Rangeomorphs

So you are stuck in the same place, but connected to your nearest and dearest via an all important filament which ties everyone together?  This might feel strange, but it is actually an exceedingly ancient arrangement.  Consider the lives of rangeomorphs, an ancient sessile marine animal which lived PRIOR to the Cambrian era, about 550 million years ago.  Many of the fundamental categories of multicellular life which are familiar to us originated in the Cambrian.  The rangeomorphs lived before that time…so it is a bit difficult to pin down their taxonomy.  Paleontologists have suggested that they are related to various groups of living suspension feeders and protists, but all of these attempts to pin down their exact place in the tree of life have been rejected.  Rangeomorphs lived during the last 30 million years of the Ediacaran Period (which was 635-542 million years ago) before the great phyla of life emerged.  Perhaps they were an extinct stem group somewhere between animals and fungi (!).

At any rate, even if rangeomorphs were their own weird kingdom of life, they were roughly analogous in form to many to many soft corals, glass sponges, ferns, luungworts and what have you.  They were mouthless (!) animals with no clear internal organs.  Their bodies consisted of many branching fronds a centimeter or so long.  They had no reproductive organs.

If you have been grinding your teeth wondering why I am writing about some blobby fronds which lived on the ocean bottom a half a billion years ago, you should open your mind.  There are some compelling questions here already.  If rangeomorphs had no mouths or organs how did they eat or sustain themselves?  They certainly were not photosynthetic.  How did they reproduce without reproductive organs?  Were they purely asexual?  We don’t know…However thanks to an exceedingly well-preserved bed of fossils just discovered in Newfoundland, we now know that rangeomorphs also had filaments which ranged in length from 2 centimeters (.75 inches) to 4 meters (12 feet) in length.  With these filaments the rangeomorphs (which formed vast monoculture colonies at the sea bottom) could conceivably communicate, transport nutrients, or even bud (as seen in the ‘suckering’ of plants like the infernal tree of heaven).  Maybe rangeomorphs were even subtly like syphonophorae–colony animals where different individuals (zooids) perform different functions in the manner of organs.

Not only is it interesting to speculate about the first great “forests” or “reefs” of life (assuming you don’t assign that role to the stromatolites of 3.5 billion years ago), it is also worth thinking about how different and alien the basal forms of life at the bottom of the animal “trunk” really were (assuming these guys were even animals).  I imagine them as colorful gardens of fleshy creatures not unlike seapens swaying in the currents of the ancient ocean, yet all strangely operating together like a clonal colony (which they almost certainly were–since how else would they come into existence?).  It forms a soothing mental picture during these tumultuous times.

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Tunicates

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)

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

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

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

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)