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Description

The second period of the Paleozoic and started around 485 millions of years ago, it was a new chapter on the early history of earth, doing a turn of the dominant and varied clades that existed during the cambrian event and adding another new variety of fauna across the marine landscape. This period saw many new innovations and radiations including the gradual expansion of forms and sizes on cephalopods nautiloids and trilobites, the rise of eurypterids, many new forms of sponges, cnidarians corals, bryozoans and the appearance of more advance true vertebrates, with a variety of jawless and armored forms, the spread of conodonts, and possible the first gnathostomes, and probably the most transcendental event since the evolution of complex life: the conquest of land by unknown groups of plants speculated to be similar to moss and fungi growing on a layer of ground, which would eventually will expand and settle for animals to colonize dozens of millions of years after.

Although at the end of the period most of these prolific life trends faced what will be known as the first mass extinction event earth passed: the Ordovician-Silurian Extinction Event which resulted in the death of more than 80% of families, harming a lot of clades in the process and marking a before and after of life history. Even with that end, this period hold some wonderful trends of biodiversity, as well an increase of size on some clades, including a variety of bizarre large arthropods and mollusks time before fishes would take over.

Even though most Panarthropods were still alive after the cambrian, their remains became very scare due to the absence of similar preservation conditions as on the cambrian, but thanks to the known specimens found we know they were still thriving on the Ordovician seas, specially the titanic Radiodonts, relatives of the Anomalocaris, with species that reached even bigger sizes that their former antecessors giving a last glory of gigantism before falling into the shadows (or at least into the shadows of unknown for the lack of a more complete fossil remains); the Hurdiid Aegirocassis benmoulai known by one the best well preserved remains of this period, in life this filter feeder reached a length of around 2 meters being half of it just its head, and during the early Ordovician it was the biggest animal of its time to swim in the ocean, alongside the first small cephalopods and others early pelagic animals.

The Arthropods didn’t get behind during this time, exploring further on divergence as during this period these would not only evolve and dominate the marine landscape, but also would finally establish some of the prominent clades that could prevail for the next periods.

Chelicerata were among the first major extant arthropod clades to rise in the cambrian, with different early relatives very different in morphology to its actual relatives, but during the Ordovician they established the main bauplan of this clade as we know it, as well the evolution of some of known groups such Xiphosura (Horseshoe crab) and one of the most outstanding clades of the Paleozoic: the sea scorpions or Eurypterida.

These large arthropods have a very obscure early history with little knowledge of how they evolved, is only theorized that they appeared around the late cambrian/early Ordovician due to the oldest known species of this clade had already established its basic shape, also this oldest known species correspond to one of the biggest eurypterid of the period, Pentecopterus decorahensis, a great Megalograptid of around 1.7 meters long, it lived around the middle Ordovician showing not only were quite advance at that point of time but also were already pretty prominent as the large marine fauna.

Trilobites were still thriving being widespread and successful in many niches prior to their downgrade after the Or/S mass extinction, and compared to its cambrian relative they experience a lot more of morphological variations with different species developing peculiar features that helped them to defend from the new predatory sea scorpions and nautiloids, or properly adapt to certain lifestyles. The Ordovician would be the peak on their size development as it came with some very large species in the ranges of 20 to 40 cm long which are quite common in some fossil records with complete specimens founds for the dozens, and around these large forms includes the biggest of all this clade, like the most known Asaphid Isotelus rex, a large benthonic species which is one of the most complete large species found with an individual reaching up to 72 cm or Ogyginus forteyi which reached sizes of about 45 to 70 cm in length, but there are some even possible bigger species (although from fragmentary remains) that potentially exceed these; Hungioides bohemicus, a Dikelokephalinid trilobite, based on a insolated pygidium if it is scaled with most complete specimens reach up a length of 90 cm long, which would make it the biggest trilobite ever.

Mollusks started bright in this period with the eventual rising diversification and complexity on different ecological levels, becoming widespread on the benthonic and nektonic levels with bivalves, gastropods and even further took important megafauna niches.

