bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 Reconstructing the ecology of a Jurassic pseudoplanktonic megaraft
2 colony
3
4 Short title: A Jurassic megaraft ecosystem
5
6 Aaron W. Hunter a,b,1, David Casenovec,1, Emily G. Mitchella, and Celia Mayersd
7
8 aDepartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK.
9 bSchool of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.
10 cALESS Program Center for Global Communication Strategies, Komaba International Building for Education and Research
11 (KIBER), Komaba Campus, The University of Tokyo, Meguro-ku, Tokyo, 153-8902, Japan
12 dDepartment of Applied Geology, Curtin University, Perth, Western Australia
13 1 To whom correspondence may be addressed. Email: [email protected] (A.W.H.).
14
15 Keywords: Transoceanic rafting | Holzmaden Lagerstätte | Spatial Analysis | Diffusion analyses |
16 Crinoidea
17
18 Author contributions: A.W.H., D.C. and E.G.M. contributed equally to research, discussion, and
19 manuscript preparation. A.W.H designed the research project, collected the museum data, photographed
20 fossil material and prepared the specimen figures including the spatial analysis plots. D.C. designed and
21 ran the diffusion analyses, and prepared the corresponding diagrams. E.G.M. designed and ran the
22 spatial analysis, and prepared the corresponding diagrams. C.M. prepared the reconstruction.
23 The authors declare no conflict of interest.
24
1 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 Abstract
2 Pseudoplanktonic crinoid megaraft colonies are an enigma of the Jurassic. They are among the largest in-situ
3 invertebrate accumulations ever to exist in the Phanerozoic fossil record. These megaraft colonies and are
4 thought to have developed as floating filter-feeding communities due to an exceptionally rich relatively predator
5 free oceanic niche, high in the water column enabling them to reach high densities on these log rafts. However,
6 this pseudoplanktonic hypothesis has never actually been quantitatively tested and some researchers have cast
7 doubt that this mode of life was even possible. The ecological structure of the crinoid colony is resolved using
8 spatial point process techniques and its longevity using moisture diffusion models. Using spatial analysis we
9 found that the crinoids would have trailed preferentially positioned at the back of migrating structures in the
10 regions of least resistance, consistent with a floating, not benthic ecology. Additionally, we found using a series
11 of moisture diffusion models at different log densities and sizes that ecosystem collapse did not take place solely
12 due to colonies becoming overladen as previously assumed. We have found that these crinoid colonies studied
13 could have existed for greater than 10 years, even up to 20 years exceeding the life expectancy of modern
14 documented megaraft systems with implications for the role of modern raft communities in the biotic
15 colonisation of oceanic islands and intercontinental dispersal of marine and terrestrial species.
16
17 Significance statement
18 Transoceanic rafting is the principle mechanism for the biotic colonisation of oceanic island ecosystems.
19 However, no historic records exist of how long such biotic systems lasted. Here, we use a deep-time example
20 from the Early Jurassic to test the viability of these pseudoplanktonic systems, resolving for the first time whether
21 these systems were truly free floating planktonic and viable for long enough to allow its inhabitants to grow to
22 maturity. Using spatial methods we show that these colonies have a comparable structure to modern marine
23 pesudoplankton on maritime structures, whilst the application of methods normally used in commercial logging is
24 used to demonstrate the viability of the system which was capable of lasting up to 20 years.
25 2 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 Introduction
2 Transoceanic rafting is a fundamental feature of marine evolutionary biogeography and ecology, often
3 invoked to explain the origins of modern global patterns of species distributions (1, 2, 3, 4). These
4 communities have been recorded today lasting up to 6 years (5). However, the deep time ecology of
5 these communities has never been investigated in detail (6). In recent communities, such rafts have
6 included highly adapted bivalves, barnacle, limpets, bryozoans, sea anemones, amphipods, and isopods
7 (5). In the Jurassic these communities also consisted of specially adapted crinoids, whose apparent
8 maturity suggests that these communities had to have lasted longer than modern examples (>6 years)
9 (6). The structure and duration and these colonies has remained a mystery, with most studies choosing
10 to focus on how the crinoids were adapted rather than the viability of the system, prompting intense
11 debate on their lifestyle (7, 8) rather than the ecological structure and longevity of the habitat. This
12 study uses the latest ecological techniques used in paleobiology to reconstruct the ecology and duration
13 of these rafts.
