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Carbonates in thin section: Echinoderms and barnacles

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A community of grazing echinoderms have almost cleaned the bedrock surface of bryozoa and algae (depth 25 m). In the process they add fine bioclastic debris to the local sediment mass. Three Kings Islands, northernmost NZ. Image courtesy of Cam Nelson.

A community of grazing echinoderms have almost cleaned the bedrock surface of bryozoa and algae (depth 25 m). In the process they add fine bioclastic debris to the local sediment mass. Three Kings Islands, northernmost NZ. Image courtesy of Cam Nelson.

Echinoderms and barnacles are important contributors to bioclastic limestones

Echinoids are a diverse phylum with a geological history dating back to Early Cambrian. This entirely marine group of invertebrates includes the familiar Crinoids, star fish (Stelleroids and Ophiuroids), sea urchins (Echinoids) and sea cucumbers (Holothuroids), all of which have modern representatives. There are also several extinct groups. Most groups have five-fold skeletal division.

Devonian crinoid stems and disarticulated columnals in the bioclastic mix with fragmented fenestrate (f) and encrusting bryozoans (e).

Devonian crinoid stems and disarticulated columnals in the bioclastic mix with fragmented fenestrate (f) and encrusting bryozoans (e).

Barnacles are arthropods, a phylum that includes spiders, scorpions, Horseshoe Crabs, and crustaceans like crabs, shrimp and lobster. They are included in this phylum because they have up to 6 appendages that, instead of being attached to a thorax and used as legs, are delicate, feathery wands that are extended and retracted in search of food. They are fully marine and as adults are firmly attached to rocky, or shelly substrates. The oldest definitive barnacles are Carboniferous, but it wasn’t until the Late Cretaceous that species diversity increased, becoming important contributors to Cenozoic cool-temperate limestones.

Although clearly disparate groups of invertebrates, echinoderms and barnacles do have a few attributes in common:
– Both are important contributors of bioclasts to cool-temperate carbonate sediments. They also play important roles in tropical reef systems, but the volume of sediment they produce is proportionally less.
– They shed calcium carbonate plates to the sediment mass; echinoderms also shed spines.
– They are bioeroders; barnacles erode or corrode the substrate (bedrock, other shelly organisms) prior to growing their basal plate, and echinoderms produce fine-grained bioclastic material while munching coral, bryozoa and calcareous algae.
– They are marine organisms that thrive in shallow environments washed by waves and currents, although some species occur at bathyal depths (e.g., some brittle stars; and barnacles like Bathylasma). They coexist with many other invertebrates and reef-builders.

 

Echinoderms

All common echinoderms (except the sea slugs) have segmented exoskeletons. Each segment, or plate is a single crystal of calcite; each attached spine is also a single calcite crystal. Calcite in modern echinoids is high in magnesium; the Mg is replaced by Ca during recrystallization to low magnesium calcite. When the animal dies, the exoskeleton breaks up into its constituent plates and spines.

Echinoderms bioclasts, even as small fragments, are relatively distinctive in thin section:

  • In life, the plates and spines are porous. In thin section this appears as uniformly distributed, microscopic perforations. The pores are filled with calcite cement during burial diagenesis; they may also be filled by glauconite.
  • In transverse sections, spines show a distinctive radial crosshatch pattern formed by the intersection of concentric growth rings and perforations arranged in radial fashion.
  • Plates and spines consist of a single calcite crystal that will show uniform extinction under crossed polars (unless the fragments have been strained during deformation).
  • Calcite cement overgrowths are usually syntaxial, such that both plate and spine plus cement will go into extinction simultaneously (they share the same crystallographic axes).
  • Crystallographic c axes parallel the long axes of spines.
  • The magnesium content of skeletal calcites is variable, even within a single specimen. For example, spines commonly vary from 5-10% MgCO3, and plates up to 16%. Much of the Mg is replaced during diagenesis so that spines and plates plus their syntaxial cement overgrowths are low magnesium calcites (inclusions of HMC may be retained within the crystals).
Recent echinoderm, minus its spines that detached soon after the animal died. The spines were attached to the primary tubercles. The 5-fold symmetry is readily apparent. The five ambulacral and interambulacral areas each consist of interlocking calcite plates that tend to separate when the animal dies. Image credit Sheila Brown.

Recent echinoderm, minus its spines that detached soon after the animal died. The spines were attached to the primary tubercles. The 5-fold symmetry is readily apparent. The five ambulacral and interambulacral areas each consist of interlocking calcite plates that tend to separate when the animal dies. Image credit Sheila Brown.

