With Fronds Like These

I'm sure pretty much anyone who's spent time looking into rock pools along the coast will be familiar with sea anemones. These sessile animals with their squidgy bodies and crown of tentacles can be seen almost anywhere there's a rock for them to stand on and a tide to cover them. As a kid, I used to amuse myself by poking them with a finger, noting the slight velcro-ish feel as the harassed anemone would vainly attempt to sting its attaker as it withdrew for protection. In hindsight, I was perhaps just fortunate that New Zealand anemones lacked the strength of venom to affect a human.

Waratah anemones Actinia tenebrosa, copyright John Turnbull.

Many of the anemones I was encountering as a child probably belong to a particular clade known as the Actinioidea. As recognised by Rodríguez et al. (2014), familiar members of this group include the beadlet anemone Actinia equina* from the Atlantic coasts of Europe and Africa, the red sea anemone Actinia tenebrosa of eastern Australia and New Zealand, and the aggregating anemone Anthopleura elegantissima and giant green anemone Anthopleura xanthogrammica of the Pacific coast of North America. Wikipedia informs me that another actinioid, the snakelocks anemone Anemonia viridis, is eaten after being marinated in vinegar and fried in parts of the Mediterranean. Rodríguez et al. recognised their Actinioidea primarily on the basis of molecular phylogenetic analysis but most members of this group had previously been recognised as relatives due to their possession of a sphincter muscle around the edge of the gastric cavity near the top of the column. This muscle allows the body cavity to be pulled tightly closed, providing protection and, for intertidal species, holding water inside the body to protect against desiccation.

*Actinia equina, offhand, was given its species name by Carl Linnaeus who described it under the name Priapus equinus. 'Equinus' means 'of a horse' whereas 'priapus' means... exactly what you think it means. Yes, the name of this species literally means 'hung like a donkey'.

Pompom anemone Liponema brevicornis, copyright Ocean Networks Australia.

Other common features of actinioids include well-developed muscles around the base of the column and an adhesive basal disc for clinging to rocks. However, both the upper sphincter muscle and the basal muscles have been lost in various subgroups of the actinioids, often at the same time. Anemones lacking these muscles, such as the ghost anemones Haloclava, are generally deeper water forms that do not cling to rocks but instead live burrowed into sand with their tentacles extended above the surface. One such anemone, the twelve-tentacled parasitic anemone Peachia qinquecapitata, develops as a larva as a parasite on the hydrozoan medusa Clytia gregaria. The larvae gain entry to their host by being eaten as food particles but proceed to themselves feed on the contents of the host's gastric cavity and eventually on the host itself. Another group of deep-sea actinioids, including such species as the deeplet anemone Bolocera tuediae and the pompom anemone Liponema brevicornis, are able to shed their tentacles as a defence thanks to small sphincter muscles at the base of each tentacle. Bolocera tuediae, found in the North Sea, is a particularly large anemone reaching up to a foot in diameter.

Aggregating anemones Anthopleura elegantissima fighting over space, copyright Brocken Inaglory. The white 'tentacles' the anemones are extending towards each other are inflated acrorhagi (see below).

Many actinioids form symbiotic associations with microscopic algae such as zooxanthellae, containing them within their body and supplementing their own nutrition through the algae's photosynthesis. A number of species reproduce by brooding larvae within the body cavity, only releasing them when they are more developed and better equipped to survive the outside world. Finally, many species of actinioid have the column ornamented by various protuberances such as vesicles or verrucae. These structures may serve environmental protective functions, such as increasing desiccation resistance or functioning in camouflage. Members of Anthopleura and related genera often have specialised bulbous protuberances called acrorhagi around the distal part of the column (Daly et al. 2017). These acrorhagi are packed with stinging cells and are used not so much to protect against predators as against other sea anemones. The acrorhagi-equipped anemone flails its column about, pressing the acrorhagi against any competitor that gets too close and stinging it until it is forced to back off. Its a tough world out there and any anemone worth its salt has got to be willing to defend its position.

REFERENCES

Daly, M., L. M. Crowley, P. Larson, E. Rodríguez, E. H. Saucier & D. G. Fautin. 2017. Anthopleura and the phylogeny of Actinioidea (Cnidaria: Anthozoa: Actiniaria). Organisms, Diversity & Evolution 17: 545–564.

Rodríguez, E., M. S. Barbeitos, M. R. Brugler, L. M. Crowley, A. Grajales, L. Gusmão, V. Häussermann, A. Reft & M. Daly. 2014. Hidden among sea anemones: the first comprehensive phylogenetic reconstruction of the order Actiniaria (Cnidaria, Anthozoa, Hexacorallia) reveals a novel group of hexacorals. PLoS One 9 (5): e96998.