Cephalopods moved away from its simple ancestral minuscule forms of few centimeters long to diversify and spread as the major dominant animals on the shallow epicontinental seas and tropical regions of the planet during the Middle and Late Ordovician, most of these early forms were mostly Nautiloids, ancient relatives of the last living shelled cephalopod of the genus Nautilus but unlike these the shells of them were straight or slightly curved. Some of the largest species belong to the groups actinocerid and endocerid with shell sizes of around 1 to 2 meters, and from these stands out some peculiar titans like the endocerid Endoceras giganteum which holds the title of the largest Paleozoic invertebrate and one of the biggest cephalopods known, which most of its long size came from the large shell which a fragment of 3 meters long is calculated to have been complete in length around 5.7 meters; important to mark out due to the lack of soft tissue preservation of these is unknown to get the possible full size of its original soft body and anything presented here is speculative.

Note: people may know about that 9 m long shell specimen mentioned, which is always the size scale used to refer as the biggest shell, the reality about its feasibility is that this suppose specimen is just anecdotic in evidence and not very trustable due to the lack any visual proof of its existence like a photo of the fossil, this due to the destruction of the specimen on one point before it was properly collected, and without physical evidence we likely have to stick with the concrete known facts, even though there is tentative consideration of possible large individuals have existed but we have yet to find any remain of these to totally confirm these claims.

Gastropods or at least stem-clades relatives to our current clades were spreading across the seabed of marine waters with thousands of different species, of course many of these were just few centimeters long some reaching shell sizes less than 10 cm of diameter/length, although among them there was Maclurina manitobensis which although the average size is around 10 cm, there is a specimen shell with a size of about 25 cm diameter and 12 cm tall, is not only one of the biggest gastropods of the Ordovician but also is the biggest known Paleozoic gastropod (until some other even bigger shell or remain is found, honestly I would think there were large species that didn’t fossilized or even more likely aren’t published)

Chordates made a substantial evolutionary development from their small vermin forms of the cambrian and started to shape into the animals they were going to become as well experimenting with peculiar forms: Within the first armored jawless fish were the arandaspids being this simple clade of long shelled forms covered in oval armor plates and many hardened scales the most know species Arandaspis; among the largest is one of the oldest species know of the period, the peculiar tailed Sacabambaspis janvieri with a specimen able to reach out 25 cm long as well the more robust Astraspis which was measured around 20 cm.

Another group of vertebrates, one that already existed since the cambrian, Conodonts were a branch of unique eel-like chordates characterized by their teeth pieces and arrangement that varied of different structures called “elements” that helped them thrive on the oceans of earth during the Paleozoic, their number and amount of these pieces are so large that these are one of many key biostratigraphic identifications across different time periods, on the Ordovician there were found some large individuals including the one of the best preserved specimens being Promissum pulchrum with 40 cm long and characterized by its mouth apparatus formed by 19 very narrow elements of 1 cm long, but there is also another contenders that could potentially surpass this, the species Archeognathus primus which unlike Promissums it only had 6 pairs elements, and Iowagnathus grandi with 15 spiky elements, based on comparison to other more complete conodonts this could have been able to reach sizes of around 50 cm to even 1 m of length.

I think is worth to mention that during this period seems that also appeared the Thelodonti (the least armored agnathans covered by tiny special dentricles-like scales) and probably the first Gnathostomes (jawed vertebrates) which they are only known mostly for fragmentary evidence (scales) and they wouldn’t have any major role until the Silurian.

