Quick Look! An Illustrated Guide to Marine Life in the Paleozoic Era 600 Million Years Ago (Part 1)

An evolutionary boom that has yet to be fully explained kicked off the Cambrian Period, marking the beginning of the Phanerozoic Eon, which is divided into the Paleozoic, Mesozoic, and Cenozoic Eras .

The Paleozoic Era lasted from the early Cambrian period 539 million years ago to the end of the Permian period 252 million years ago. As the name suggests, this was a long period of very ancient life forms, with a large number of ancient and mysterious creatures, some of which already had the shadow of modern organisms, while others looked very strange and were completely unimaginable from the perspective of future generations.

Among them are the earliest and most primitive molting animal, the Cynocephalus, which looks like a potato with a mouth and spikes; and the earliest known animal on Earth capable of building reefs on a large scale, the Archaeocyphaga, which is like a cup with a bunch of holes punched on the inner wall. It is fixed to the seabed and filters plankton in the seawater; there are also the Cambrian overlord Anomalocaris, the unique benthic swimming trilobite Diplodocus and the floating filter feeder Stratum in the Ordovician, and the Devonian benthic fish pioneer Moon Turtle. These peculiar creatures are the brightest stars of each era .

The strange creatures of the Cambrian period, among which A is a cave fossil, B is a hard shell fossil, C is a trilobite, D is the early arthropod Marella worm, E is an echinoderm, etc.
Image source: Reference [8]

Next, we invited special Blackwater photographer Cheng Wen to show everyone her atlas of Paleozoic life shot by "traveling back to the Paleozoic Era" using the "Blackwater Photography" method.

Part 1

Lyre worms: the most powerful brain in the Cambrian period

In the Cambrian ocean, predators were scarce. After all, eating grass and mud does not require intelligence, but preying on moving animals requires precise calculation and athletic ability. In the third period of the second Cambrian period, Chengjiang in China was full of mindless ancient insects, just like other places in the world.

They are a strange animal with a bipartite body, with a large foreskin with gill slits in the front, the surface of which is covered with a hard, special plate-like structure with fine gill filaments inside for filtering food. At the back is a slender posterior body with flat sides at the end that can swing, and the digestive tract is at the very end of the tail.

This group of ancient worms, which had no heads and no decent sensory organs, swung their tails and swayed on the seabed, trying to drink seawater containing organic debris.

Lyrarapax unguispinus, a small Cambrian shrimp, photographed in the third period of the second Cambrian

Image source: drawn by the author

Lyreworms are a type of shrimp named after their resemblance to the ancient Greek lyre. They lived in the early Cambrian period 520 million years ago and belonged to the Amplectobeluidae family. They were flat, segmented marine animals and one of the earliest predators on Earth . They were widely distributed in the oceans of the Cambrian period.

In this photo, we can see through their translucent bodies to the orange brains inside and the thick nerves that transmit signals like circuits within the body. These thick nerves regulate their movements like sophisticated instruments.

The head and nervous system of a lyre beetle, with the nerve cord, brain and heart visible. You can also see the captured ancient lyre beetle

Image source: drawn by the author

In order to complete the complex neural calculation of predation, lyre worms have evolved a well-developed nervous system - the forebrain is connected to the optic system, and there is a pair of huge ganglia in front of the optic nerves to control the large claws unique to the Anomalocaris.

Like modern shrimps, their nerve cords are located on the ventral side, while the internal organs are located on the dorsal side. The brain ganglia are located in front of almost all internal organs, and the nerves are very thick and responsive.

Their sensory system was also top-notch at the time : a pair of huge, cone-shaped compound eyes with long eye stalks extended backward from both sides of the head and could move freely, helping them to locate prey visually.

The head structure of a lyre beetle

Image source: Reference [5]

The body of the lyre insect is also specialized for hunting. The front of its body has a complex head armor to protect its head and prevent itself from being injured by the struggle of its prey during hunting. In addition, its neck connecting the head and body has four body segments, and its trunk also has strong muscles to drive the swimming lobe to swim.

Their swimming lobes gradually shorten from the first segment. The first lobe is the longest and widest, and becomes smoothly narrower backwards. When swimming, the rear lobe overlaps the front lobe. The overlapping parts between the swimming lobes have a linear structure, which is used to strengthen the body strength, just like staples nailing adjacent swimming lobes together to form a single "fan" that coordinates actions and helps the body move in the water.

The most peculiar thing is their respiratory system. Anomalocaris do not have gills , but instead have two rows of bristles on the back of each segment of the trunk. The base of the bristles is connected to a base structure that is close to the boundary of the body segment. When swimming, the bristles will absorb oxygen from the passing water flow, and then pump it to the whole body through the beating heart behind the brain.

