <http://www.granadanatural.com/blog.php?codigo_blog_articulo=18>

Palmer, D, Rickards, B 1991, Graptolites : Writing in the Rocks, The Boydell Press, Suffolk.

Wisegeek 2014, assessed 4 March 2013, <http://www.wisegeek.org/what-is-the-fossil-record.htm&gt;

Taphonomy-Tomas O’Sullivan 112463322.

I have chosen to focus on graptolite preservation, I have looked at the conditions and rocks graptolites are preserved in along with the various modes of preservation.

Graptolites are an extinct group of small aquatic animals that first appeared during the Cambrian and continued to appear in the fossil record into the early carbonaceous. They were floating animals that have been most frequently preserved as stick like or curvilinear impressions (Encyclopaedia Britannica, www.britannica.com). They possessed a chitinous (finger nail like) outer layer called the periderm, which despite being organic was highly resistant to decay allowing graptolites to be preserved even though they lacked mineralised hard parts. Graptolites vary in shape but are frequently dendritic or branching and so can easily be mistaken for plant remains.

There are 3 main modes of preservation for Graptolites, they are best preserved as organic remains, usually in limestones, however such instances are very rare. They are primarily found as compressed impressions or as cores which formed due to the infill of a mould with another mineral.

There is an abundance of Graptolites in black shale and mud rocks. This is due to the fact that these rocks are formed from sediment deposited in deep water conditions where bottom circulation is poor leading to anoxic conditions which don’t support bottom dwelling life(scavengers). Graptolites would sink to the sea floor where they would become entombed in sediment. Graptolites are also found in limestones and chert, however these rocks form in warm shallow seas, conditions better suited to bottom dwelling life which would eat graptolite remains.

Graptolites come best preserved in fine argillaceous limestones (Clarkson, 1993) however such occurrences are rare. The most common and unfortunately least revealing type of graptolite preservation is where the specimen has been completely flattened and preserved as a compression, which is a carbon or alumino-silicate film on the rock. However sometimes carbonization has advanced to a stage where a lot of the microstructure and growth rings are lost.Thecael remains are often retained during this process which means the fossil can be identified. It is from these sequences that most graptolite species are known. These compressed remains are often found along the bedding planes of rocks. Graptolites dating back to the Ordovian period can be seen as carbonized impressions at Abereidy bay, Dyfed, Wales.

Graptolites are sometimes found preserved in three dimensions. This is often the case where the periderm has been completely destroyed leaving a mould of the rhabdosome which infills with Pyrite, which is a replacement pseudo morph in shales. Pyrite is derived from organic material decaying in anoxic conditions (Clarkson 1993).

Graptolites are hemi-chordates which evolved rapidly and became very widespread (Benton and Harper, 2009). This makes them ideal zone fossils! they are very useful in biostratigraphy, stratigraphic correlation of separated rock units and subdividing of Ordovian – Silurian rocks. Graptolites evolved rapidly which lead to a high rate of species turnover, this allows for the fine scale division of geological time.

References:

-Encyclopaedia Britannica, http://www.britannica.com/EBchecked/topic/242086/graptolite.

-Clarkson, Intervertebrate Palaeontology and Evolution, (Chapmann and Hall, 1993) ed. 3.

-Michael j. Benton and David T. Harper, Palaeobiology and the Fossil Record, (Wiley-Blackwell, 2009) ed. 1.

Anatomy – Seán Quaid 112437012

Graptolites belong to the Phylum Hemichordata and the Class Graptolithina. They are colonial marine deuterosomes and invertebrates. Graptolies existed between the Middle Cambrian and the Upper Carboniferous. There are two main orders of Graptolites; the order Graptoloidea and the order Dendroidea. According to Clarkson (1979, p.238)  Graptolites “may be straight or curved,sometimes spiral in form, single-branched, bifid or many-branched”  which is why Graptolites make such excellent zone fossils due to their high diversity.

Wide range of Graptolite morphology. Image 1. (www.wikimedia.org)

The key anatomical features of Graptolites are a skeleton composed of a thin sheet-like material called periderm. This Rhabdosome has an outer cortical layer and an inner fusellar layer. Other key features are the nema, sicula, thecae and thecae apertures (McNamara, M. 2014).

Anatomy of the Order Graptoloid:

The first part of the Graptoloid to form is the sicula – a conical tube with its opening facing downward and terminating in a thread-like structure called a ‘nema’ at its apex. The sicula is divided into the upper prosicula and the lower metasicula, both are distinguished by different ornamentation (Clarkson, E.N.K. 1979). The metasicula, as with other parts of the graptoloid, is ornamented with rings of periodic growth called ‘fusellae’. From the sicula grows numerous cup-like structures called ‘thecae’. These cup-like structures increase in size from the initial one to about the fifth or sixth and then the dimensions remain constant. Although there is no fossil record of the graptolite animal called ‘zooids’ it is believed that individual zooids lived in these ‘cups’. The thecal cavities are connected to that the sicula  through a ‘foramen’ and all the thecae/’cups’ linked toghether through a common canal in the sicula. The colony of a graptoloid is known as a ‘Rhabdosome’ and the branches are known as ‘stipes’ (Benton & Harper. 2009).

Anatomy of a Graptoloid. Image 2.(www.palaeo.gly.bris.ac.uk)

Anatomy of Order Denroidea:

Dendroidea are the most ancient Graptolites and are ancestral to Graptoloidea. Despite this, they have a more complex anatomy than the Graptoloidea. They have the appearance of a bush, with many branches that are connected by dissepiments The main body of the Dendroidea is  an inverted conical cone, the sicula, which stood upright on the sea floor and the apex of the cone ended in a holdfast that connected to Dendroidea to the sea floor. About halfway up the sicula formed a ‘stolotheca’ – a theca which encloses the main stolon -and ,along with daughter stolothecae, forms a continuous closed chain all up the Rhabdosome. A main feature of a Dendroidea is havng two types of thecae, a large autothecae and a smaller bithecae which both arise from the stolothecae. The opening of the bithecae is on the opposite side of its origin due to it looping around the stolothecae. As with the Graptoloidea, these cup-like structures housed the Graptolite animals. In every Rhabdosome there is a internal system composed of tubes which runs from the sicula all the way along the stolotheca which is believed to have carried internal systems such as a nervous system. Unlike Graptoloids, these Dendroidea have a cortical tissue that is laid down in flat sheets that cover the earlier thecae and form a holdfast that fixes the colony to the sea floor. (Clarkson, E.N.K. 1979).

Recreation of a Dendroidea. Image 3. (www.geodz.com)

References:

Benton, M.J and Harper, D.A.T. (2009) Introduction to Paleobiology and the Fossil Record   Wiley-Blackwell, Sussex. P.412 – 416

Clarkson, E.N.K. London (1979) Invertebrate Palaeontology and Evolution. George Allen & Unwin, London. P.238 – 249

UCC Blackboard – Maria McNamara Lecture notes – Graptolites 22/01/14.

Image 1 – http://upload.wikimedia.org/wikipedia/commons/9/9c/Graptolites-EncBrit.jpg

Image 2 – http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/GRAPTO/graptoweb/thumbnail1.html

Image 3 – http://www.geodz.com/deu/d/images/1154_graptolithen.png