Tuesday, April 27, 2010

The world's OLDEST MULTI-ARMED Starfish!! A lesson in Starfish Evolution!


(note that the scale bar is 10.0 mm)

Today, a cool post with contributions from my colleague Dr. Liam Herringshaw who blogs at the Daily Liamand he's written two very interesting papers on the OLDEST known multi-armed starfish! Most of this blog post is based on this paper in the Zoological Journal of the Linnean Society. But he's also written this paper in Paleontology about the fossil starfish diversity from the Silurian of England.

So people know that MOST starfish have five arms. But a bunch of starfish species all over the world have 6 ore even more arms..many have about 10-15 arms..up to 50 in the Antarctic Labidiaster.

Among the better known of these is the predatory Sunflower star (Pycnopodia helianthoides) from the west coat of North America....
Multi-armed starfish occur in tropical (for example, Coronaster) and cold-water (for example, Labidiaster or Solaster) environments as well as in shallow-water (Coscinasterias or Pycnopodia) and deep-water habitats (for example, the Pacific California Rathbunaster or the brisingids).

Liam's PhD work focused on, among other things, fossil starfish from Silurian deposits (about 444 million years ago) in the English Midlands and Welsh Borderlands. His work introduced him to this amazing beast..
The legendary Manx naturalist Edwin Forbes described the fossil in 1850 (this is the Forbes for whom the Atlantic starfish Asterias forbesi is named), calling it Lepidaster grayi.

The name is composed of the Ancient Greek for "scale"...Lepidos and -aster for "star" for the scale-like body elements. Forbes named the species for John Gray, one of Forbes' colleagues and fellow scientists.

Based on Liam's accounts, it apparently not clear to Forbes if his fossil was even a starfish!
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As author-illustrator of the definitive British guide, Forbes knew his living starfish, but this fossil baffled him. At first it reminded him of the common sun star Crossaster papposus, individuals of which often have 13 arms, but Forbes began to have his doubts. Was this pal├Žontological oddball even a starfish at all? Could it be the missing link between starfish and sea lilies, their many-armed cousins?

Liam's paper breaks down the evolutionary importance of Lepidaster and what it means to be a PALEOZOIC multi-armed starfish! Let's go through the major points!

1. This is the earliest fossil where we find multi- armed starfish in the Fossil Record!
If we were to compare this to vertebrates, this might be like looking at all of the mammals, turtles, lizards, snakes, dinosaurs, crocodiles, etc. and trying to find the very FIRST time that something stood up and walked around regularly on two legs. A minority of mammals do it today-but wouldn't it be interesting if some OTHER species did it before??

So, multiple arms is a recurring evolutionary theme in sea stars. Most have five-but it occurs over and over across very different evolutionary lineages. More on this below under #2...

This tree basically shows APPROXIMATELY where Lepidaster is placed relative to modern starfishes and how far away from the starfishes you see in the oceans today would be from something fossil like Lepidaster.


2. Multiple Arms appear Multiple Times in the Evolution of Starfishes!

So, not only did multiple arms appear first, but it is the first of MANY times that we see multiple arms throughout starfish evolution.

A tree of starfish is below...the "starbursts" on the right give you an idea of all the different evolutionary branches where the multi-armed condition is seen..
This modified from Blake (1987) taken from the TOL Asteroidea page.


...So bottom line is that having multiple arms is the result of what's called CONVERGENT evolution also known as homoplasy.
That is to say that the SAME body form or structure is seen in two or more completely separate evolutionary lineages. For example, dolphins and whales, superficially resemble fish-with fins and the teardrop body shape, because they live in open ocean/water, etc.... But aside from both being vertebrates, they are NOT directly related.

The same is often observed in starfish! Having multiple arms works in starfish evolution. A hit that keeps on coming back!


3. The Ecology. Why have multiple arms?? Was Lepidaster the first??

Liam has provided some intriguing speculation on the nature of the multi-armed condition...

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Does having extra arms help?
If the ratio of body size to arm length is kept consistent, but eight extra arms are added, the volume (or biomass) of a multiradiate starfish is roughly double that of a 'normal' form. This means a marked increase in the energy required to keep the animal functioning. If you need more food, there are two ways you can go about getting it. You can out-muscle your competitors for existing food sources, or you can start exploiting new ones. Many starfish with supernumerary rays seem to have taken the latter course of action, from consuming coral,
to eating other echinoderms, even to scoffing (=eat) fellow starfish.
Perhaps Gray’s scaly star was the pioneer. With its mouth in the middle of a much larger and more flexible body than that of its contemporaries, it would certainly have been capable of doing something different.
Herringshaw and his colleagues speculated that Lepidaster might be similar to the solasterid Crossaster. Here for comparison against the mouth shown above is a related solasterid, Heterozonias, (a deep-sea predator) which looks similar...
To further explore Liam's notion's we often invoke a principle known as uniformitarianism which basically assumes that the "present is the key to the past". In other words, processes that are at work today are those SAME processes that occurred in the past.
And so, we look to modern starfishes for possible ecology, feeding strategy, and evolutionary ideas about how the multi-armed condition arose.

Specifically, we look at the functional morphology of modern forms, in other words, any kind of direct behavioral or ecological inference based on the external morphology-like how sharp teeth are used to tear and eat flesh.....something that we hope will surrender clues or provide inference of what the fossil animal's life mode might have been like...

A good example of different multi-armed species that have a specific ecological function associated with their form are taxa like the Brisingids, who use their arms to pluck food out of the water as suspension feeders. I wrote these up here.
If we follow up with an example of what a modern predator like Solaster looks like...
Compared to Lepidaster...
There's a LOT of differences in the fundamental skeletal structure between these two animals. Bear in mind that ANY similarlities are almost certainly because of convergence! (Solaster is a highly derived modern starfish)

That being said, the overall form of the body between Lepidaster and solasterids, such as this predatory Arctic sun star Solaster endeca is curiously similar.
This sort of implies that Lepidaster probably was some kind of predator. But the similar body form COULD be due to some entirely OTHER evolutionary adaptation!

Thus, the mystery may remain unresolved, barring better fossils preserving catching Lepidaster eating prey (or the unlikely discovery of a living Lepidaster somewhere), BUT we have a much better idea of what this mysterious and intriguing beast was like and considerations about its lifestyle and evolutionary "story".....

My, grandma starfish, what a lot of arms you have!

All the better for grabbing you with!

Why grandma starfish, what a flexible mouth you have!

All the better for extruding my stomach and smothering you with!

Thanks again to Liam Herringshaw for his pix and insights-and prose above! (Responsibility for all errors is all mine...)

1 comment:

soliussymbiosus said...

Love the paleoposts!

While asteroids are very rare in the Ord. of my region, they are known from an outcrop that produces various rare echinoderms and trilobites.

Unfortunately, the strata that produces the rare forms is at the base of the column. As such, good exposures are few and far in-between.

One of these days...