The fossil pictured below is an echinoid (sea urchin) some 84 to 90 million years old from the middle of the Upper Cretaceous period. This post explores how this particular specimen, as well as most of the other members of its specific taxon, served as a "canvas" that was intricately "decorated" postmortem. It concludes with a very plausible explanation of why.
The first photo shows the apical (apex) side of the fossil (the side where, in life, waste products were expelled); the second is of the oral side.
Echinoids originate in the fossil record in the Middle Ordovician period (471.3 to 458.2 million years ago) and are with us still. They come in a variety of spherical, usually globular, shapes. A calcium carbonate skeleton or test made up of interlocking plates encloses the internal organs of the animal. In life, the test is covered with spines giving the animal the name "urchin" which, in Middle English, means "hedgehog." (Sea Urchins: Strange and Spiny Wonders of the Ocean, Holly Chetan-Welsh, Natural History Museum, London.) The spines are typically lost when the animal dies.
In my well worn (and surely dated) copy of Invertebrate Fossils (1952), geologist Alfred G. Fischer notes that the echinoids are divided into two groups: Regularia and Irregularia. The former have a clear and distinctive pentagonal symmetry; the latter show a bilateral symmetry. The two groups differ as to their life style. Regular urchins live on the surface of the sea bottom. Fischer quaintly says of the regular urchins: "some wander about on their spines, as on stilts, while others clamber over submarine cliffs by means of their prehensile tube feet, or nestle in rock cavities." (p. 705) In contrast, the irregular urchins are adapted to soft, muddy sea floor sediments, often burrowing into the substrate.
The echinoid pictured above has the distinctive overall heart shape that marks it as an irregular urchin of the spatangoida order. The distinguishing heart shape evolved, according to Fischer, to facilitate the animal's deep burrowing into the sea floor.
The apical side of this specimen displays a set of radiating plates, all but one in a quite distinctive petal shape. That outlier, the one pointing to the top of the picture and leading to the indentation of the test, appears slightly less well defined. This specimen is of the genus Micraster, and I accept the original collector's identification of it as a M. coranguinum.
This specimen was collected in Spain from a portion of the Olazagutía Formation that spans the Coniacian (89.9 to 85.7 mya) and Santonian (85.7 to 83.6 mya) ages of the Cretaceous period. These fossils are very abundant in this formation which is composed of sedimentary layers of marl and marly limestones. In general, marls are a blend of carbonate (usually calcite) and siliciclastic (silt and clay) muds: the marl tends to marly mudstone as the siliciclastic proportion increases and to marly limestone as the carbonate proportion increases. (Samuele Papeschi's "Marl" entry on his Geology is the Way website is an excellent introduction to marls.) The ocean floor in the Olazagutía area during the Upper Cretaceous was soft and muddy, ideal for a burrowing irregular echinoid like Micraster.
I noted at the opening of this post that this echinoid and others of its ilk were thought to have been decorated after death (more on that timing below). The "artists" in question were so-called sclerozoans. These are animals that, in this instance, attached themselves to the surface of the test after the urchin died, some to drill into or otherwise penetrate the urchin plates, presumably in search of whatever remained of the dead urchin, and some to secure a solid perch.
The term sclerozoan was proposed in 2002 by paleontologists Paul D. Taylor and Mark A. Wilson to describe an animal "fouling any kind of hard substrate." (A New Terminology for Marine Organisms Inhabiting Hard Substrates, PALAIOS, Volume 17, Number 5, October 2002.) I like the term "fouling" applied to what these animals and their plant counterparts (sclerophytes) do to these surfaces.
In the images below, I have focused on several of the "ornamentations" left by the sclerozoans on this particular Micraster specimen. In general, I can only suggest the broad taxa from to which these animals belong.
The first shows a close up of the worm tube on the oral side of the test. This could be the work of worms in the Serpulidae or Spirorbidae families.
This next picture is the signature of a bivalve that encrusted the test on the edge of apical side of the test. The species responsible could be Atreta sp. I do not know what the little oblong object is that appears above and to the right of the shell, but it would appear that it dug into the urchin test.
This last picture is of ichnofossils, that is, the traces left by the action of some living organism. What entity is responsible for an ichnofossil is not often known with any certainty. In this case, the little cuts into the urchin test were left by some boring animal and are identified by paleontologists as of the ichnogenus Rogerella. If evidence from extant animals is any guide, the actor in question was probably a barnacle of some sort.
These are only a very few of the traces of sclerozoan activity on this test. A fascinating study of Micraster echinoids from the Olazagutía Formation by researcher Samuel Zamora and his colleagues lays out the wide range of signs of sclerozoan activity upon and in Micraster tests from this formation. In their paper titled The Infaunal Echinoid Micraster: Taphonomic Pathways Indicated by Sclerozoan Trace and Body Fossils From the Upper Cretaceous of Northern Spain (Geobios, first available online January 11, 2008), Zamora et al. present the results of analysis of 100 Micraster specimens collected from the Cementos Portland quarry near the village of Olazagutía, Spain. These fossils were found in a layer dating from the Lower Santonian age.
I would note that it is a section of the Olazagutía Formation exposed in this particular quarry which has been accepted as defining the base of the Santonian age. The bivalve Platyceramus undulatoplicatus first occurs there. (M.A. Lamolda, et al., The Global Boundary Stratogype and Section Point (GSSP) for the Base of the Santonian Stage, "Cantera de Margas", Olazagutia, Northern Spain, Episodes, Volume 37, Number 1, March, 2014.)
What I found remarkable about the study by Zamora and colleagues was that 95 (95 percent) of the 100 Micraster echinoids they analyzed showed evidence of activity by sclerozoans, either living on the test surface (94) or digging into the test (71). Though my specimen shown above may not be from precisely the same location as those considered in the study, it certainly reflects the same evidence of sclerozoan activity as those analyzed by Zamora et al.
This raises the question:
Why did these urchin tests prove so irresistible to sclerozoans - those encrusting or drilling animals - in this general time and place?
Zamora and colleagues proffer an answer I find elegant and convincing. First, as I noted earlier, this activity on and in the urchin tests likely occurred after the death of the echinoids. Zamora et al. posit this timing is true because, while alive, these urchins lived burrowed into the muddy ocean floor and were protected by spines. Both of these attributes ward off sclerozoans. Second, after death, the echinoids' tests lost their spines and the tests were likely to have been uncovered at some point under a variety of circumstances. Third, in a key assertion, the authors write:
In the argillaceous-carbonate, muddy bottoms of the Upper Cretaceous marine platform of the Olazagutía area, exhumed endobenthic echinoids tests constituted small, yet stable island environments on which several biological groups found a place for settling (94% of the tests found are colonised). (p. 23)
"Small, yet stable island environments . . . ." Lovely image. Given the abundance of Micraster echinoids, their tests constituted many, many such small islands.
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