14 Crinoids or sea lilies were a major part of the Jurassic shallow sea ecosystem, with crinoids found
15 in a diverse suite of shallow marine environments (9). The monospecific crinoid colonies preserved on
16 wood rafts are one of the most enigmatic and iconic of these communities (10). Found globally, they
17 represent one of the largest in-situ invertebrate accumulations found in the fossil record (6), the only
18 fossil example of transoceanic rafting with up to 100 individuals covering oyster-encrusted logs up to
19 14m long (11). An ongoing debate prevails on whether these crinoids could have colonized and
20 persisted on these floating log habitats or they were instead part of benthic islands systems typical of the
21 Mesozoic (12). Previous studies have not quantitatively addressed this quandary. In the present study
22 we use spatial statistics and diffusion modelling in a novel approach to test whether this
23 pseudoplanktonic mode of life existed. Spatial statistics are used to test whether the logs were colonized
24 in open water or on the substrate, and diffusion models then quantify floating-log mechanics to test how
25 long a floating system could have existed. 3 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 The spatial positions of Seirocrinus on one of the largest and best-preserved Early Jurassic floating
2 wood examples known, the giant ‘Hauff Specimen’ from Holzmaden, Germany (13) were mapped (Fig.
3 1 and SI Appendix, Figs. S1–S3). The spatial patterns of benthic organisms depend on the dispersal of
4 larvae (14, 15), the environmental conditions in which they settled, and whether the conditions were
5 favorable for them to grow to adulthood (16, 17). Therefore, the spatial patterns of the crinoids can be
6 used to try to “reverse engineer” the conditions in which they settled in order to deduce the
7 environmental conditions (what part of the water column the log was in) when colonized (18, 19).
8
9 10 Fig. 1. Crinoid fossil megaraft, the ‘Hauff Specimen’ from Holzmaden (G1). (A) Log with spatial analysis data points; key: 11 blue= crinoid crowns, black= attachment discs. (B) Spatial analysis plot. (C) Close up view of crinoid crown and stem 12 sections. (D) PCF distance plot.
13
14
15
4 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 This analysis was complemented by analyses using density/diffusion models (20, 21) to constrain
2 the length of time logs of differing sizes and densities, with a full colony, could stay afloat as water
3 infiltrates the wood over time. This diffusion analysis used three sets of size defined colonies (SI
4 Appendix, Figs. S4–S9)), including small colony specimens (S1 and S2) (SI Appendix, Figs. S4 and S5)
5 medium colonies (M1 and M2) (SI Appendix, Figs. S6 and S7) and finally massive giant colonies from
6 Holzmaden (G1 “The Hauff Specimen” and G2) (SI Appendix, Figs. S8 and S9).
7
8 9 Fig. 2. Wood diffusion gradient plots. (A) S1 unloaded. (B) S1 loaded. (C) M1 unloaded. (D) M1 loaded. 10 (E) G1 unloaded. (F) G1 loaded.
11 5 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 Results
2 The spatial analyses clearly show an ecological signal of directionality along the log with the highest
3 density on the left hand side, with decreasing density along the log (Fig. 1B and SI Appendix, Figs. S2
4 and S3). This directionality can be modelled by heterogeneous Poisson model depending on the x
5 co-ordinate (SI Appendix, Table S1). The diffusion analysis is represented by two models, which
6 constantly show that the largest of the log systems could have survived for a minimum of 2 years and a
7 maximum of 20 years. (Fig. 2, and SI Appendix, Table S1) Thus allowing the crinoid colony to grow to
8 maturity and confirming the viability of the pseudoplanktonic hypothesis. Firstly, the diffusion model
9 assumes that the system is not adequately sealed. When the diffusion model is applied to the small logs
10 (S1 and S2) (SI Appendix, Figs. S4 and S5) without the population, the log would sink after 800 days,
11 however when the colony is added it would sink within 1 day (SI Appendix, Fig. S5). The apparent
12 longevity of the logs is increased by their size, with the medium sized logs (M1 and M2) (SI Appendix,
