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The anus!
It might just be one of the greatest inventions
animals ever stumbled into evolving.
You put food in your mouth, and the waste can go
out of the other end of your digestive tube.
And the anus has been around for hundreds of millions of years,
but sometimes, a species evolves out of having one.
Last week in the journal Nature, scientists revealed that a long-extinct critter
once thought to be the ancient ancestor of you, me, and starfish,
is actually an anus-lacking relative to a bunch of very different creepy crawlies.
And that changes our picture of how these groups evolved.
The subject in question is a roundish,
millimeter-sized critter by the name of Saccorhytus coronarius.
It lived during the Cambrian Explosion, between 531 and 537 million years ago,
when the fossil record shows a dramatic increase in the diversity of animal life.
And scientists are always trying to find a way to sort
these ancient animals into life’s family tree.
Which animal’s ancestors led to which animal’s descendants?
How did life actually evolve?
Where does the anus arise?
One of the most important branches in that tree of life is the distinction
between protostomes and deuterostomes,
which may have formed a hundred million years before the Cambrian Explosion.
The difference lies in how they form their mouths
and anuses during embryonic development.
Humans are deuterostomes, along with every other animal with a spinal cord,
plus sea stars and a few other animal groups.
Most other animals, including arthropods, are protostomes.
Previous analysis of Saccorhytus fossils revealed a very obvious mouth,
and no obvious anus.
Which makes it hard to sort into one of those two groups,
or place it as something outside them.
But there were also a bunch of other structures
around the mouth that were a little more difficult to identify.
Some were kind of wrinkly, some kind of spiky.
Some even looked like they had holes in them.
A previous report interpreted those holes as pharyngeal openings,
which is a basic body part that evolved into more modern structures like gills.
And because that structure is a key feature in almost all deuterostomes,
they put Saccorhytus near the base of that branch.
But there were other hypotheses about where this animal fell on the family tree.
So in new research, another team took an even closer look at hundreds of new,
and better preserved, fossilized specimens…
firing x-rays and electrons at them to construct 3D models.
And those models revealed that Saccorhytus probably isn’t
an early ancestor we share with sea stars.
Those holes that the earlier research identified as pharyngeal openings?
They were only there because a bunch of spines had been broken off.
According to these more detailed models, plus how well their outer tissues
seem to survive the process of fossilization,
this tiny life form seems to be an early protostome.
That gives it a very different place in the tree of life,
and changes our understanding of who it’s related to.
More specifically, Saccorhytus is an ecdysone, which means it
belongs to a group of animals that includes arthropods and nematode worms.
But here’s the kicker: the team did confirm that the mouth didn’t lead to any exit.
There was still no anus to be found.
And as you may have realized, ecdysones generally do have anuses.
So it is no longer one of the first deuterostomes,
but Saccorhytus can still help scientists explain why some animals
whose ancestors have anuses lost theirs over time.
And given how useful they have been over evolutionary time,
it’s a question well worth answering.
In other ancient, tube-based news, paleontologists have been
on the hunt for the very first fruit eaters.
I guess it’s also other evolution news, because they want to understand how
fruit-bearing plants and fruit-eating animals could have helped each other evolve.
And according to a paper published last week in the journal eLife,
they have found a new suspect:
One of the oldest known birds in the fossil record,
which lived about 120 million years ago.
That’s the first half of the Cretaceous Period,
which doesn’t seem like it was all that long ago,
at least relative to the history of life on Earth.
But that’s also the same period where flowering,
fruit-bearing plants were really starting to get going.
It’s a tale as old as time: Bird eats fruit, bird poops seeds.
Bird gets fed, and baby plants get a new place to grow.
And the fossil record suggests that relationship really is about as old as it can be.
Our fruit-eating suspect is the raven-sized dinosaur Jeholornis.
While it is technically an early bird, it isn’t a direct ancestor to modern birds.
The first fossil was found twenty years ago,
and scientists claimed the stomach contents included seeds.
But whether or not those seeds were the bird’s food,
or merely hiding inside its actual food source, was up to debate.
Had the Jeholornis not died when it did, would it have digested those seeds
for energy or would the seeds have been pooped out for a new plant to grow?
To try and settle that debate, in this new study, a team of scientists
analyzed one of the best preserved skulls we have of this species.
Notably, when comparing the jaw structure to those of modern birds
with seed-based diets, it didn’t look like Jehelornis
would have cracked open seeds before swallowing them.
And that suggests they were fruit-eaters, not seed-eaters.
But modern birds that rely on seeds for food
don’t just use their beaks to break seeds down.
They swallow gizzard stones so their guts can grind them up for them.
And some Jeholornis fossils do appear to have such stones in their guts.
But there was one clue that the team noted:
no fossil has been found with both seeds and stones.
And when there were seeds, they were intact.
So they hypothesize that Jeholornis had a diet
that varied over the course of the year.
During some seasons, it ate whole fruits, and during others, it ate something else.
They weren’t able to figure out what that something else was,
because gizzard stones are common in a lot of different birds with different diets.
If that’s true, it’s one of the first pieces of evidence that dinosaurs
could adapt their diet to the seasons,
which we still observe in some bird species today.
And knowing that there was at least this one bird out there,
spreading seeds in those ancient days,
could help explain the spread and surge in population of fruiting plants.
Or as scientists call it, the Cretaceous Terrestrial Revolution.
Given that birds and fruit-bearing plants seem to appear in the fossil record
around the same time, they may have helped each other evolve.
Modern birds could owe their amazing flying skills
to their ancestors needing to move from tree to tree for more food.
And modern plants could owe their big juicy fruits to their ancestors needing to
make themselves as attractive to seed-spreading birds as possible.
Dinosaurs and fruits… a friendship that lasts to this day.
If you want to keep learning about birds and discovering new animals,
Wondrium has The National Geographic Guide to Birding in North America.
This 24-part series takes you through bird colors and sizes to look out for,
as well as the time of year and locations you’re most likely to see them.
For example, lesson 11, “Essentials of Bird Migration,”
taught me that North America has four major flyways or migration zones.
But that’s only part of birding.
This series also has episodes dedicated to the equipment you might find useful,
like binoculars and cameras.
And they teach you how to use them!
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