Ordovician and Silurian Communities

Ordovician and Silurian Communities
By: Linda Zantout

Taxon Count Life Habit Complete or Broken Body or Mold Period
Trepostomes 2 Epifaunal Suspension Feeder Broken Body Ordovician
Platystropha 4 Epifaunal Suspension Feeder Complete Mold Ordovician
Homotrypa 1 Epifaunal Suspension Feeder Broken Mold Ordovician
Batostomella 3 Epifaunal Suspension Feeder Broke Mold Ordovician
Strophomena 1 Epifaunal Suspension Feeder Broken Mold Ordovician
Crinoids 3 Epifaunal Suspension Feeder Broken Body Ordovician
Laptaena 1 Epifaunal Suspension Feeder Broken Mold Ordovician

 

 

Data Analysis and Interpretation

 

1. Depending on the sample size being measured, different sets of individuals will be studied which may lead to different results in measuring species richness. Thus, results regarding species richness are most valid when the sampling sizes are for the most part equated. For example, within the class data we see 20 different types of brachiopods and 17 different types of bryozoa and thus a valid deduction would be that the species richness is higher in brachiopods. However, in a different sample in which the number of individual species in each phylum is added, there are 131 brachiopods and 20y bryozoa. In this sample set, it would be valid to say that the species richness of bryozoa is greater than that of brachiopods.

 

2.

Phylum Genera                           Count            Simpson Index
Bivalve Ambonychia

1

0.00212766

Bivalve Carotidens

1

0.00212766

Brachiopod Platystrophia

31

0.065957447

Brachiopod Lepidocyclus

16

0.034042553

Brachiopod Strophomena

14

0.029787234

Brachiopod Hypsiptycha

12

0.025531915

Brachiopod Thaerodonta

11

0.023404255

Brachiopod Hebertella

8

0.017021277

Brachiopod Zygospira

7

0.014893617

Brachiopod Megamyonia

5

0.010638298

Brachiopod Hesperorthis

4

0.008510638

Brachiopod Diceromyonia

3

0.006382979

Brachiopod Hiscobeccus

3

0.006382979

Brachiopod Rafinesquina

3

0.006382979

Brachiopod Retrisirostra

3

0.006382979

Brachiopod Glyptorthis

2

0.004255319

Brachiopod Leptaena

2

0.004255319

Brachiopod Rhyncotrema

2

0.004255319

Brachiopod Sowerbyella

2

0.004255319

Brachiopod Orbiculoidea

1

0.00212766

Brachiopod Pentamerus

1

0.00212766

Brachiopod Reserella

1

0.00212766

Bryozoa “trepostome”

83

0.176595745

Bryozoa Batostomella

36

0.076595745

Bryozoa Homotrypa

17

0.036170213

Bryozoa Dekayella

14

0.029787234

Bryozoa Prasapora

12

0.025531915

Bryozoa Rhombotrypa

8

0.017021277

Bryozoa Monticulipora

6

0.012765957

Bryozoa Parvohallopora

6

0.012765957

Bryozoa Dimesopora

5

0.010638298

Bryozoa Aspidopora

4

0.008510638

Bryozoa Fistulipora

4

0.008510638

Bryozoa Homotrypella

3

0.006382979

Bryozoa fenestrate

2

0.004255319

Bryozoa Heterotrypa

2

0.004255319

Bryozoa Rhimdictya

2

0.004255319

Bryozoa Dekayia

2

0.004255319

Bryozoa Batostoma

1

0.00212766

Conularid Cornulites

6

0.012765957

Coral Calapoecia

4

0.008510638

Coral Rugose coral – unid

3

0.006382979

Coral Favosites

1

0.00212766

Coral Grewingkia

1

0.00212766

Coral Protarea

1

0.00212766

Coral Streptalasma

3

0.006382979

Crinoid unidentified

75

0.159574468

Monoplacaphoran

1

0.00212766

Porifera Astylospongia

6

0.012765957

Porifera Hindia

3

0.006382979

Stromatoporoids

17

0.036170213

Tentaculitid Tentaculites

3

0.006382979

Trilobites

6

0.012765957

TOTAL

470

1

 

Taphonomy and Life Habits

1. The community seems to be heavily dominated by epifaunal suspension feeders.

2. The fossils can be divided into two sizes: small ones such as the trepostomes and other bryozoans and then bigger ones such as the brachiopods. However amongst each one of these divisions, the sizes are evenly distributed. Also, there are outliers, those that are significantly smaller or bigger, but that is to be expected.

3. What’s missing from the biota is more specimens of coral and porifera and some of cephalopods although they are generally harder to preserve so it makes sense.

4. The mode of preservation was generally mold or cast in that there was an imprint left behind in the rock in which the shape and texture of the shell could be seen. Other than this, there were some that had the body preserved but those were mainly bryozoans, which had hard body parts that were able to withstand the time and preservation.

5. Based on observations, it can be inferred that the organisms resided in a warm shallow marine environment and were deposited and preserved  at the bottom in sediments or mud after they died.

6. To a certain extent, the collection does represent the original living community. Of course however, there are a number of species missing due to the fact that they’re usually not preserved well or rare in general but there was a large percentage of brachiopods and bryozoans which is what mostly would have been making up the original living community.

7. The main difference between the Ordovician and Silurian communities is that the organisms of the Ordovician period were all marine organisms whereas the Silurian brought with it the appearance of the first few organisms able to start their way onto land. This makes sense since all the organisms we found were marine based and we were looking in Ordovician beds.