Coral or bryozoan?

[PaleoOrdo]

I found this stone with several circles and some pattern. Could it be a coral or is it a bryozoan? Seems too small for bryozoan.

Anybody can help identify it? Size of diameter about 3cm. Late Ordovician.

 

[PaleoOrdo]

I mean seems too big to be a bryozoan.

Martin

[PaleoOrdo]

I found two other stones which have a similar pattern, which can illuminate more what it could be. 

It seems by this picture that chain coral can make circles from the chain.

 

[PaleoOrdo]

Another stone indicate that rugose coral have similar form or pattern as chain coral. Could that be an important fact, possibly the one developed into the other? First picture shows many circles inside a rugose coral, in second picture we see the similarity of the parrtns of rugose coral and "these circles" which however are made like in a chain. These are my thought, maybe I am on thin ground, as I am not an expert.

[PaleoOrdo]

Since nobody have any suggestion to these patterns, I propose it is ruguse (horn) corals, where we see only the outer circular edges, and also the internal structure of those edges. That could make sense, because those parts are often most or best preserved in these kind of coral (as is seen in the last picture above). It is however puzzling that these edges themselves are composed of many circular patters or structures, as in these pictures below (se the right upper corner coral, in the second picture). Maybe that is a general attribute of horn corals top edge structure?

Then again is the question for me what is the circular pattern in the third picture below, this maybe is something different, chain coral? Size of last rings are less than 1 cm, while the other circles in the other pictures are about 2-3 cm in diameter. 

 

Martin

 

[aek]

Yes, looks like some type of colonial rugose.

[PaleoOrdo]

Agreed, all of them. The pictures reveal the edge structure nicely.

[PaleoOrdo]

Would you likewise say that all and each of these alone, although of a different family, are rugose colonial corals, or just rugose solitary? Maybe the last 3 below is different, namely part of tabulate coral?

[PaleoOrdo]

The size of the corals is from 7 mm to 17 mm, all late ordovician (katian).

[PaleoOrdo]

Among the last series of pictures, I think that the last 3 is colonial corals. They have same form as those is this picture, which show a pattern, as parallell lines. These have parallell lines in two direction. The seemingly two directions may be due to where the center of the whole coral is located. I think then, in general,  that if we see these kinds parallell lines of corals, it is a proof that they are colonial. Any individual "coral-like" form in the stone which break the parallell pattern may be something else. It is a little strang though that the last picture above shows a bended coral. So maybe that one is not part of a colony?

[digit]

The amount of space between the coral polyps likely suggest this is not a clonal coral colony (a single clone that has formed multiple individuals by asexual budding to form a colony). Rugose (horn) corals were a common inhabitant of many habitats back when corals first appeared in the Ordovician and it is not difficult to envision a shallow muddy or sandy ocean bottom populated by a high density of solitary individuals. I'd say your specimen likely represents a nice example of a coral community rather than a colony. Keep in mind that I know a lot more about modern coral communities (soon to follow their Ordovician ancestors into the fossil record and become extinct). Other members more familiar with rugose corals (no Ordovician outcrops in Florida) may chime in with their thoughts.

 

 

Cheers.

 

-Ken

[PaleoOrdo]

Thank you for your interesting thought Ken. So you think the parallell directions of their positions are another reason than they be colonial?

Martin

[digit]

Picture a shallow sea bed back in the Ordovician. There was not yet the diversity of life that is found on a modern coral reef and so large numbers of a few types of organisms dominated the seascape. Hermatypic (reef building) corals probably did not exist yet (or at least were not dominant). Without 3D reef structure formed by the accretion of calcium carbonate the seascape was likely a lot more flat than today's reefs with only bioherms created by microbial crusts and stromatolites providing any structure. The bottom would have been dominated by the same organisms that we find when hunting Ordovician fossil sites--crinoids, bryozoans, rugose corals, trilobites. I've seen matrix slabs that are composed of virtually nothing but crinoid columnals and some enviable slabs with dozens of crinoid stems frozen into very beautiful fossil plates. Instead of a diverse habitat with an abundance of different species the reefs back in the Ordovician would likely have been simpler in species diversity but not necessarily less dense in individuals. Rugose corals would likely have been able to colonize fresh areas of open bottom (say after a storm or a mudslide that buried the former benthic inhabitants). In the same way that some weed species (like dandelions) can rapidly fill a disturbed area I can easily envision lots of individual rugose corals staking their claim on a tiny portion of the bottom forming an extensive community of individual (not colonial) coral polyps all planted in the bottom and thus oriented in the same general direction.

 

You can easily do internet searches for "reefs of the ordovician" and see some interesting reproductions of what they might have looked like. Here's one to get you started:

 

https://www.digitalatlasofancientlife.org/learn/cnidaria/anthozoa/rugosa/

 

It would be really great (if a bit monotonous) to have a time machine and be able to scuba dive the Ordovician.

 

 

Cheers.