Close-up of the back of a lyre insect, showing the bristle strips attached to the segments, the slightly opaque muscles, and the yellow-white intestine.

Image source: drawn by the author

The most obvious thing on the back of the lyre worm's body is its digestive system , which is mainly composed of an enlarged intestine. This well-developed tube carries the secret of the rapid growth of the Anomalocaris larvae: the more they eat, the faster they digest, and the faster they grow.

Its front part is the foregut, and the back part is the undifferentiated midgut and hindgut carrying digestive glands. When the Anomalocaris larvae grow to 6~7cm, this intestine will become thinner, which means that they have sufficient mobility and do not have to worry about being eaten by Cambrian creatures with low intelligence, sensory and motor abilities. Therefore, they no longer need to grow quickly and can live a peaceful life.

Part 2

Diplocera: The earliest user of ground-effect flight

Although the agile Anomalocaris stole the show in the Cambrian period, the most common life forms in the Paleozoic Era were trilobites, graptolites, and brachiopods . Most trilobites were poor swimmers, but Hypodicranotus of the order Ctenophora took a different approach and evolved a swimming function similar to modern ground-effect vehicles.

Diplodocus lived in the Middle Ordovician. Like other trilobites, its body was segmented, with grooves dividing the body into three vertical lobes. In addition, it had a huge, forked suboral plate growing under its head. The suboral plate was prominent, smooth and round, giving the shell a rounded and streamlined shape instead of only a semicircular dorsal shell like other trilobites.

Diplocera, a trilobite of the genus Ctenophora, from the Middle Ordovician

Image source: drawn by the author

The suboral plate of the diplodoceid is close to the seabed. This structure is like a wing. When seawater flows through the suboral plate, it not only reduces the induced drag in the seawater, but also increases the pressure difference between the upper and lower parts of the trilobite, thereby suddenly increasing the lift.

Especially when the distance from the seabed is only half the thickness of the trilobite body, its lift is the largest, making it easier for the bilaterian to swim. This principle is very similar to the current ground effect vehicle, which is both a coincidence and a masterpiece in biological evolution .

The hypostome of the bilaterian (blue part) can guide water flow

Image source: Cradle CFD The University Museum at the University of Tokyo｜List

In addition, the suboral plate of the bilaterians can also create vortices to concentrate organic-rich seawater into the mouth. Their filter-feeding habits and swimming ability allowed them to be widely distributed in shallow seas and oceans in the Middle Ordovician.

How the subostial plate guides water flow

Image source: Cradle CFD The University Museum at the University of Tokyo｜List

In the waters of Ontario, Canada, there are diplodoceida. They swim near the bottom of the ocean. The red and white edges outline the contours of each segment against the black background. The carapace composed of calcium and chitin is decorated with white, making them shine like stars.

When the mouth of the diplodoceidae faces downward, creating a vortex, the water will flow through the primitive two-limbed appendages under the body, half of which are feathery gill filaments and the other half are ordinary appendages. Food particles will be enriched through the dense gill filaments on the gills and then sent into the mouth.

Looking up at the Diplodocerus from below, you can see the bipedal appendages with gills and the V-shaped suboral plate.

Image source: drawn by the author

Here, the diplodocephaga can fully utilize the lifting advantage brought by the suboral plate and reduce physical energy consumption.

Even so, the bipunctate worms will still swim for a while and then stop on the seabed to rest. This may be because their muscle tissue, like other arthropods, is mainly composed of fast muscle fibers, and their endurance is relatively poor, so they feel tired after swimming for a while.

In addition to these arthropods, other classes of creatures in the Paleozoic Era were equally beautiful. In the next issue, we will use Straight Graptolites and Lunar Softshells as examples to talk about Graptolites and fishes—very interesting creatures from the Ordovician-Silurian and Devonian periods, respectively.

References:

[1]Shiino Y, Kuwazuru O, Suzuki Y, Ono S. Swimming capability of the remopleuridid ​​trilobite Hypodicranotus striatus: hydrodynamic functions of the exoskeleton and the long, forked hypostome. J Theor Biol. 2012 May 7;300:29-38.

[2] Shiino Y , Kuwazuru O , Suzuki Y , et al. Pelagic or benthic? Mode of life of the remopleuridid ​​trilobite Hypodicranotus striatulus[J]. Bulletin of Geosciences, 2014, 89(89):207-218.

[5]Brain structure resolves the segmental affinity of anomalocaridid ​​appendages (2014)

[6]Origin of raptorial feeding in juvenile euarthropods revealed by a Cambrian radiodontan (2018).

[7]Morphology of the Radiodontan Lyrarapax from the Early Cambrian Chengjiang Biota (2016).

[8] Erwin DH, Laflamme M, Tweedt SM, et al. The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals[J]. Science, 2011, 334(6059):1091-7.

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