Fusulinellidae, -inae, summat like that...

In an earlier post, I introduced you all to the fusulinids, a group of complex foraminiferans that were abundant during the later Palaeozoic. In that post, I alluded to the complex array of terminology that can be used when describing fusulinids but said that I would rather not cover it at that time. Well, this time I'm going to be dredging some of it up because I've drawn the Fusulinellidae as the topic for today's post.

Sectioned reconstruction of Fusulinella, from here. Labels: нк = primary chamber, са = septal folds, с = septa, сб = septal furrows, х = chomata, у = septal aperture, т = tunnel.

The Fusulinellidae as recognised by Vachard et al. (2013) are a family of fusulinids with fusiform or oblong tests known from the Middle to Late Pennsylvanian (during the later part of the Carboniferous). One genus, Pseudofusulinella, persists into the early Permian (Ross 1999). They are a part of the larger superfamily Fusulinoidea, a group of fusulinids characterised by what is known as a diaphanotheca. This is a thick, more or less translucent layer in the test wall. As noted in my earlier post, such a test structure may have functioned to allow light through to symbiotic microalgae (or possibly captured chloroplasts from algal prey) sheltered within. Fusulinellids are distinguished from other fusulinoids by the structure of the septa dividing chambers within the test, which are mostly flat except for some folding near the poles of the test (in the Fusulinidae, in contrast, the septal walls were folded throughout). As the test developed, sections of the septa were resorbed to form tunnels connecting adjacent chabers (and presumably allowing the transmission of materials between chambers in life). The course of the tunnels is commonly delimited within the chambers by chomata, discrete ridges of shell material. In other species, the chomata are absent but axial fillings of calcite were formed in the chambers instead.

How fusulinids are more commonly seen: sections of fusulinellid Dagmarella iowensis from Vachard et al. (2013). Image on left = subaxial section (scale bar = 0.1 mm); image on right, larger individual = tangential section (scale = 0.5 mm). The smaller individual on the right is a juvenile Profusulinella cf. fittsi, which depending on the author may or may not be considered a fusulinellid.

Being so widespread and abundant when they lived, fusulinellids are commonly used as index fossils for identifying when a deposit was formed. However, this process is complicated somewhat by ongoing debates about fusulinid systematics. Rauzer-Chernousova et al. (1996) proposed a classification of fusulinids that represented an extensive modification from previous systems. Part of this was simply a question of ranking, with Rauzer-Chernousova et al. recognising many groups at higher ranks than previously (so, for instance, recognising the separate family Fusulinellidae as opposed to its previous recognition as a subfamily of Fusulinidae). Nevertheless, some subsequent authors have felt that Rauzer-Chernousova et al. and their followers attribute too much significance to relatively minor variations. For instance, Kobayashi (2011) synonymised several genera under Profusulinella that Rauzer-Chernousova et al. regarded as belonging to distinct families (and Vachard et al. 2013 even placed in separate superfamilies). Some of the features regarded by Rauzer-Chernousova et al. as indicating separate genera were regarded by Kobayashi as representing variation within a single species. Indeed, there have even been arguments that some 'significant' features may represent post-mortem preservation artefacts (I've come across the term 'taphotaxa' used to refer to taxa based on such features). At present, my impression is that there is something of a geographical divide in preferred systems with eastern European authors following the lead of Rauzer-Chernousova et al. whereas authors from elsewhere may keep to a more conservative arrangement. The Berlin Wall may be down but the Fusulinid Cold War continues.

REFERENCES

Kobayashi, F. 2011. Two species of Profusulinella (P. aljutovica and P. ovata), early Moscovian (Pennsylvanian) fusulines from southern Turkey and subdivision of primitive groups of the family Fusulinidae. Rivista Italiana di Paleontologia e Stratigrafia 117 (1): 29–37.

Rauzer-Chernousova, D. M., F. R. Bensh, M. V. Vdovenko, N. B. Gibshman, E. Y. Leven, O. A. Lipina, E. A. Reitlinger, M. N. Solovieva & I. O. Chedija. 1996. Spravočnik po Sistematike Foraminifer Paleozoâ (Èndotiroidy, Fuzulinoidy). Rossijskaâ Akademiâ Nauk, Geologičeskij Institut, Moskva "Nauka".

Ross, C. A. 1999. Classification of the Upper Paleozoic superorders Endothyroida and Fusulinoida as part of the class Foraminifera. Journal of Foraminiferal Research 29 (3): 291–305.

Vachard, D., K. Krainer & S. G. Lucas. 2013. Pennsylvanian (Late Carboniferous) calcareous microfossils from Cedro Peak (New Mexico, USA). Part 2: smaller foraminifers and fusulinids. Annales de Paléontologie 99: 1–42.