The Poriferans/Sponges were among the basic reef builder group on this period alongside other benthos organisms like brachiopods and bryozoans, these were able to develop into large forms in shapes and forms being some of the most prominent the stromatoporoids, a variety of demospongids that prevailed though most reefs of the early Paleozoic. Among these ones comes some of the tallest species, belongs to the genus Aulacera, a pillar-shaped sponge which in live were able to reach about 1 to 2 meters in height and around 20 cm in diameter, although some specimens from the species A. undulata were measured to reach heights of around 3 m tall and 30 cm of diameter (with some estimations of 5 m tall but due to how old are these estimations I have to take these with a little of doubt)

Just like many of the mentioned clades, the Echinoderms came with a very remarkable diversification across the oceans after the Cambrian establishing most groups, and similar to many other seabed animals they became a major component of the marine environments diversity, establishing some peculiar groups like blastozoans and the current living clades such as crinoids, asterozoans (starfishes and relatives) among others; most of these were just less than few centimeters in body size, most not exceeding lengths of 10 cm, only being the crinoids the largest of all with most of their sizes being partially by their columnar stems that hold their small cups and tentacles at the top, such as, Ectenocrinus simplex of 70 cm stem long, and the more lanky Cincinnaticrinus varibrachialus and Merocrinus curtus, which both are characterized for have very small narrow cups of 0.5 cm but a stem able to reach 1 m in height, although some of the largest individuals belongs to the robust Anomalocrinus incurvus, with a cup and body length of almost 10 cm and a stem height of 60 cm to 1 meter.

Due to the conditions of fossilization most Cnidarians of soft body forms didn’t preserve but those that were able to create a stony body which was able to leave marks in the fossil record of this period in great, especially those that thrived as reef builders in ways similar to current coral reefs, being the common colonial forms and many other solitary corals, most belongs to the ancient group of Rugosans and Tabulates, and the Conulariids, a peculiar clade of still disputable affinity to cnidarians, being long very triangular/pyramid bodies corresponding to just part of its life cycle as they are considered to be part of the scyphozoans (true jellyfishes), their stony remains often are around the sizes of 10 to 20 cm long, but Archaeoconularia was a genus with specimens able to reach sizes of around 50 cm of height, making one of the biggest conulariids known.

References

-Van Roy, P., Briggs, D. E., & Gaines, R. R. (2015). The Fezouata fossils of Morocco; an extraordinary record of marine life in the Early Ordovician. Journal of the Geological Society, 172(5), 541-549.

-Van Roy, P., Daley, A. C., & Briggs, D. E. (2015). Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps. Nature, 522(7554), 77-80.

- Lamsdell, J. C., Briggs, D. E., Liu, H. P., Witzke, B. J., & McKay, R. M. (2015). The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa. BMC Evolutionary Biology, 15(1), 169.

- Rudkin, David M., et al. "The world's biggest trilobite—Isotelus rex new species from the Upper Ordovician of northern Manitoba, Canada." Journal of Paleontology 77.1 (2003): 99-112.

-Corbacho, J., & Vela, J. A. (2010). Giant trilobites from lower Ordovician of Morocco. Batalleria, 15, 3-32.

- Gutiérrez-Marco, J. C., Sá, A. A., García-Bellido, D. C., Rábano, I., & Valério, M. (2009). Giant trilobites and trilobite clusters from the Ordovician of Portugal. Geology, 37(5), 443-446.

-Klug, C., De Baets, K., Kröger, B., Bell, M. A., Korn, D., & Payne, J. L. (2015). Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change. Lethaia, 48(2), 267-288.

-Teiciiert, C., & Kummel, B. (1963). SIZE OF ENDOCEROID CEPHALOPODS1. Breviora, (121-178).

- Pradel, A., Sansom, I. J., Gagnier, P. Y., Cespedes, R., & Janvier, P. (2007). The tail of the Ordovician fish Sacabambaspis. Biology Letters, 3(1), 73-76.

-Sansom, I. J., Smith, M. P., Smith, M. M., & Turner, P. (1997). Astraspis-the anatomy and histology of an Ordovician fish. Palaeontology, 40(3), 625-644.

- Gabbott, S. E., Aldridge, R. J., & Theron, J. N. (1995). A giant conodont with preserved muscle tissue from the Upper Ordovician of South Africa. Nature, 374(6525), 800-803.