An abraded echinoderm plate in modern beachrock, Hawaii. The perforated structure is easily identified. Under crossed polars the grain extinguishes as a single crystal. Plain polarized light.

An abraded echinoderm plate in modern beachrock, Hawaii. The perforated structure is easily identified. Under crossed polars the grain extinguishes as a single crystal. Plain polarized light.

 

Recent echinoderm plate, slightly rounded by abrasion, sampled from temperate water bioclastic sand (that includes benthic forams (f), bryozoa, barnacle, and mollusc fragments). The perforations are unfilled. Left: Plain polarized light; Right: crossed polars with the grain in full extinction. Grain mount thin section. Three Kings Islands, northernmost NZ.

Recent echinoderm plate, slightly rounded by abrasion, sampled from temperate water bioclastic sand (that includes benthic forams (f), bryozoa, barnacle, and mollusc fragments). The perforations are unfilled. Left: Plain polarized light; Right: crossed polars with the grain in full extinction. Grain mount thin section. Three Kings Islands, northernmost NZ.

 

Grain mount thin sections of bioclastic sand sampled from about 30 m water depth, Three Kings Islands, northernmost NZ. Left: Small echinoderm spines with the basal tubercle attachment intact. Right: Longitudinal section of a broken and partly abraded echinoderm spine (s) showing the characteristic arrangement of perforations parallel to the spine long axis. Specimen occurs with unfilled foram tests (f) and abraded barnacle fragments (b). Both images plain polarized light.

Grain mount thin sections of bioclastic sand sampled from about 30 m water depth, Three Kings Islands, northernmost NZ. Left: Small echinoderm spines with the basal tubercle attachment intact. Right: Longitudinal section of a broken and partly abraded echinoderm spine (s) showing the characteristic arrangement of perforations parallel to the spine long axis. Specimen occurs with unfilled foram tests (f) and abraded barnacle fragments (b). Both images plain polarized light.

 

Bioclastic beachrock (Hawaii) showing an abraded and bored echinoderm plate (top left). The margins of the plate and the borings (b) are lined with a thin rind of pale brown micrite. Two transverse sections of echinoderm spines (lower center and right) display typical radial fenestrate-like textures.

Bioclastic beachrock (Hawaii) showing an abraded and bored echinoderm plate (top left). The margins of the plate and the borings (b) are lined with a thin rind of pale brown micrite. Two transverse sections of echinoderm spines (lower center and right) display typical radial fenestrate-like textures.

 

An Oligocene bioclastic, glauconitic limestone containing abundant abraded and broken echinoderm plates (e), most of which are encased in syntaxial calcite cement (s). Also present are numerous benthic forams (f), bryozoa fragments (b - some with glauconite filling), one small solitary coral (c - transverse section across septa), barnacle fragments with large pores filled by calcite cement (bc), and the occasional faecal pellet replaced by glauconite. Left: plain polarized light; Right: crossed polars. The syntaxial overgrowths extinguish at different stages for each echinoderm fragment.

An Oligocene bioclastic, glauconitic limestone containing abundant abraded and broken echinoderm plates (e), most of which are encased in syntaxial calcite cement (s). Also present are numerous benthic forams (f), bryozoa fragments (b – some with glauconite filling), one small solitary coral (c – transverse section across septa), barnacle fragments with large pores filled by calcite cement (bc), and the occasional faecal pellet replaced by glauconite. Left: plain polarized light; Right: crossed polars. The syntaxial overgrowths extinguish at different stages for each echinoderm fragment.

Barnacles

 Sketches of the two barnacle morphologies based on common modern forms. Left is the acorn barnacle species Balanus with operculum intact, attached by a basal calcite plate; Right is the common Goose barnacle Lepas that attaches to substrates with a soft pedicle that allows the animal to move with waves and currents.

Sketches of the two barnacle morphologies based on common modern forms. Left is the acorn barnacle species Balanus with operculum intact, attached by a basal calcite plate; Right is the common Goose barnacle Lepas that attaches to substrates with a soft pedicle that allows the animal to move with waves and currents.

All barnacles live attached to hard substrates (rock, shells, wood). The majority of species reside in shallow, agitated water although a few have evolved to survive the darkness at bathyal depths (e.g., Bathylasma). One group, the goose barnacles, are attached by a long leathery tube (a peduncle) to rocks, floating logs, and other flotsam. However, the most familiar species (the acorn barnacles) attach themselves with a basal calcite plate to rocks, other shells, whales, and ship’s hulls.

