Is this a bone?

[bridgetloud]

Hello everyone! Just yesterday by a river in south central Indiana I found this. It did not look like a normal rock so I decided to keep it. I think it is a bone, but I cannot tell what kind of bone it is or how old it is. Is it a bone and if so, what kind?

[Lone Hunter]

It's some kind of coral, maybe horn coral?

[Kane]

Agreed. Coral.

[Rockwood]

Looks like it must be a rugose coral. It's a little hard to be certain if it is a solitary horn or part of a branching colony, but the bulge is typical of the growth cycles seen in some horn corals.

[IsaacTheFossilMan]

Not a bone, but something equally as cool, if you're into invertebrate stuff!

 

What you have there is a rugose or "horn" coral. They were abundant in the seas of the Middle Ordovician to the Late Permian, going extinct around 250 million years ago! The Paleobiology Database (PBDB) recognises 1337 species of them - that's quite a lot...

 

Several diagnostic features are present in your specimen (I can count three!) that help us distinguish this from a tabulate coral. 

1. The complex septa radiation in your specimen is unlike that of tabulate corals
2. The remains of a columella is present
3. Bilaterian symmetry is shown from the fossula

 

Number 2 and 3 are interesting - a columella is a spine-like structure in the corallite "skeleton" of the rugose coral - this was necessary as the rugosa were variably solitary and colonial. Forming reef-like structures, the stability of a coral is less important, as it can rely on others to stabilise itself - think of it like a cola can in a box - because there are several of them, its almost impossible for a middle one to fall over.^{[citation needed ]}

However, when they are solitary, its very easy to push a can over - as the rugose corals could be either solitary or in a reef, it was necessary to have a columella present in every critter, as otherwise some of them would just fall over!

 

Number 3 is also interesting - the fossula is a crease-like dip in the radiations of the septa - this displays bilateral symmetry (symmetry on two sides, much like us humans) - which is different to the other corals tabulate and scleractinian.

 

Blue = septa,

Red = columella

Green = fossula (highlighted) and bilateral symmetry (dotted)

 

 

Here is a diagnostic rugose coral:

 

The identification of whether a specimen is either solitary or colonial comes from the overall morphology of the coral.

 

 

Here are reconstructions of colonial rugose corals (left), and solitary corals (right). Can you spot the difference? 

 

Colonial rugose corals don't taper off to a point as drastically as solitary corals - to conserve space, it's better for colonial corals to be more cylindrical in nature - but to attach to the floor, it's better for solitary corals to form the typical "horn" shape expected of a rugose coral. The diagnostic specimen above is thus solitary, and, as yours is colonial.

 

As @Rockwood suggested, the bulge in your specimen is also typical of rugosa - the growth lines being present in the epitheca (outer wall). If you look at all above pictures, you'll see the same bulge!

 

A fourth and final diagnostic feature will be present - but I will leave that up to you... By counting the septa, it's observable that the amount of septa in a rugose coral is almost always a multiple of four - hence the other name of rugosa - "Tetracorallia", distinct from modern "Hexacorallia".

 

Isaac

[Rockwood]

 I wonder, do branching rugose corals bud, or divide ? 

[IsaacTheFossilMan]

3 hours ago, Rockwood said:

 I wonder, do branching rugose corals bud, or divide ? 

 

Very interesting question!

 

In the formation of coral colonies, two supertypes of branching or budding is known. It is also useful at this point to make distinctions - a "polyp" is the living part of a single coral, and its skeleton is known as the "corallite" - the term "coral" refers to both of these parts.

 

Now, onto the types of branching! (I'm going to attempt to create some diagrams...)

 

There are two "supertypes" of branching - those with shared epitheca, and those with separate epitheca.

Separate epitheca are less common in modern corals, but more abundant in older corals, like the rugose corals. We will be looking at these types first!

 

A separate epitheca causes each corallite to be exsert - meaning it has the tendency to "push outwards", and grow away from the other neighbouring polyps. There are two main types of these, distinguished in a somewhat wishy-washy relative definition. If the corallites are more elongate and tubular, they are known as "phaceloid". A diagram of this is below:

 

 

Light pink = corallite + epitheca; purple = polyp; dark pink = costa

 

However, if the corallites are more dome-shaped and less elongate, they are know as "plocoid" - they still have their own epitheca, though!

Here is a plocoid coral:

 

 Light pink = whole colony, purple = polyp, dark pink = corallite + epitheca

 

Now, we move onto the corals with conjoined, or shared epitheca. These do not tend to push away from each other because, well, they can't, as they are connected, but instead form really pretty colonial shapes. The different two types here are defined by whether they form "valleys" or not. If the corallites are less circular, they form meandering valleys and hills - these are aptly named "meandroid" corals. Shown below is a typical meandroid.

