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A painting of Climatius

A painting of Climatius

I promised this blog would feature more fish this year, but thus far, all we have seen is the remarkable ocean sunfish…so today we travel way back in time to the oceans of the Paleozoic world to check out some spiny sharks. However these “sharks” are really different from what you are probably expecting! During the Late Silurian and Early Devonian the oceans were filled with Climatius reticulatus a fish which took up a niche analogous to great schools of anchovies & sardines which swim in today’s oceans. Climatius reticulatus grew to be only 7.5 centimetres (3 inches) long! Some of these remarkable illustrations are bigger than they were! I am calling them “sharks” because they are indeed commonly known as spiny sharks, but they are more properly acanthodians—an early order of jawed vertebrates which shared some features with both bony fish and cartilaginous fish. Climatius reticulatus did have a cartilaginous skeleton, so don’t go thinking I am completely misleading you with quotation marks and paleontological hokum.

pelagic Climatius

pelagic Climatius

Although this fish was tiny with a squishy skeleton, it was not defenseless: each little Climatius sported fifteen razor sharp spines. Presumably they also swam together in great schools which would dazzle and mislead predators of long ago just as shoals of fish do today. Speaking of which, the predators of 420 million years ago were most likely anomalocaridids (horrifying giant arthropods, which were on their way out) cephalopods, and scary new vertebrate predators like Dunkleosteus.

Life in the Early Devonian (by Gogosardina)

Life in the Early Devonian (by Gogosardina)

Climatius was itself a predator too. It had a powerful caudal fin and large complicated eyes in order to find and capture the little animals swimming in the plankton of the ancient seas. The first acanthodians had appeared in the ocean back during the Ordovician (the age of cephalopods). They predate sharks and bony fish and were probably related to a basal ancestor of both. By the early Devonian, however the bony fishes were coming into their own and fierce competition from these magnificent teleosts soon drove the thriving schools of Climatius (and other similar acanthodians) into oblivion.

A school of Climatius (by NTamura)

A school of Climatius (by NTamura)

A large single tunicate (blue) with a colony of smaller tunicates (saffron)

A large single tunicate (blue) with a colony of smaller tunicates (saffron)

It’s time to talk tunicates! Many people blithely dismiss tunicates as primitive sack-like marine invertebrates which derive sustenance from filter-feeding. Although that is technically true, it is a very reductive and dismissive way to think about this ancient, ancient subphylum of animals. Tunicates are chordates…barely, but they are also classified as invertebrates. Because they mostly consist of delicate tissue sacks filled with fluids, the fossil record of tunicates is understandably exiguous, but it is believed they existed in Ediacaran times (circa 550 million years ago) and were part of the mysterious soft Ediacaran biota which blossomed into the Cambrian era’s suffusion of life forms. Tunicates probably closely resemble the basal organisms from which Pikaia and all other vertebrates (lynxes, caecilians, hummingbirds, triggerfish, humans, ichthyosaurs, turkeys, moeritheriums, and suchlike animals) sprang. Of course tunicates also resemble hydrozoans, mollusks, worms, and even arthropods—so they may be very basal indeed!

A free-swimming larval tunicate (microphoto by Wim van Egmond of Rotterdam)

A free-swimming larval tunicate (microphoto by Wim van Egmond of Rotterdam)

All–or very nearly all–tunicates are hermaphrodites with both male and female reproductive organs (a single ovary and a lone testis). Not only do tunicates keep their romantic options open, they also metamorphose into different forms throughout their lives. The majority of tunicates have a free-swimming larval stage when they are motile (and have a little sliver of nerve chord). As they reach sexual maturity, their nerve chords disintegrate and they settle down to become sessile—attaching to a permanent base. Some tunicates live their entire lives as solitary individuals whereas others form colonies (like corals or siphonophores).

Not only do they have multiple genders they have multiple methods of reproduction

Not only do they have multiple genders they have multiple methods of reproduction

Colonial tunicates integrate at different levels depending on the species. In some, the zooids (the individual living organisms) merely live next to each other like coral or Brooklynites, whereas other tunicate colonies grow entwined and share common organs and anatomical structures. There are many different tunicates going by many different lifestyles and they have all sorts of crazy names. Wikipedia poetically avers that “…various species are commonly known as sea squirts, sea pork, sea livers, or sea tulips.”

Komodo National Park sea squirt (Polycarpa aurata) by nick Hobgood

Komodo National Park sea squirt (Polycarpa aurata) by nick Hobgood

As you could guess from these names, tunicates have an otherworldly beauty. Here is one which looks like a diseased zombie heart! Others look like transparent alien shrimp, fluorescent pens, or strangely hieroglyphed eyes. There are bioluminescent tunicates of the deep ocean, and pelagic tunicates that form long chains (with a single digestive tract running through the individual zooids).  They live in coastal waters, pelagic waters, and in the depths.

Colonial tunicate with multiple openings in each zooid

Colonial tunicate with multiple openings in each zooid

Most of these zoology articles end with a sad coda about how the subject organism is threatened in the modern world–no so for tunicates!  As humans overfish the oceans and drive countless teleosts to the edge of extinction, so-called primitive species like jellyfish and tunicates are flourishing! Acidification, climate change, and pollution seem to be resetting the great worldsea back to Neoproterzoic times. Additionally tunicates easily travel the world in ballast water and numerous species are becoming invasive pests (like the evocatively named carpet tunicate).  In this troubled era, there is raw power in being a primitive protean organism with only a wisp of a nervous system (as we should have known just by looking at successful late-night comedians).  Get used to the tunicate–not just an incredibly distant ancestor, but the once and future (and always) avatar of animal life in the oceans.