13 Figs. S6 and S7) surviving 100 days (800 without the colony) and the massive colonies (G1 and G2) (SI
14 Appendix, Figs. S8 and S9) surviving up to 400 days (800 days without the colony) (SI Appendix, Fig.
15 S9). Our results suggest that without any natural sealing the largest megarafts could only have survived
16 for just over two years with the smaller systems unviable despite being relatively common. We propose
17 that in order for the system to survive for long enough for the animals to grow to maturity the natural
18 sealing of the wood structure would be needed with the oysters and the crinoids being part of efficient
19 sealing of the system. The population model incorporated both the spatial distribution along the log as
20 well as life-history estimates (fecundity, mortality, settling rates and maturation time) for a complete life
21 cycle of extant oyster Ostrea chilensis. Simulations for the growth of oyster communities including
22 spawning rate, per capita reproductive rate and mortality rate correspond to average values taken from
23 the literature about Ostrea chilensis. The model allowed reconstruction of the number of years required
24 to reach sinking density for the whole log for various values of K (SI Appendix, Tables S2 and S3). In
25 some cases, the dimensions of the log combined with the density of the wood material would allow the 6 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 oyster and crinoid loaded log never to reach sinking threshold within the 20-year window (Fig. 2).
2
3 Discussion
4 The crinoids attached preferentially to one end of the log structure (Fig. 1B, and SI Appendix, Figs. S2
5 and S3), with other parts of the log more sparsely populated and attachment discs barely developed on
6 the upper surface (Fig. 3).
7
8
9 Fig. 3. Reconstruction of the Holzmaden ‘Hauff Specimen’ crinoid megaraft colony (G1).
10 7 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 These spatial analyses support the hypothesis that our largest colony was structured as a floating body
2 not a benthic structure. This feature is demonstrated by modern shipworms, for example, which
3 preferentially accumulate at one end of the structure (22). In the case of a ship, this would be the back
4 or stern. If the log were moving through the water even at a low rate the bow of the structure would still
5 be subject to the most current pressure, so the most hospitable part of the structure would be the area of
6 least resistance, the stern. However, this asymmetric distribution, combined with the observation that
7 the vast majority of crinoids are attached to the underside of the log (Figs. 1B and 3 and SI Appendix,
8 Figs. S2 and S3), would make the log unbalanced towards one end, bringing the colony’s stability into
9 question. The conventional view is that the community would have rapidly sunk and been preserved at
10 anoxic depths due to becoming unstable or overladen (13). However, our diffusion model does not
11 support rapid sinking for three key reasons.
12 Firstly, the growing community of oysters and crinoids even at the climax state would still be a
13 minor component of the total weight of the system. This total maximum weight (in very rare cases)
14 would have varied from 630 to 880 kg depending on carry capacity and density compared to estimates
15 of between 1100 kg to 15,000 kg for the total mas of the log depending on wood type (SI Appendix).
16 Secondly, without efficient sealing our catastrophic models suggest the log could have supported the
17 population of crinoids for a substantial amount of time. Even with a system designed to be a
18 catastrophic scenario taking on water at a constant rate until the log raft system failed, the largest
19 examples (G1 and G2) (SI Appendix, Figs. S8 and S9) could have survived for up to two years (SI
20 Appendix, Table S3). Preserved fossil examples in this study were close to their maximum point
21 viability as a mobile substrate. This point is evident from smaller logs (S1 and S2) (SI Appendix, Figs.
22 S4 and S5)whose population was simply too big to be supported by the smaller log structure. However,
23 the model looks at the viability of the climax community that would take time to develop, and would
24 expand the life of the colony within the parameters of the model >2 years. However, modern growth
25 rates for recent isocrinid crinoids 30-40 cm (23) in length suggests it would have taken at least 10 years 8 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 for Seirocrinus to reach maturity. Although the growth rates of this crinoid may have been faster due to
2 the unusual structure of the stem (24). For these communities to exist to maturity the log needs to float
3 for much longer than 2 years (SI Appendix, Figs. S8 and S9). Our oyster community growth model in
4 contrast, predicts that the megaraft was viable for up to and exceeding 20 years (Fig 2 and SI Appendix,
5 Table S3). This length of time would require the structure to be sealed by an oyster coating to preserve
6 any failures in the wood structure. The key to understanding how this lifespan of the megaraft was
7 possible, is firstly by identifying areas of weaknesses were the structure is most likely to fail, this would
8 include areas on the surface not coated in bark and secondly the ends of the log system that expose the
9 wood xylem (25). However, it is clear from all the examples examined that most of the surface is
10 covered in oysters (Fig. 1) and as our results suggest that the crinoids themselves are preferentially
11 distributed at one terminal end of the system (Figs. 1B and 3 and SI Appendix, Figs. S2 and S3),
12 contributing to the natural sealing of that end of the log system. Not all areas of the log were sealed and
13 this would have shortened the lifespan of the community.