 

  

Light green = whole colony, dark green = epitheca, yellow-green = single corallite and polyp 

 

But, what about things like Favosites spp.? They share a common epitheca, too! Well, that's the second type of common epitheca corals - without valleys - they are known as "cerioid" corals. The geometrically sound among them form epitheca of tileable shapes, such as hexagons, but some may just squish and warp epitheca around to make them fit.

 

Light green = whole colony, dark green = epitheca, yellow-green = single corallite and polyp

So, that should be everything, right? Wellllll...

 

Evolution is a cruel mistress. 

 

It decided to throw us a third main form, which is... sort of in the middle. We've named it "flabello-meandroid", from the Latin for a small fan - 'flabellum'.

These are similar to the original meandroids, but the epitheca isn't shared across multiple valleys of corals - these have alternating valleys of empty and filled.

 

Light blue= whole colony, dark green = blue, green-blue = single corallite and polyp

 

Most corals have vast tissue connecting each valley or corallite - spare from the flabello-meandroids and the phaceloid corals, which have little to none. I'll show you a picture of an actual flabello-meandroid, as the diagram I drew doesn't cut it.

 

Lobophyllia serratus, Brunei. Photograph: Emre Turak and Lyndon DeVantier

 

Notable are the gaps between each valley of polyps - the tissue interconnecting valleys is very limited.

Phaceloids and flabello-meandroids, without this interconnected tissue, often compete for space and resources - some parts of the colony overgrow other.

 

Some colonies may even exhibit two types of growth - towards the centre of the colony, space to grow is small, sometimes constraining growth of meandroidal growth, such as in Euphyllia and Lobophyllia - towards the centre they are phaceloid, but at the outside, they are flabello-meandroid. Specimina of Symphyllia have both meandroid and flabello-meandroid, Favia plocoid and meandroid, Goniastrea and Favites plocoid and cerioid, etc. There are also a few forms that are a mix of two - known as intermediate forms. An example of this are between plocoid and phaceloid, and, frequently, cerioid and meandroid. 

 

There are also several more types of colonial growth, but these are more rare, so have been omitted from this post.

So now we see, there are several types of branching corals! Selected rugose corals form with phaceloid colonialism - so, let's look at how that forms.

Well, we don't even have to look at phaceloids, as every single type grows the same way!

 

The multiplication of each polyp is called "budding". There are two main ways this occurs, and a third way that is less common.

First, its good to note that the method of budding doesn't necessarily define which type of colonial growth the colony will exhibit, as they can mix together - but in some genera, the words "meandroid" are used to describe both the overall look, and the method of budding - however, 99% of the time, you will use different terms - these are as follows:

 

Every form of budding works via cloning - the parent polyp divides by splitting, or dividing, itself into 2 or more daughter polyps - which are genetically identical to the original polyp. Note that evolution still takes place, as corals also sexually reproduce, known as "spawning".

 

Photograph: G.P. Schmahl/FGBNMS

 

This is a single polyp - it has a ring of tentacles, known as the tentacular ring. This polyp may now either asexually clone itself via intratentacular budding, or extratentacular budding. In intratentacular budding, the polyp clones itself, and the daughter polyps will now sit in the same tentacle ring. In extratentacular budding, the daughter polyps will sit alongside each other, in separate tentacular rings!

 

Photograph: https://marinelabresearch.wordpress.com/2014/08/12/its-the-most-wonderful-time-of-the-year-coral-spawning/

 

Note that colonies almost always exhibit both main forms of budding. 

 

These two forms of budding occur in plocoids, cerioids, flabello-meandroid, and phaceloid corals. Note that flabello-meandroid corals are just formed with lots of intratentacular budding.

 

However, in the valleys of meandroid corals, another form of budding occurs. This is known as "meandroid budding" - as I mentioned earlier, meandroid refers to both the budding and overall look, as the form of budding determines what the overall look will be. This is different to other corals, where, as shown in image above, both forms can take place with no impact on the overall look or form of the coral! In this budding, corallites no longer have separate walls that are complete.

 

 

 

So, in branching rugose corals, known as a phaceloid rugose colony, the polyps bud via asexual cloning, where they divide into two or more!

 

I hope this answers your question

 

Isaac

 

 

Edited August 7, 2022 by IsaacTheFossilMan

[Rockwood]

Wow. Thanks. I'll have to read this over a few times to digest it all. Hope everyone enjoys it.

[IsaacTheFossilMan]

2 minutes ago, Rockwood said:

Wow. Thanks. I'll have to read this over a few times to digest it all. Hope everyone enjoys it.

 

Haha, you're welcome! I'm thinking of writing some post somewhere discussing systematic analyses of different groups of life, I quite enjoy it!

[Meganeura]

7 minutes ago, IsaacTheFossilMan said:

 

Haha, you're welcome! I'm thinking of writing some post somewhere discussing systematic analyses of different groups of life, I quite enjoy it!

You absolutely should!

[IsaacTheFossilMan]

4 minutes ago, Meganeura said:

You absolutely should!

 

I appreciate the encouragement, I'll go for it!