Belemnitellidae: Reaching the End of an Era

Fossil cephalopods have featured on this site numerous times in the past. I've talked about nautiloids, I've talked about ammonoids. But one group of cephalopods that I haven't given that much time to to date is the group including the majority of living species: the coleoids. In coleoids, the ancestral cephalopod shell has become reduced and internalised (one group, the octopods, has lost the shell entirely) so it should not come as much of a surprise that their fossil record is more limited than that of other cephalopod groups. Nevertheless, the coleoid lineage does include at least one group known from an abundant fossil record: the Mesozoic belemnites.

Fossil guard of Belemnitella americana, from here, in ventral view with the ventral opening of the alveolus visible as a longitudinal fissure.

Belemnites were a significant part of the marine fauna during the Jurassic and Cretaceous. Externally, they were similar in overall appearance to modern squid, as demonstrated by rare finds of specimens with preserved soft body parts. However, whereas squid have the internal shell reduced to the thin, non-calcified pen, belemnites possessed a well-developed internal shell. The posterior end of the shell was a solid, bullet-shaped rostrum or guard, in front of which was a chambered section known as the phragmocone. Being completely calcified, the rostrum of a belemnite was readily preserved and isolated rostra make up the greater part of the belemnite fossil record (the more delicate phragmocone was less likely to survive the fossilisation process). Different belemnite taxa may be recognised by variations in rostral shape and structure and several families are recognised from various parts of the Mesozoic. The latest surviving belemnite family was the Belemnitellidae.

Reconstruction of a typical belemnite showing the life position of the shell (not actually visible externally), copyright Charlotte Miller.

Belemnitellids are characterised by rostra with an alveolus or pseudoalveolus (an anterior conical depression into which the phragmocone would have originally fit) that opens through a ventral fissure, and longitudinal dorsolateral impressions (Christensen 1997, 2002). The earliest belemnitellids appeared during the early part of the Cenomanian epoch of the Cretaceous period, about 98 million years ago (Christensen 1997). They reached their peak of diversity during the lower Santonian, about 86 million years ago, but they persisted in one form or another right up to the end of the Cretaceous, eventually disappearing in the giant colossal environmental clusterbump that brought that period to a close. Throughout their history, belemnitellids were restricted to the Northern Hemisphere, being known from what is now Europe and North America. By the late Cretaceous, of course, the modern continents were definitely approaching their modern forms and positions but were not quite there yet. For a large chunk of this period, sea levels were higher than they are now so much of modern Europe and the central part of North America were covered by shallow seas. The North Atlantic was still a developing prospect; it looks like there still would have been something of a continental shelf connection between what is now its two sides during the Santonian. This continental shelf and shallow seas was the habitat of the belemnitellids; it appears that they never made the shift to deeper waters. Hence their geographical restriction as the deeper Tethys Ocean still separated Eurasia from Africa and India. When the belemnitellids first appeared, these deeper Tethys waters were home to another belemnite family, the Belemnopseidae (the belemnitellids would make some inroads to the northern coast of the Tethys after the belemnopseids became extinct during the Cenomanian but never anything extensive). A third family, the Dimitobelidae, occupied the position of the belemnitellids in the Southern Hemisphere.

The earliest belemnitellids are known from northern Europe where they presumably evolved from belemnopseid ancestors (Christensen 1997). There do appear to be some questions about whether the belemnitellids as currently recognised represent a monophyletic group or whether the belemnopseid invasion happened more than once. However it be, northern Europe would remain the centre of diversity for the group. They reached North America during the Turonian, about ninety million years ago, but for whatever reason never quite diversified there as much as they did in their homeland. During the Campanian, from about 83 million years ago, there is a period of close to ten million years where belemnitellids disappeared from the North American fossil record entirely. Presumably this represents a local extinction followed by a later recolonisation from Europe.

North American belemnitellids also failed to quite make it to the end of the Cretaceous, dropping out about one or two million years earlier. In Europe, however, three species are known from the period's closing hours. Though not at their earlier levels of success, belemnitellids were diversifying right to the end: the distinctive Fusiteuthis polonica appears well within the last couple of million years. Nevertheless, there was precious little from that part of the world at that time in history that did not have the word DOOM stamped firmly on its forehead and belemnitellids were no exception. Their passing marked the final end of the belemnite hegemony and the stage was now completely clear for the more modern coleoids to rise.

REFERENCES

Christensen, W. K. 1997. The Late Cretaceous belemnite family Belemnitellidae: taxonomy and evolutionary history. Bulletin of the Geological Society of Denmark 44: 59–88.

Christensen, W. K. 2002. Fusiteuthis polonica, a rare and unusual belemnite from the Maastrichtian. Acta Palaeontologica Polonica 47 (4): 679-683.