Chain of fluorescent tunicates. (photo by Francis Abbott/Nature Picture Library)

Chain of fluorescent tunicates. (photo by Francis Abbott/Nature Picture Library)

Monito del monte (Dromiciops gliroides)

Monito del monte (Dromiciops gliroides)

Monito del monte (Dromiciops gliroides) is a tiny arboreal marsupial native to the temperate rainforests of Chile and Argentina.  The name “Monito del monte” means “little monkey of the mountain” and although the tiny marsupials are not even remotely related to primates, they are clever and deft.  During the cold winter months the animals hibernate in little ball-like nests which they build out of waterproof leaves and line with moss.  Like the more familiar marsupials of Australia, the females have pouches where they nurse their litters of up to four offspring.

Monito del monte (Dromiciops gliroides) with tree snail

Monito del monte (Dromiciops gliroides) with tree snail

The adult animals prey on small invertebrates which live in the trees but they also supplement their diets with fruits and seeds.  A particular species of Loranthacous mistletoe (Tristerix corymbosus) has evolved in conjunction with the monito del monte and relies entirely on the animal to spread its seeds.  This is noteworthy because “scientists speculate that the coevolution of these two species could have begun 60–70 million years ago.”  The monito del monte is not some rodentlike offshoot of the marsupial line, it is a close analog (and direct descendent) of the basal line from which all marsupials spring.

Monito del monte (Dromiciops gliroides) with human for scale

Monito del monte (Dromiciops gliroides) with human for scale

In fact, like something out of a gothic novel, the monito del monte is the only species of the sole genus of the last family of the exceedingly ancient order Microbiotheria.    During the dawn of the dinosaurs, South America, Antarctica, and Australia were amalgamated together as a supercontinent Gondwana.  The offspring of the original marsupials spread from South America, across Antarctica, to Australia, but then the continents drifted away from each other and evolution took a different direction in each ecoysytem.  The monito del monte remained in the same sort of forest as its ancestors and changed least over the years.

journal.pbio.1000436.g002

Speaking of which, the Valdivian temperate rain forests where the monito del monte lives today are themselves a remnant of the great forests of Gonwana.  The trees and plants which live there now are most closely related to the living plants of Australia, New Zealand, & New Caledonia, but they are closer still to the fossilized forests which lie beneath the glaciers of Antarctica.  The Valdivian forest is the closest thing surviving to the great forests which once covered the iced over southern continent.

Valdivian Temperate Rainforest

Valdivian Temperate Rainforest

The ancestors of the monita del monte—and of all other marsupials—originated in South America and spread through the Antarctic forests to Australia before the continents drifted apart during the Cretaceous.  When the continent broke from Australia and drifted south into the prison of the circumpolar current during the Eocene, the forests died and Antarctica became an otherworldly landscape of ice.   Yet if you wish to know what the sweeping temperate forests of Antarctica were like you can visit Chile and watch the most ancient marsupial among the tree ferns and araucaria trees of the Valdivian forest.

map-80-mya-402

Cast your imagination down to the bottom of the ocean—not at a beach or a bright coral reef just offshore, but the true ocean floor—the abyssal plains which cover much of Earth’s surface.  Here vast flat swaths of mud lie in black silence.  Only the occasional seamount or shipwreck breaks the monotony of plains as big as continents.  Tides do not particularly affect the bottom of the ocean.  The most violent storms do not perturb the waters.  Even humankind’s restless activities, which have so much affected the rest of the planet, mean little here.  At first it seems bleak, but soon enough you realize that life is everywhere here.  There are spiderfish, lizardfish, deep sea octopuses, bizarre roving sea cucumbers, and all sorts of strange creatures, but we are not here for them.  Instead we are concentrating on an inconspicuous worm-like animal.  The tiny cylindrical creatures are only 5 cm (2 inches long) and they shimmer strangely when exposed to light.  It would be reasonable to assume that they were worms or tiny sea cucumbers, but they are not.  The benthic beasts are members of the aplacophora class of mollusks—the naked mollusks.  They are presumed to be similar in appearance and nature to the basal mollusks from which the other classes of mollusks have evolved (although both fossil and molecular evidence is frustratingly exiguous).  To look at aplacophorans is to see back to the Cambrian (540 million years ago) and to glimpse an even earlier era when the ancestors of the mollusks diverged from the annelids.

Two specimens of the aplacophoran Simrothiellidae (photographed by G. Rouse)

The aplacophoran shine because of tiny calcareous spicules embedded in their skin.  There are about 320 known species split between two clades: the caudofoveates and solenogasters.  To quote the University of California Museum of Paleontology website, “Caudofoveates are burrowers that feed on detritus and bottom-dwelling microorganisms, while solenogasters feed on cnidarians. Both groups have a radula and lack true nephridia.”  There is an even more important distinction between the two different clades: whereas solenogasters are hermaphrodites, caudofoveates have two genders, and reproduce by external fertilization.

Epimenia australis (Photo by R. Willen)

The depths of the ocean are known to harbor animals which have vanished from the rest of the Earth long ago, and such is believed to be the case with aplacophorans.  For a half billion years they have gone about their business in a part of the world which is resistant to outside change.  The next time you fly across an ocean, imagine all of the naked mollusks in the muck at the very bottom and think about the vast amount of time they have been there.

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