Predation Through Geological Time: Evidence from Gastropod Shell Repair
Predation Through Geological Time: Evidence from Gastropod Shell Repair
Lizon Cenac & Victoria Urrutia
This
article discusses research conducted on Gastropod shells
that inhabited warm shallow water, soft bottom habitats, during 3 periods and 2 Epochs of the Phanerozoic Eon: The Pennsylvanian, The Triassic, the Cretaceous, the Miocene and the Present; to determine the
rise in the selection of breakage-resistant shells. It was hypothesized that shell breakage is a more common and important selecting agent today than in the past, research was conducted to explain increased frequency of break-resistant shells in the Middle Mesozoic and decline of structurally weak features. Selection can only
occur if individual gastropod in a population survived a shell
breaking attack and reproduced. Higher incidence of shell breaking
attacks allow for selection of hard shell armament. The shells were
divided into 4 size classes 5-9 mm,10-19mm, 20-29mm and 30-39mm, with
each time unit after the Triassic and The Pennsylvanian showing
higher frequencies in repairs. The Triassic has no shells for size class 20-29mm and 30 to 39mm. Reasons offered for increase in repairs include
significantly weaker predators of Gastropods than their descendants and higher frequency of lethal attacks on smaller shells. The hypothesis for this study was found to be supported as increased frequency of shell repair was observed in larger size classes, in turn larger size samples were correlated with the diversification of shell breaking fish and crustaceans during the Jurassic period showing increased significance to the selecting agent.
What
may be the cause as to why there is no representation of size classes
20-29 mm and 30-39 mm in the Triassic period, But the Pennsylvanian
period shows some representation of size class 20-29 and the later
periods also show representation of the larger classes as well. Could
this be due to the sample areas?
Does being Thin-lipped or Thick- lipped correlates with the size classes and survival chances?
Considering
that frequency repair is less common in thick lipped species, do you
think that there should be a higher frequency of representation in the
Triassic and Pennsylvanian periods as they are less susceptible to
predation?
The possible reasons for why there is no representation of size class 20-29 mm in the Triassic and then some in the Pennsylvanian could be that predators in the Triassic were too weak to leave any sort of scaring on larger shells. Then they evolved over time and became stronger leaving some scaring on shells during the Pennsylvanian.
ReplyDeleteThe paper says that being thick-lipped seems to correlate with with less need for repair of fractures.
Perhaps there would be more representation of thick-lipped shells in the more recent time because predators evolved and became stronger, possibly strong enough to leave scars on thick-lipped species.
I think the results show a gradual evolution of predators over time. Later on in history we don't see predation evidence on larger gastropods because natural selection hadn't selected for predators that could take advantage of this food source. Over time ecological niches were filled and larger predators or more adapted predators were able to exploit these niches even better as time went on. There could be some bias in the fossil record however. We obviously don't have a full fossil record for almost any animals let alone a diverse ecosystem consisting of prey and predators. More fossil evidence or lack of evidence is needed to be found to conclude these hypotheses
ReplyDeleteI agree with what has already been said, this is more telling of the predators at the time and shows how they evolved to eventually be able to eat the larger gastropods. In that respect then i think that we should see a larger number of large gastropod fossils from the earlier time periods since they were not being predated as much or even at all, but then again maybe not since they had no fast way to heal fractures so maybe when they did die there shells would be more accessible to erosion over time leading to little to no fossilization.
ReplyDeleteI would like to agree that this article seems to be about the evolution of predators as well as the evolution of gastropods throughout time. The difference in shell repair shown in the Pennsylvanian time period and not Triassic could be the difference in predation among the predators among the shallow shores where gastropods lived. There may have been a vast difference in selection for the gastropods in the Triassic compared to the Pennsylvanian due to predators being able to damage gastropods without ability to heal or even not effect it at all since the article stated that as time grew on predators were able to eat larger gastropods. To add on since thick lipped species had less repair frequency it should correlate to predation occurring less probably due to the predator knowledge that the smaller gastropods were easier to eat during the Triassic and Pennsylvanian periods.
ReplyDeleteI think that there is a correlation between increase predation through time and the rise of thick lipped shells. The more that predators broke thin shells, the more likely that thick lipped shells would develop to survive. Think lipped shells are common from the Cenozoic to the Recent while thin lipped shells are common during the Paleozoic. I am interested to know why there would still be thin shelled creatures in the Recent if they are less likely to survive?
ReplyDeleteAs pointed out in some of the other comments, the Vermeij et al. paper suggests that survival odds for organisms seem to improve as a result of being thick-lipped as compared to their thin-lipped counterparts. Given that the thick lipped organisms do have a lower repair frequency, owing to their increased resistance to predators and other agents of damage, I would expect a greater degree of representation in the fossil record relative to thin-lipped and smaller organisms. However, this is just one factor that can influence representation; other forces and qualities of these organisms may also play a role in their representation in the fossil record. Relatedly, there may be a fitness tradeoff in these thick-lipped organisms that could, potentially, influence the degree to which they are represented in the fossil record.
ReplyDeleteI believe most of my classmates agree that this paper shows an evolution of the gastropods predators. Andrew put it very well with this being a fitness trade off, where the thin-lipped organisms would have several advantages over the thick-lipped and as pointed out by the paper that would be the repair frequency that was seen. The thick-lipped organisms had a slower repair rate than the thin-lipped. But the thin-lipped organisms would have been highly predated on due to the fact that they were easier to consume than the thicker lipped organism.