-Liu, H. P., Bergström, S. M., Witzke, B. J., Briggs, D. E., McKay, R. M., & Ferretti, A. (2017). Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA. Journal of Paleontology, 91(3), 493-511.

- Briggs, D. E., Liu, H. P., McKay, R. M., & Witzke, B. J. (2018). The Winneshiek biota: exceptionally well-preserved fossils in a Middle Ordovician impact crater. Journal of the Geological Society, 175(6), 865-874.

-Aldridge, R. J., Purnell, R. J., Gabbott, S. E., & Theron, J. N. (1995). The apparatus architecture and function of Promissum pulchrum Kovács-Endrödy (Conodonta, Upper Ordovician) and the prioniodontid plan. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 347(1321), 275-291.

- Rohr, D. M., Blodgett, R. B., & Furnish, W. M. (1992). Maclurina manitobensis (Whiteaves)(Ordovician Gastropoda): the largest known Paleozoic gastropod. Journal of Paleontology, 66(6), 880-884.

- Galloway, J. J., & Jean, J. S. (1961). Ordovician Stromatoporoidea of North America. Paleontological Research Institution.

- Meyer, D. L., Miller, A. I., Holland, S. M., & Dattilo, B. F. (2002). Crinoid distribution and feeding morphology through a depositional sequence: Kope and Fairview Formations, Upper Ordovician, Cincinnati Arch region. Journal of Paleontology, 76(4), 725-732.i

- Deline, B. L. (2006). Inter-and Intraspecific Morphological Variation of Crinoid Columnals in Relation to Water Depth in the Type Cincinnatian (upper Ordovician) (Doctoral dissertation, University of Cincinnati).

-Van Iten, H., Gutiérrez-Marco, J. C., Muir, L. A., Simões, M. G., & Leme, J. M. (2018). Ordovician conulariids (Scyphozoa) from the Upper Tiouririne Formation (Katian), eastern Anti-Atlas Mountains, southern Morocco. Geological Society, London, Special Publications, 485, SP485-5.

Description

The second period of the Paleozoic and started around 485 millions of years ago, it was a new chapter on the early history of earth, doing a turn of the dominant and varied clades that existed during the cambrian event and adding another new variety of fauna across the marine landscape. This period saw many new innovations and radiations including the gradual expansion of forms and sizes on cephalopods nautiloids and trilobites, the rise of eurypterids, many new forms of sponges, cnidarians corals, bryozoans and the appearance of more advance true vertebrates, with a variety of jawless and armored forms, the spread of conodonts, and possible the first gnathostomes, and probably the most transcendental event since the evolution of complex life: the conquest of land by unknown groups of plants speculated to be similar to moss and fungi growing on a layer of ground, which would eventually will expand and settle for animals to colonize dozens of millions of years after.

Although at the end of the period most of these prolific life trends faced what will be known as the first mass extinction event earth passed: the Ordovician-Silurian Extinction Event which resulted in the death of more than 80% of families, harming a lot of clades in the process and marking a before and after of life history. Even with that end, this period hold some wonderful trends of biodiversity, as well an increase of size on some clades, including a variety of bizarre large arthropods and mollusks time before fishes would take over.

Even though most Panarthropods were still alive after the cambrian, their remains became very scare due to the absence of similar preservation conditions as on the cambrian, but thanks to the known specimens found we know they were still thriving on the Ordovician seas, specially the titanic Radiodonts, relatives of the Anomalocaris, with species that reached even bigger sizes that their former antecessors giving a last glory of gigantism before falling into the shadows (or at least into the shadows of unknown for the lack of a more complete fossil remains); the Hurdiid Aegirocassis benmoulai known by one the best well preserved remains of this period, in life this filter feeder reached a length of around 2 meters being half of it just its head, and during the early Ordovician it was the biggest animal of its time to swim in the ocean, alongside the first small cephalopods and others early pelagic animals.

The Arthropods didn’t get behind during this time, exploring further on divergence as during this period these would not only evolve and dominate the marine landscape, but also would finally establish some of the prominent clades that could prevail for the next periods.

