Two publications just appeared online from students we supervised.
Where did all the missing squids go ?!
Squids and their ten-armed kin (Decabrachia) are the sister-group of octoposes and their eight-armed kin (Vampyropoda) and should appear in the fossil record at the same time. Interestingly, only soft-bodied remains of vampyropods have been found from Cretaceous Lagerstätten (i.e. Hâkel and Hâdjoula Lagerstätten, Cretaceous, Lebanon), while no reliable fossil decabrachian labile soft-tissues are known.
Experiments performed by Thomas Clements during his MSc Project (that K. De Baets co-supervised with Jakob Vinther) showed that during 14 observed days, the squid carcasses did not reach a pH low enough to enter the calcium phosphate window, while octopus and fish carcasses reached the threshold for calcium phosphate precipitation within 4 days and remained below pH 6.38 until after the termination of the experiment. Both negatively and neutrally buoyant decabrachians use chemical buoyancy control (ammonia) whereas vampyropods do not. We propose that a hitherto unknown taphonomic bias pertaining to the differing methods of buoyancy control within coleoid groups limits preservation potential. In the event of rapid burial in an environment conducive to exceptional preservation, ammonia dramatically decreases the ability of the decabrachian carcass to generate the required pH for authigenic calcium phosphate replacement, limiting its preservation potential. Moreover, the greater surface area and comparatively fragile dermis further decrease the potential for fossilization. This might also have broader implications for the preservation of soft-tissues in other groups of cephalopods like ammonoids. It largely remains a mystery why so far no labile soft-tissues (e.g., arms, etc.) of ammonoids have been found. This has been attributed to various factors like their pelagic ecology (cannot by surprised by sudden turbidites), anatomy (e.g., non-muscular or thin arms might not fossilize) or taphonomy (float post-mortem or lie around too long on the seafloor). These do not really explain why soft-tissues are not even found in places where coleoids are well preserved and externally shelled cephalopods like ammonoids and nautiloids are found with jaws and stomach contents indicating in situ preservation. Their high ammonium-content might also have made it more difficult to generate the required pH for authigenic calcium phosphate replacement. It remains difficult to test this hypothesis today as the only living externally cephalopod Nautilus is vulnerable to over-exploitation, which merits it getting protected not further used for experiments.
It is openly available for anyone to read:
Clements, T., Colleary, C., De Baets, K., Vinther, J. (2016), Buoyancy mechanisms limit preservation of coleoid cephalopod soft tissues in Mesozoic Lagerstätten. Palaeontology. doi: 10.1111/pala.12267
Natural or pathological variation?
Julia Stilkerich examined the only known non-planispirally coiled early Devonian ammonoid, the holotype specimen of Ivoites opitzi, during her Bachelor Project (supervised by Kenneth De Baets) to investigate if the host was encrusted in vivo and if these sclerobionts were responsible for the trochospiral coiling observed on this unique specimen. To test if the presence of runner-like sclerobionts infested the historically collected specimen of Ivoites opitzi during its life, we used microCT to produce a three-dimensional model of the surface of the specimen. Our results indicate that sclerobionts grew across the outer rim (venter) on both sides of the ammonoid conch at exactly the location where the deviation from the planispiral was recognized, and where subsequent ammonoid growth would likely preclude encrustation. This indicates in vivo encrustation of the I. opitzi specimen, and represents the earliest documentation of the phenomenon. Further, this suggests that non-planispiral coiling in I. opitzi was pathologically induced and does not represent natural morphological variation in the species. Despite the observed anomalies in coiling, the specimen reached adulthood and retains important identifying morphological features, suggesting the ammonoid was minimally impacted by encrustation in life. As such, appointing a new type specimen – as suggested by some authors – for the species is not necessary. In addition, we identify the sclerobionts responsible for modifying the coiling of this specimen as hederelloids, a peculiar group of sclerobionts likely related to phoronids. Hederelloids in the Devonian are commonly found encrusting on fossils collected in moderately deep environments within the photic zone and are rarely documented in dysphotic and aphotic samples. This indicates that when the ammonoid was encrusted it lived within the euphotic zone and supports the latest interpretations of the Hunsrück Slate depositional environment.
The work is currently published as a PeerJ Preprint:
Stilkerich J, Smrecak TA, De Baets K. (2016) 3D-Analysis of a non-planispiral ammonoid from the Hunsrück Slate: natural or pathological variation? PeerJ Preprints 4:e2479v1 https://doi.org/10.7287/peerj.preprints.2479v1
We must admit it is our first venture into Preprints. We are bit anxious and hope that the reviewers/public will like our results as much as we do so it can eventually get published in the peer-reviewed journal PeerJ.
I am a paleobiologist into fossil cephalopods, parasites and movies. My main research focuses on macroevolution, particularly on the relative contributions of biotic interactions (e.g., parasitism) and abiotic factors (e.g., climate) in driving these large-scale patterns.