A typical acorn barnacle consists of robust calcite and aragonite wall plates fitted to the margins of the basal plate. Wall plates are commonly plicated, or folded, presenting a ribbed appearance. The front door, or operculum, consists of two smaller triangular shaped calcium carbonate plates (the scutum and tergum) that open during feeding.

When the animal dies, the operculum disaggregates. The wall plates may be overgrown by a new generation of barnacles or dislodged by waves or moving sediment to become part of the bioclast sediment mass.

A recent temperate to subtropical NZ gastropod, Cookia sulcata, covered by the common acorn barnacle, Balanus. Most of the barnacles have lost their opercula.

A recent temperate to subtropical NZ gastropod, Cookia sulcata, covered by the common acorn barnacle, Balanus. Most of the barnacles have lost their opercula.

Live goose barnacles (Lepas) attached to a log that was washed ashore during a storm, their cirri searching vainly for food. The calcite plates will join the local sediment mass when the animals die or are predated by marauding sea gulls. Raglan, northwest New Zealand.

 

Apical view of a small, Pliocene acorn barnacle attached to a large oyster shell. The operculum and animal gut has been replaced by cemented carbonate sand. N.Z, Matemateonga Formation.

Apical view of a small, Pliocene acorn barnacle attached to a large oyster shell. The operculum and animal gut has been replaced by cemented carbonate sand. N.Z, Matemateaonga Formation.

 

 

Thin section cut transverse to a recent acorn barnacle plate, showing the array or pores at the base of the plate. Note the finely layered calcite that extends upwards into the characteristic plications, or folds (see image below). Plain polarized light.

Thin section cut transverse to a recent acorn barnacle plate, showing the array or pores at the base of the plate. Note the finely layered calcite that extends upwards into the characteristic plications, or folds (see image below). Plain polarized light.

 

A modern Balanid barnacle. Note the rib-like plications (left side, and the longitudinal pores exposed on the abraded plate (centre). Specimen from Parengarenga, northernmost New Zealand

A modern Balanid barnacle. Note the rib-like plications (left side, and the longitudinal pores exposed on the abraded plate (centre). Specimen from Parengarenga, northernmost New Zealand

 

Thin section cut approximately parallel to a recent acorn barnacle plate, showing the characteristic plicate (folded) calcite structure. The left margin corresponds to the base of the plate (cf. the image above). Plain polarized light.

Thin section cut approximately parallel to a recent acorn barnacle plate, showing the characteristic plicate (folded) calcite structure. The left margin corresponds to the base of the plate (cf. the image above). Plain polarized light.

 

 

Fragments of Oligocene barnacle plates (centre and top left) showing typical plicate structure. The central fragment has a thin, outer marginal layer; the upper fragment has remnants of the large basal pores. The plates are surrounded by small dark brown pelloids. Crossed polars. Image courtesy of Cam Nelson.

Fragments of Oligocene barnacle plates (centre and top left) showing typical plicate structure. The central fragment has a thin, outer marginal layer; the upper fragment has remnants of the large basal pores. The plates are surrounded by small dark brown pelloids. Crossed polars. Image courtesy of Cam Nelson.

Acknowledgement

Many thanks to Kirsty Vincent, Earth Sciences, Waikato University for access to the petrographic microscope.

Other posts in this series

Brachiopod morphology for sedimentologists

Bivalve shell morphology for sedimentologists

Gastropod shell morphology for sedimentologists

Cephalopod morphology for sedimentologists

Optical mineralogy: Some terminology

Carbonates in thin section: Molluscan bioclasts

Carbonates in thin section: Bryozoa

Carbonates in thin section: Forams and sponges

Neomorphic textures in thin section

Sandstones in thin section

Greywackes in thin section

Mineralogy of carbonates; skeletal grains

Bivalve morphology for sedimentologists

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; lime mud

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates; cements

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; deep sea diagenesis

Mineralogy of carbonates; meteoric hydrogeology

Mineralogy of carbonates; Karst

Mineralogy of carbonates; Burial diagenesis

Mineralogy of carbonates; Neomorphism

Mineralogy of carbonates; Pressure solution

Mineralogy of carbonates: Stromatolite reefs

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A Gaggle of Goose Barnacles

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A gaggle of Goose Barnacles (Lepas anatifera), west coast New Zealand, near Raglan.  The inset emphasizes the shelly plates of individuals and their long 
fleshy stalks

A gaggle of Goose Barnacles (Lepas anatifera), west coast New Zealand, near Raglan. The inset emphasizes the shelly plates of individuals and their long fleshy stalks

You never know what new treasures will be discovered strolling along a beach after a good storm.  The beach may have changed shape; cusps, ruts and rills smoothed, some of the sand moved offshore beneath the waves, a few sand dunes cut in half.  There’s flotsam and jetsam, a few bedraggled seabirds.  And there are shells, mostly devoid of their original inhabitants.