14 Thirdly, the viably of the community would have been influenced by the natural properties of the
15 wood itself. Although it is hard to model the wood structure with no fossil remains available in the
16 Posidonia Shale (26), the gymnosperm wood structure with the addition of the oyster coating and other
17 agents such as aquatic fungus and algal slime must have allowed for sealing of the log and increased its
18 longevity substantially beyond our models of continuous infiltration (27). Estimates vary for how long
19 modern gymnosperm pine driftwood can survive (see SI Appendix for full discussion). Some estimates
20 are as little as 17 months (28) while others suggest that the wood could have remained buoyant up to 5
21 years after entering the marine environment (see SI Appendix). The exceptional properties of the
22 Jurassic gymnosperm wood with natural sealing by the oysters and crinoids, and without modern
23 aggressive wood boring predators, which evolved later in the Mesozoic (29) the wood would have
24 likely floated for longer than the 5 years observed in today’s driftwood. The colony could therefore have
25 survived for the 10 years needed for a community to reach maturity, and possibly longer, until the decay 9 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 and water infiltration of the wood reached a limit that could no longer support any further community
2 growth. At which point the log sank and the crinoids died, being perfectly preserved by the anoxic
3 conditions on the seabed (11) (Fig. 1A and 1C). Evidence suggests that fragments might have broken off
4 while the system was in a state of decay which might explain the occurrence of single mature
5 individuals attached to relatively small fragments of wood observed across the museum collections
6 examined.
7 Pseudoplanktonic organisms are highly significant for global ecology and biodiversity, as a
8 mechanism for global colonization (30), typically attaching themselves to necktic and planktonic
9 organisms or other floating objects such as driftwood or flotsam. These are significant in today’s
10 ecosystems and were responsible for the colonisation of oceanic islands such as Hawaii (31). These
11 temporary rafts can become a permanent home to self-contained long-term communities such as, in
12 modern ecosystems, external goose barnacles, tunicates and bryozoans (32), with wood boring bivalves
13 such as shipworms inhabiting the internal structures (33).
14 Our analysis suggests that the exceptionally preserved Jurassic crinoid megarafts are the longest
15 surviving communities to exist in the fossil record. Our results not only demonstrate that they have an
16 ecological structure consistent with a recent living megaraft colony (22, 34, 35, 36) (see SI Appendix)
17 but with efficient sealing these communities could have survived for up to and exceeding 20 years. In
18 contrast, those recorded today have survived for up to 6 years. These extant megarafts of marine debris
19 deliver substantial communities of adult organisms capable of reproduction or colonization by
20 zooplankton in marginal marine environments from one continental margin to another (5). Rafts tend to
21 be one-way arrival and deposition events which limit their journey time and life span before they
22 encounter another system. These colonies are slow moving (1 to 2 knots) compared with modern
23 commercial vessels (20 to >25 knots). These data are consistent with palaeoenvionmental
24 interpretations suggesting an inhospitable seabed (37), indicating that these colonies remained afloat to
25 survive and developed largely in isolation and could easily have spread around the globe. Our data 10 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 suggests that the life of the colony was far more dependent on the wood structure than previously
2 thought. Although the total weight of the crinoids would finally contribute to sinking of the system, they
3 are comparatively lightweight organisms compared to the oysters.
4
5 Conclusions
6 We have demonstrated that Jurassic crinoids did inhabit this unique pseudoplanktonic niche, becoming
7 highly adapted, fast growing, very lightweight and self-sufficient viable communities. In the early
8 Mesozoic seas, these rafts were home to far larger and more complex, now lost, ecosystems whose
9 existence was a necessity as a result of a high number of anoxic shallow water basins (37, 38).
10 Development of this lifestyle ensured the continued success of the group. The appearance of wood
11 boring bivalves, and the prevalence of angiosperm wood along with the restoration of healthy benthic
12 environments by the Middle Jurassic (9, 13) meant that this unique ecological phenomenon vanished
13 from the fossil record forever.
