Quick Look! An Illustrated Guide to Marine Life in the Paleozoic Era 600 Million Years Ago (Part 2)

In the last issue, I introduced two arthropods, Lyra and Diplodocus, from the Cambrian and Ordovician periods respectively. They are the epitome of the era when arthropods dominated the world. In the following era, the rich species of graptolites and fishes could not be ignored . In this issue, we might as well take the examples of Straight Graptolites and Lunar Softshells to take a peek at their prosperity at that time.

Part 1

Straight Graptolites: Physalia from the Ordovician

Graptolites belong to the class Ptychobranchia of the phylum Hemichordata . Like vertebrates, they are deuterostomes, but are more closely related to echinoderms. In addition, like crinoids, they have chosen to degenerate their mobility and survive by filtering food.

The shape of a graptolite is like a small tube with tentacles.

Image source: Reference [5]

Individual graptolites are called "members". They live in interconnected collagen pipe systems secreted by themselves, just like coral polyps. The pipes are connected to each other to form colonies . When they die, they leave thin, shiny carbon traces on the surface of the rock, hence the name "graptolite".

The earliest graptolites either lived on the seabed, attached to boulders, or formed upright clumps, but in the early Ordovician period, some graptolites became free-floating organisms.

To exploit food reserves and cope with the new challenges of floating life, different species evolved rapidly, including Orthograptus quadrimucronatus in the shallow seas of the Ordovician.

Straight stylolites floating on the water, taken during the Ordovician period

Image source: drawn by the author

Like the benthic species of Rhabdopleura that still survive today, this huge colony of graptolites has a polymeric collagen shell in the form of orange tubes, each of which contains a living, nearly white individual graptolite with extended hollow tentacles.

Close-up of the sac and individual members of the Stratum Graptolite, showing the inflated sac and the living Graptolite with white tentacles

Image source: drawn by the author

Straight graptolites belong to the diploidean graptolites, a group formed by two rows of tubules arranged back to back and open on both sides. There is a well-developed spicule at the top of the group. This needle is not used to resist predators, but to create greater resistance with its thorny and mesh-like structure to prevent the group from sinking, allowing the graptolites to filter nutrients that rise with the water flow to the water surface in the upwelling current.

In addition, the Stratographium has a secret to maintaining buoyancy: many of its small tubes live together in groups, connected to the light blue floating sacs growing above by hanging filaments. The floating sacs are connected together and filled with secreted gas or low-density fat, allowing them to float half above the water like buoys, just like today's Portuguese man-of-war jellyfish .

Modern jellyfish, similar in form to Stratograptes, also have similar bladders and, however, their structure is quite different.

Image source: Reference [1]

When feeding, they slowly rotate in the water with the flow, filtering small organisms and organic particles in the water, like a Lantern Festival lantern.

Part 2

Moon Turtle: The earliest bottom-dwelling predator vertebrate

The first period of the Late Paleozoic Era, the Devonian , lasted from 419 to 359 million years ago and is known as the "Age of Fishes." The Devonian fishes ranged from placoderms like Dunkleosteus to lobe-finned fishes that eventually gave rise to the first tetrapods, including this lazing Lunaspis broilii.

Lunaspis broilii lying on the seafloor, showing yellow and black body coloration

Image source: drawn by the author

The moon turtle is a flat bottom-dwelling animal belonging to the order Cyprinidae of the class Placoderms . Like other Cyprinidae, this moon turtle has a well-developed head armor on its head, which is flat and has special groove-ridge decorations on the head armor, making it easy to distinguish from other Cyprinidae.

The moon turtle had elongated horn plates on both sides of its head armor, forming backward-curved spines. Its eyes were facing dorsally, which provided both stability and visibility of the environment above. The rest of its body was covered with tiny scales. Its tail was long and thin, like a whip, and it looked stupid, just like modern flat rays.

Close-up of the head of a moon turtle, showing the rough, ridged head armor and the serrated spines of the pectoral fin horns

Image source: drawn by the author

This form is very suitable for lying on the sand or waiting in the sand to prey on small creatures passing by to fill their stomachs. Of course, the moon turtle does this.

The long horn plate of the moon turtle's head armor is located slightly in front of its pectoral fins. There are many small spines along the front ridge of the horn plate, forming a serrated leading edge. This structure is similar to the pectoral fins of the Taole catfish, known as the "modern armored fish."

Modern "platoderms" of the family Taurenidae have thick, almost naked bone armor and are covered with spikes and serrations, very similar to moon turtles.

Image source: AquaInfo

The use of the horn plates is similar to that of the pectoral fins of the Taole catfish: the long horn plates of the moon turtle make them look large, rough and unpalatable in the eyes of predators, who cannot swallow them and can only leave in disappointment.

If a greedy predator insists on swallowing them, the serrated spines of the moon turtle will get stuck in the enemy's throat, making it unable to swallow or being stabbed. Sometimes, the small moon turtle can even die together with the huge predator, so they have almost no natural enemies in the sea.

Starting with the moon turtle, various armored fish and later more advanced fish such as bony fish began to live a benthic life , squeezing out the ecological niches of small jawless fish and horseshoe crabs, which were the main organisms in the benthic environment in the past, and ushered in an era in which fish dominated the seabed.

Okay, that’s all for this time. Our Blackwater paleontological photography trip is coming to an end.

In the long geological age of the past, in the ocean, which was as vast as the universe and as long as time, there lived countless beautiful, lively, agile and exquisite creatures. They were like bright stars, twinkling against the dark background, performing their own life legends.

References:

[1]Munro C, Vue Z, Behringer RR, et al. Morphology and development of the Portuguese man of war, Physalia physalis[J]. Scientific reports, 2019, 9(1): 1-12.

[2]Young G C. Further petalichthyid remains (placoderm fishes, Early Devonian) from the Taemas-wee Jasper region, New South Wales[J]. Bureau of Mineral Resources Journal of Australian Geology and Geophysics, 1985, 9: 121-131.

[3]Goldman D. Taxonomy, evolution, and biostratigraphy of the Orthograptus quadrimucronatus species group (Ordovician, Graptolithina)[J]. Journal of Paleontology, 1995, 69(3): 516-540.

[4] Castiello M, Jerve A, Burton MG, et al. Endocranial morphology of the petalichthyid placoderm Ellopetalichthys scheii from the Middle Devonian of Arctic Canada, with remarks on the inner ear and neck joint morphology of placoderms[J]. Canadian Journal of Earth Sciences.

[5]Maletz, Joerg. Graptolite reconstructions and interpretations[J]. Palaontologische Zeitschrift, 2015.

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