Chelicerata were among the first major extant arthropod clades to rise in the cambrian, with different early relatives very different in morphology to its actual relatives, but during the Ordovician they established the main bauplan of this clade as we know it, as well the evolution of some of known groups such Xiphosura (Horseshoe crab) and one of the most outstanding clades of the Paleozoic: the sea scorpions or Eurypterida.

These large arthropods have a very obscure early history with little knowledge of how they evolved, is only theorized that they appeared around the late cambrian/early Ordovician due to the oldest known species of this clade had already established its basic shape, also this oldest known species correspond to one of the biggest eurypterid of the period, Pentecopterus decorahensis, a great Megalograptid of around 1.7 meters long, it lived around the middle Ordovician showing not only were quite advance at that point of time but also were already pretty prominent as the large marine fauna.

Trilobites were still thriving being widespread and successful in many niches prior to their downgrade after the Or/S mass extinction, and compared to its cambrian relative they experience a lot more of morphological variations with different species developing peculiar features that helped them to defend from the new predatory sea scorpions and nautiloids, or properly adapt to certain lifestyles. The Ordovician would be the peak on their size development as it came with some very large species in the ranges of 20 to 40 cm long which are quite common in some fossil records with complete specimens founds for the dozens, and around these large forms includes the biggest of all this clade, like the most known Asaphid Isotelus rex, a large benthonic species which is one of the most complete large species found with an individual reaching up to 72 cm or Ogyginus forteyi which reached sizes of about 45 to 70 cm in length, but there are some even possible bigger species (although from fragmentary remains) that potentially exceed these; Hungioides bohemicus, a Dikelokephalinid trilobite, based on a insolated pygidium if it is scaled with most complete specimens reach up a length of 90 cm long, which would make it the biggest trilobite ever.

Mollusks started bright in this period with the eventual rising diversification and complexity on different ecological levels, becoming widespread on the benthonic and nektonic levels with bivalves, gastropods and even further took important megafauna niches.

Cephalopods moved away from its simple ancestral minuscule forms of few centimeters long to diversify and spread as the major dominant animals on the shallow epicontinental seas and tropical regions of the planet during the Middle and Late Ordovician, most of these early forms were mostly Nautiloids, ancient relatives of the last living shelled cephalopod of the genus Nautilus but unlike these the shells of them were straight or slightly curved. Some of the largest species belong to the groups actinocerid and endocerid with shell sizes of around 1 to 2 meters, and from these stands out some peculiar titans like the endocerid Endoceras giganteum which holds the title of the largest Paleozoic invertebrate and one of the biggest cephalopods known, which most of its long size came from the large shell which a fragment of 3 meters long is calculated to have been complete in length around 5.7 meters; important to mark out due to the lack of soft tissue preservation of these is unknown to get the possible full size of its original soft body and anything presented here is speculative.

Note: people may know about that 9 m long shell specimen mentioned, which is always the size scale used to refer as the biggest shell, the reality about its feasibility is that this suppose specimen is just anecdotic in evidence and not very trustable due to the lack any visual proof of its existence like a photo of the fossil, this due to the destruction of the specimen on one point before it was properly collected, and without physical evidence we likely have to stick with the concrete known facts, even though there is tentative consideration of possible large individuals have existed but we have yet to find any remain of these to totally confirm these claims.

Gastropods or at least stem-clades relatives to our current clades were spreading across the seabed of marine waters with thousands of different species, of course many of these were just few centimeters long some reaching shell sizes less than 10 cm of diameter/length, although among them there was Maclurina manitobensis which although the average size is around 10 cm, there is a specimen shell with a size of about 25 cm diameter and 12 cm tall, is not only one of the biggest gastropods of the Ordovician but also is the biggest known Paleozoic gastropod (until some other even bigger shell or remain is found, honestly I would think there were large species that didn’t fossilized or even more likely aren’t published)

Chordates made a substantial evolutionary development from their small vermin forms of the cambrian and started to shape into the animals they were going to become as well experimenting with peculiar forms: Within the first armored jawless fish were the arandaspids being this simple clade of long shelled forms covered in oval armor plates and many hardened scales the most know species Arandaspis; among the largest is one of the oldest species know of the period, the peculiar tailed Sacabambaspis janvieri with a specimen able to reach out 25 cm long as well the more robust Astraspis which was measured around 20 cm.