14
15 Methods
16 Data collection. Photographs were taken from six specimens and composite images produced. These
17 include small, medium and finally massive giant colonies from the following German museums and
18 collections: Geoscience Centre of the University of Göttingen; Werkforum Museum, Dotternhausen;
19 Naturmuseum Senckenberg in Frankfurt am Main; Geological Institute, University of Tuebingen;
20 Staatliches Museum für Naturkunde, Stuttgart; and Urweltmuseum Hauff, Holzmaden.
21
22 Spatial analysis. Randomly distributed points (Fig. 1A) can be modelled using homogeneous Poisson
23 processes, and are found, for example, when larvae attach to an substrate not impacted by strong
24 currents nor patchy environmental variables (39). While spatial processes that depend on position on the
25 substrate can be modelled using heterogeneous Poisson processes, with density changing according to a 11 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1 given formula. To assess how disc density changed along the log, disc density was modelled as a
2 heterogeneous Poisson process dependent on the x co-ordinate and then the y-co-ordinate. Model fit was
3 assessed using the model residuals by plotting Q–Q and smoothed residual plots. If the observed line in
4 the Q–Q plot fell outside two standard deviations of the model, the model was rejected (18, 40). Akaike
5 information criterion values (41) were used to compare the relative quality of the statistical models that
6 fitted the data.
7
8 Moisture diffusion models. As it is not possible to know the actual properties of the fossil wood (due
9 to the lack of fossil examples) or the actual growth rate of the oysters (as these species are now extinct),
10 two types of models were developed. The first is a set of ‘catastrophic’ models (SI Appendix, Figs. S4–
11 S9) which look at how long a series of size defined structures can survive with no natural sealing (see
12 Diffusion analyses and SI Appendix). The second model is predictive, and proposes how long the
13 mega-raft could have lasted if the system was efficiently sealed and a population of oysters (based in
14 extant oysters) was growing on its surface (see Oyster community growth model and SI Appendix).
15
16 Diffusion analyses. For the first model, three sets of size defined colonies (SI Appendix, Figs. S4–S9)),
17 including small colony specimens (S1 and S2) (SI Appendix, Figs. S4 and S5) medium colonies (M1
18 and M2) (SI Appendix, Figs. S6 and S7) and finally massive giant colonies from Holzmaden (G1 “The
19 Hauff Specimen” and G2) (SI Appendix, Figs. S8 and S9) were tested. To address the stability of the log
20 and colony, the density/diffusion model is constructed for each of the colony three-size classes using the
21 log properties without any maximum growth population attached, then the crinoid community is added,
22 along with a layer of oysters (11). The models use a colony weight based on modern isocrinid
23 Metacrinus and Japanese oysters (Crassostrea gigas). They assume there is no natural sealing of the
24 wood and it begins to absorb water, and therefore breakdown, from the onset of the colony (see SI
25 Appendix). 12 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
1
2 Oyster community growth model. The second model considers the growth of the oyster community on
3 the surface of the same logs and estimates how long the log can remain afloat while bearing the weight
4 of the colony. It relies on oyster population data about extant communities of Ostrea chilensis and
5 Crassostrea virginica. The model of population growth assumes the settlement on one single individual
6 when the log contacts sea water and estimates the added mass of the community to the completely
7 sealed log every 1-year increment until the whole log and colony sinks due to its density (see SI
8 Appendix, Fig. S2 and Table S3).
9
10 ACKNOWLEDGEMENTS. We thank the following institutions for access to the specimens:
11 Geoscience Centre of the University of Göttingen; Werkforum Museum, Dotternhausen; Naturmuseum
12 Senckenberg, Frankfurt am Main; Geological Institute, University of Tuebingen; Staatliches Museum
13 für Naturkunde, Stuttgart; and Urweltmuseum Hauff, Holzmaden. We thank T Ewin, Natural History
14 Museum, London, for his help with the early stages of the project. Also special thanks to R Haude,
15 University of Göttingen, for providing the composite image of Holzmaden Log (G1) (Fig. 1A and SI
16 Appendix, Fig. S1). We acknowledge support from DAAD funding enabling collection of data (A.W.H.)
17 and a Junior Research Fellowship from Murray Edwards College, University of Cambridge (E.G.M.).
18
19 Data availability: Extended methods for the spatial analysis and the diffusion analysis as well as
20 properties, definitions and equations are discussed in detail in SI Appendix.
21
22
23
13 bioRxiv preprint doi: https://doi.org/10.1101/566844; this version posted April 25, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
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