 

-Ken

[PaleoOrdo]

Thank you very much for the interesting explanation and imaginative reproduction and weblink. I googled ordovician reefs and found a relevant article by Barry Webby: Ordovician reefs and climate: a review. https://paleoarchive.com/literature/Bruton1984-10-Webby-OrdovicianReefsClimate.pdf

The article is interesting not only for our discussion on coral structures, but also for topics on climate change and extinction of coral reefs, a concern which I share with you. According to Mr. Webby, concerning the latter issue, he write for instance that "Ordovician climate (with the possible exception of the Tremadoc which was climatically similar to the Late Cambrian) was more like that of the present than that of the Cambrian or Silurian". 

I understand that the ordovician reefs were different, as you mentioned, than the modern ones and in general smaller. The ordovician reefs were usually small sizes and usually lacked the differentiation into fore-reef, reef-core and back-reef facies as seen in most modem reefs. But the Ordovician reefs still show a remarkable degree of variation in form, size and organic composition in the reefs went throug a remarkable and revolutionary development during the ordovician age.

I have, even if I agree with you on your comments about coral communities versus colonial corals, some questions.

1. You wrote "Without 3D reef structure formed by the accretion of calcium carbonate the seascape was likely a lot more flat than today's reefs with only bioherms created by microbial crusts and stromatolites providing any structure." I agree that the modern reefs are higher and longer in diameter. But how high are the modern reefs in comparison really? 

 

2. When I did see the picture in your link, as in many other popular websites on ordovician sea bottom, you get the impression that the bottom was almost flat. That cannot be right where there was reefs. I guess that is not your opinion also ...

Mr. Webby reports "In the Swedish Boda Limestone there are much larger stromatactis-bearing carbonate mud mounds, 100-140 m high and up to 1 km across (Jaanusson 1982). They appear to lack any sort of organic frame. Similar reef-like bodies of limestone are represented in the Keisley Limestone of northern England (Ingham & Wright 1972), in the Chair of Kildare Lime stone of Ireland (Wright 1968), the Pirgu stage of Estonia (Männil 1966) and the Sa-limestones of Norway (Brenchley & Newall 1 980)."  He mention that in Norway, where I found the stones, the reefs was in some places up to 15 meters in late ordovicium, but much lower in early ordovicium. Of course, it may be that my stones's site is not those reefs.  Still, I found several reef-builders in the site: many kinds of tabulate and rugose corals, stromatolites and stromatoporoids, often side by side, especially I found several stones of stromatoporoids (known important reef-builders in ordov.) together with tabulate corals. One surface stone I found of tabulate coral is 50cm in diameter, too big to bring home :-)

Mr. Webby says that during ordovicium (MIDDLE-LATE ASHGILL) North of Europe moved northwards towards Equator, into a warmer climate area where reef structures grows. 

I also read somewhere that ordovicium can be seen as the age of reef-building, laying the ground for the later development of the habitat for development fishes in the silurian and devonian. I understand the differences that the reef builders at that time was not just corals, but also sponges, stromatolites and bryozoans. In my site it it also a lot of bryozoans and all these reef-structures, some of them smaller and other bigger, are fused togheter with organisms like bivalves, corals, brachiopods and gastropods made of calcium carbonate exoscheletons. Was the corals using these structures as a substrate? It seems so.

Martin

Martin

[PaleoOrdo]

I would like to add that the picture in your link, Ken, seems to be relevant to show a "scattered" coral environment, as my stone above exemplifies. On the other hand, according to mr. Webby, the ordovician reefs manifests a wide variety of reef and reef-like structures, and the stone may be situated in one of these. He mentions "carbonate mud mounds", "patch reefs" and "reef complexe". The first alternative is not possible for the site I found the stone, which could seem to be a reef complex, and also in a shallow water location, due to the location is far from the modern coastline (today about 500-600 m above sea level), and by the fact that I found many stromatoporoids in the site. Stomatoporoids are know to live in shallow water. The dominant reef-bulider in the site, however, as far as the number of stones I have seen there so far is tabulate corals. Mr. Webber notes that "Harland (1981) has described relatively small on-shelf coral-stromatoporoid patch reefs and a larger shelf-edge reef complex in late Caradoc successions of the Oslo region." I have not read that article yet, but he writes about middle ordovicium, while my site is late ordovicium (Katian). What makes it difficult to determine the kind of reef is that ordovician reefs in Skandinavia underwent much erosion, not only by the modern ice-ages, but also in ordovicium.

 

It may perhaps be a smaller patch reef, because in general small patch reefs dominated by corals and stromatoporoids occur in the Sb-limestones of the Oslo region and in general in northern Europe in the late ordovicium (Hanken & Owen 1982), of a of bahamitic type.

 

Anyway, it is very interesting to compare with modern reefs, which I know little about. Concerning extiction at that time, mostly due to changes in temperature and sea-level, it seems that sponges survived the late ordovicium extinction event more than most lifeforms. What could be the reasons for that? I will come back to that issue later, when I present fossils I found in an early silurian site in the Oslo region.

Martin