Another group of vertebrates, one that already existed since the cambrian, Conodonts were a branch of unique eel-like chordates characterized by their teeth pieces and arrangement that varied of different structures called “elements” that helped them thrive on the oceans of earth during the Paleozoic, their number and amount of these pieces are so large that these are one of many key biostratigraphic identifications across different time periods, on the Ordovician there were found some large individuals including the one of the best preserved specimens being Promissum pulchrum with 40 cm long and characterized by its mouth apparatus formed by 19 very narrow elements of 1 cm long, but there is also another contenders that could potentially surpass this, the species Archeognathus primus which unlike Promissums it only had 6 pairs elements, and Iowagnathus grandi with 15 spiky elements, based on comparison to other more complete conodonts this could have been able to reach sizes of around 50 cm to even 1 m of length.

I think is worth to mention that during this period seems that also appeared the Thelodonti (the least armored agnathans covered by tiny special dentricles-like scales) and probably the first Gnathostomes (jawed vertebrates) which they are only known mostly for fragmentary evidence (scales) and they wouldn’t have any major role until the Silurian.

The Poriferans/Sponges were among the basic reef builder group on this period alongside other benthos organisms like brachiopods and bryozoans, these were able to develop into large forms in shapes and forms being some of the most prominent the stromatoporoids, a variety of demospongids that prevailed though most reefs of the early Paleozoic. Among these ones comes some of the tallest species, belongs to the genus Aulacera, a pillar-shaped sponge which in live were able to reach about 1 to 2 meters in height and around 20 cm in diameter, although some specimens from the species A. undulata were measured to reach heights of around 3 m tall and 30 cm of diameter (with some estimations of 5 m tall but due to how old are these estimations I have to take these with a little of doubt)

Just like many of the mentioned clades, the Echinoderms came with a very remarkable diversification across the oceans after the Cambrian establishing most groups, and similar to many other seabed animals they became a major component of the marine environments diversity, establishing some peculiar groups like blastozoans and the current living clades such as crinoids, asterozoans (starfishes and relatives) among others; most of these were just less than few centimeters in body size, most not exceeding lengths of 10 cm, only being the crinoids the largest of all with most of their sizes being partially by their columnar stems that hold their small cups and tentacles at the top, such as, Ectenocrinus simplex of 70 cm stem long, and the more lanky Cincinnaticrinus varibrachialus and Merocrinus curtus, which both are characterized for have very small narrow cups of 0.5 cm but a stem able to reach 1 m in height, although some of the largest individuals belongs to the robust Anomalocrinus incurvus, with a cup and body length of almost 10 cm and a stem height of 60 cm to 1 meter.

Due to the conditions of fossilization most Cnidarians of soft body forms didn’t preserve but those that were able to create a stony body which was able to leave marks in the fossil record of this period in great, especially those that thrived as reef builders in ways similar to current coral reefs, being the common colonial forms and many other solitary corals, most belongs to the ancient group of Rugosans and Tabulates, and the Conulariids, a peculiar clade of still disputable affinity to cnidarians, being long very triangular/pyramid bodies corresponding to just part of its life cycle as they are considered to be part of the scyphozoans (true jellyfishes), their stony remains often are around the sizes of 10 to 20 cm long, but Archaeoconularia was a genus with specimens able to reach sizes of around 50 cm of height, making one of the biggest conulariids known.

References

-Van Roy, P., Briggs, D. E., & Gaines, R. R. (2015). The Fezouata fossils of Morocco; an extraordinary record of marine life in the Early Ordovician. Journal of the Geological Society, 172(5), 541-549.