Raglan (west coast New Zealand – i.e. the coast facing Tasman Sea) was a bit like that this week.  One particularly neat find on our jaunt was a largish log completely covered in Goose Barnacles.  It is usually the case that critters like these are dead by the time they wash up the beach.  But this time all were still alive.  The log was a slowly-seething mass of stalked shells, parched, and all looking for a way out of their predicament.

Goose barnacles, other than being fascinating to watch up close, have served the science of evolution.  Charles Darwin’s book about them, published in 1851, contains many of the ideas he was formulating about species variations and embryonic development, laying some of the foundations for his ‘Origin of Species’.

 

Lepas anatifera

Yes, that’s its zoological name.  The common name ‘Goose barnacle’ has an interesting history that from a 21stC perspective seems slightly weird.  The word derives from a 13th century usage for a seabird – the so-called Barnacle Goose, an Arctic migrant.  Gaggles breed in the Arctic then migrate to spend a balmy winter on British shores.  Coastal Brits, those that hadn’t been press-ganged into the Crusades, were never quite sure where the birds came from (they never saw the eggs).  They surmised that the actual stalked barnacle looked a bit like the actual bird, and that the birds hatched in much the same way, from the planks of ships, whereupon they would fly off to join their gaggle.

Lepas attaches with a long fleshy stalk (a peduncle) to flotsam, logs, basically anything that floats; the Raglan examples were up to 20cm long.  The stalk is part of the animal that can move the shell to take advantage of currents, light, or food.  The animals live cheek-by-jowl, as you can see in the image.  They are crustaceans like crabs and shrimp.

Barnacle guts are contained within five shelly plates.   They feed by filtering microscopic particles, plankton, and algae from seawater using delicate, feathery protrusions called cirri (hence the general classification as Cirripedes).  In the video, our Raglan examples are extending their cirri in air – perhaps they can sense the incoming tide.

Darwin’s barnacles; sources of invention

He wrote four books on these critters; two on living groups (the stalked group and the sessile-attached group), and two volumes on fossil representatives.  The first was on the stalked variety, including Lepas. A second volume on (living) barnacles that are more commonly cemented to rocks was published in 1854. His studies of these creatures provided him with insights into species variation and embryonic development.  As Martin Rudwick illustrates in his wonderful book ‘The Meaning of Fossils; Episodes in the History of Palaeontology, Darwin understood that both phenomenon would require cogent explanation to convince his audience of the central theme of his ‘Origins’; natural selection.   Thus, his studious and systematic observations of barnacles, seemingly a dry topic, provided both the data and the wherewithal for creative thinking.

Diagram of goose barnacles from Darwin's monograph 1851.

 

Prevailing 19th century thought on species development, postulated by pioneer biologist Jean-Baptiste Lamarck (1744-1829), was that species tended to progress toward improvement and complexity.  Darwin’s recognized that regression was also an important adaptive process in evolution.  He based this challenge to the status quo on the well-known fact that free-swimming barnacle larvae have legs (like other crustaceans), and that these appendages are converted “into an intricate food-collecting device, and lost many of the functions and organs associated with a free-swimming life.” (Martin Rudwick, p233).  This feeding device is the cirri.

As is so often the case in science, the seemingly innocuous, tedious, but deliberate gathering of data can lead to startling invention and discovery. The humble Goose Barnacle has certainly done its part in shaping our ideas on the biological world. With our barnacle-covered log, we were witness to a microcosm struggling for survival; hundreds of individuals and a single community. Some days later, most are dead, scavenged by seagulls and demolished by waves. Perhaps all that’s left are a few broken, disarticulated shells.

**********************

Martin J.S. RudwickThe Meaning of Fossils; Episodes in the History of Palaeontology. Second Edition, 1976, Science History Publications, Neale Watson Academic Publications Inc, New York.

There is a nice essay by Marsha Richmond (2007) on Darwin’s barnacles, written for Darwin on Line.

You can also find lots of interesting general information and teaching resources on Darwin, including his voluminous correspondence (more than 2000 letters), on Cambridge University’s Darwin Correspondence Project

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