ReplyDeleteThis paper suggests that the influence predation has on gastropods through time is evident in the thickness and repair frequencies of the shell. As predators break a shell (nonlethal), directional selection would favor more of a thick shell for survival. Having a thick shell must have its trade offs though because why would there still be thin-shelled gastropods today?
ReplyDeleteThe paper clearly explains its findings and the reasoning behind them in such a way as to have everyone in agreement. In light of this, I would like to move forward; how did the mass and structure of the shells themselves change over time? Knowing this would provide insight to the limit on the shells' base strength before they became immobile, and how that was responded to.
ReplyDeleteI feel like I am just repeating what everyone else has agreed on; that predators have evolved over time. Predators from the Triassic were most likely unable to leave any sort of marking on gastropods or evolution had not allowed them to find gastropods as a food source yet. Natural selection has a large part in why there was no evidence in the Triassic that predators trying to eat the gastropods, but later in time natural selection either selected for stronger predators to leave a marking on gastropods or selected for predators to find gastropods as a food source.
ReplyDeleteThis paper seems to show two main factors concerning molluscan adaptation to prevent predation over time. The first of these is that mollusks seemed to increase their body size over time, especially since the Cretaceous. The second is the transition of frequency of repair. During the Pennsylvanian and Triassic, the smaller Mollusks had a higher relative repair frequency than the larger ones. However, the larger Mollusks have much higher repair frequencies in more recent times. It is also worth noting that the absolute repair frequency of the smallest Mollusks stays relatively constant throughout the entire time sampled. This suggests that larger Mollusks tend to invest more energy and resources into shell repair mechanisms.
ReplyDeletelarger molluscs do seem to be adaptations for greater predator defense. however the trend toward larger molluscs using more energy on shell repair may, in modern samples, indicate a possible life style strategy difference. smaller molluscs may not invest in shell repair as it may not be their primary defense. larger molluscs likely rely on their shell for defense more than smaller molluscs which may be able to avoid predators more easily than their larger counterparts. moving under a rock, or being less desirable than the larger molluscs may provide them an alternate defense than shell development. this seems especially plausible if you consider fishing as humans will most likely select for the larger more accessible molluscs.
ReplyDeleteReading CHD's comment about thick vs. thin-lipped gastropods, and I'm not clear on whether these thin-lipped gastropods are becoming thick-lipped or if these are two different groups of gastropods affecting the data. Does shell repairment create thick-lipped gastropods from thin-lipped ones, or does it just make the thin-lipped shells stronger? Wasn't really on my mind until I read CHD's comment.
ReplyDeleteI agree with what most have already stated on evolution of predators and gastropods had an influence on the shell. Thick lipped gastropods didn't seem to need to rely on repairing frequently as much as thin-lipped.
ReplyDeleteI liked Nick's explanation about larger mollusks would invest more of their energy for shell repair with looking at changes over time.
I tend to greatly appreciate the articles we are assigned week after week because they lead me to think about evolution and predator/prey interactions in ways I hadn't thought about before. After reading this article, it makes perfect sense that Vermeij et al. (1981) would look at shell breakage and frequency in repair to as selective evolutionary agents. Figure one made it clearer to see that the Triassic was missing size classes of 20-29 mm and the Pensylvanian was missing the 30-39 mm category of shells. I think this could potentially be attributed to the species that had harder-to-break shells being able to fare better within their environments and eventually give rise to shells that were bigger and better able to survive. For example, if my ancestors were able to survive and live for longer, then eventually I will also have more opportunities and resources to grow larger and stronger. Just a thought!
ReplyDeleteI think a possible explanation as to why there was poor representation of large (20-39mm) sized mollusks in the Triassic is because they were hit comparatively harder during the Permian Extinction than smaller mollusks. It could be that thick-lipped mollusks had more representation in the Triassic and Pennsylvanian periods as nothing was around that was adapted to crushing their thicker shells. Or it could be that during those periods there was another selection factor coming into play. We should also consider if Atmospheric CO2 concentration had any correlation in shell thickness, as today's mollusks are threatened by the acidification of the oceans by increasing CO2 concentrations which theoretically would favor thicker shells.
ReplyDeleteI found this article difficult to unpack and to understand, I was unclear on the differentiation between the thick lipped mollusks and thin shelled mollusks as well as why they varied. At first I thought it was a result of protection - when nothing is around to crush the mollusks then they did not need to dedicate resources into producing thick shells. However the CO2 hypothesis having correlations with what thickness the shell would be really threw me. However after reading through prior comments it makes a bit more sense why there is a positive correlation between the thickness of the shell and increasing CO2 concentrations in the atmosphere.
ReplyDeleteProbably a difference in defense; small mollusks focus on avoiding/evading predators at higher risk of death upon engagement. Where as the larger ones evolved to be bigger and tougher with higher focus on self repair compared to the smaller ones. Could me that at different times the different strategies of survival were better suited/selected based on predator type/quantities as well as other biotic and abiotic factors. The thickening of the shells in response to increases in CO2 concentrations point to other variables excluding immediate predatory threats as a focal reason.
ReplyDeletethe best explanation for the changes is a evolutionary arms race. as predators evolved to prey on mollusks the mollusks changed to either provide better armor or evade predators.
ReplyDeleteotherwise i dont know what to say other thn what other people have said.
ReplyDelete