-Van Roy, P., Daley, A. C., & Briggs, D. E. (2015). Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps. Nature, 522(7554), 77-80.

- Lamsdell, J. C., Briggs, D. E., Liu, H. P., Witzke, B. J., & McKay, R. M. (2015). The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa. BMC Evolutionary Biology, 15(1), 169.

- Rudkin, David M., et al. "The world's biggest trilobite—Isotelus rex new species from the Upper Ordovician of northern Manitoba, Canada." Journal of Paleontology 77.1 (2003): 99-112.

-Corbacho, J., & Vela, J. A. (2010). Giant trilobites from lower Ordovician of Morocco. Batalleria, 15, 3-32.

- Gutiérrez-Marco, J. C., Sá, A. A., García-Bellido, D. C., Rábano, I., & Valério, M. (2009). Giant trilobites and trilobite clusters from the Ordovician of Portugal. Geology, 37(5), 443-446.

-Klug, C., De Baets, K., Kröger, B., Bell, M. A., Korn, D., & Payne, J. L. (2015). Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change. Lethaia, 48(2), 267-288.

-Teiciiert, C., & Kummel, B. (1963). SIZE OF ENDOCEROID CEPHALOPODS1. Breviora, (121-178).

- Pradel, A., Sansom, I. J., Gagnier, P. Y., Cespedes, R., & Janvier, P. (2007). The tail of the Ordovician fish Sacabambaspis. Biology Letters, 3(1), 73-76.

-Sansom, I. J., Smith, M. P., Smith, M. M., & Turner, P. (1997). Astraspis-the anatomy and histology of an Ordovician fish. Palaeontology, 40(3), 625-644.

- Gabbott, S. E., Aldridge, R. J., & Theron, J. N. (1995). A giant conodont with preserved muscle tissue from the Upper Ordovician of South Africa. Nature, 374(6525), 800-803.

-Liu, H. P., Bergström, S. M., Witzke, B. J., Briggs, D. E., McKay, R. M., & Ferretti, A. (2017). Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA. Journal of Paleontology, 91(3), 493-511.

- Briggs, D. E., Liu, H. P., McKay, R. M., & Witzke, B. J. (2018). The Winneshiek biota: exceptionally well-preserved fossils in a Middle Ordovician impact crater. Journal of the Geological Society, 175(6), 865-874.

-Aldridge, R. J., Purnell, R. J., Gabbott, S. E., & Theron, J. N. (1995). The apparatus architecture and function of Promissum pulchrum Kovács-Endrödy (Conodonta, Upper Ordovician) and the prioniodontid plan. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 347(1321), 275-291.

- Rohr, D. M., Blodgett, R. B., & Furnish, W. M. (1992). Maclurina manitobensis (Whiteaves)(Ordovician Gastropoda): the largest known Paleozoic gastropod. Journal of Paleontology, 66(6), 880-884.

- Galloway, J. J., & Jean, J. S. (1961). Ordovician Stromatoporoidea of North America. Paleontological Research Institution.

- Meyer, D. L., Miller, A. I., Holland, S. M., & Dattilo, B. F. (2002). Crinoid distribution and feeding morphology through a depositional sequence: Kope and Fairview Formations, Upper Ordovician, Cincinnati Arch region. Journal of Paleontology, 76(4), 725-732.i

- Deline, B. L. (2006). Inter-and Intraspecific Morphological Variation of Crinoid Columnals in Relation to Water Depth in the Type Cincinnatian (upper Ordovician) (Doctoral dissertation, University of Cincinnati).

-Van Iten, H., Gutiérrez-Marco, J. C., Muir, L. A., Simões, M. G., & Leme, J. M. (2018). Ordovician conulariids (Scyphozoa) from the Upper Tiouririne Formation (Katian), eastern Anti-Atlas Mountains, southern Morocco. Geological Society, London, Special Publications, 485, SP485-5.