Education, Geology, Paleontology, Science

Know Your Bones: January 2015

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Last month’s challenge appeared to give everyone a hard time since it went a whole two days before someone took a guess. However, after a few days of silence, Isotelus once again correctly named this critter.

 

Rhamphorhynchus muensteri

 

This critter is indeed Rhamphorhynchus muensteri.

 

(Taken at The New Mexico Museum of Natural History and Science)

R. muensteri lived during the late Jurassic 156 to 145.5 million years ago. Its fossils are found in Germany, with several possible specimens of Rhamphorhynchus found in England, Spain, and Tanzania. R. muensteri reached a size of ~1.26 meters from snout to tail with a wingspan of ~1.81 meters. However, the smallest known specimen is a (hatchling) ~290 millimeters in length, but would have still been capable of flight. R. muensteri was a long-tailed genus of pterosaur, and was less specialized than the contemporary short-tailed pterosaurs.

 

R. muensteri had needle-like teeth that were forward facing, the tip of its beak was sharp and curved up, and lacking teeth. When the jaws were closed, the teeth fit together like a closed zipper. This suggests that R. muensteri had a diet mostly made up of fish and other marine animals. Adult R. muensteri had a diamond-shaped vane at the end of their tail, which possibly was used to signal for mates; the diamond-shaped vane does not appear on smaller specimens assumed to be juveniles, and their beaks are not as sharp and curved.

 

Moving on to next week’s challenge:

 

(Taken at The New Mexico Museum of Natural History and Science)

 

Good luck to everyone who is playing and I promise that this one will be the last difficult one for a while.

Education, Geology, Paleontology, Science

Know Your Bones: December 2014

1 Comment

Last month’s challenge was not very challenging seeing as how Isotelus was able to give the correct answer within hours of when the blog was posted. She said it gave her some trouble, but I actually highly doubt that.

 

Partial skull of Parasaurolophus. I would say P. tubicen because the crest is a bit different from P.walkeri, and it’s definitely not P. cyrtocristatus. Also it’s from New Mexico so it makes sense to be in the NM museum. SCIENCE.

 

This skull did indeed belong to Parasaurolophus tubicen, which stands for trumpeter-crested lizard.

 

(Taken at The New Mexico Museum of Natural History and Science)

 

Parasaurolophus are extremely rare animals in the fossil record. There are three species known to science found in Alberta (Canada), New Mexico (USA), and Utah (USA). In each area, only a few specimens have been found and all specimens are incomplete. P. tubicen is only known from New Mexico with three specimens discovered. Parasaurolophus lived during the Late Cretaceous 76 to 73 million years ago. P. tubicen reached a size of ~9.5 meters in length and weighed ~ 2.5 tons. P. tubicen was an herbivore and most likely walked on four legs, but was able to run, walk, and brows on its hind limbs.

 

Even though it is rare, it is still one of the most famous dinosaurs, and that is most likely due to the eye-catching aspect of P. tubicen. The crest that grows from the rear of its skull is fairly unique. The crest is hollow and allowed air to be pushed through it. This would have allowed P. tubicen to make very loud trumpeting noises. The crests were also most likely colorful and could have acted as visual displays. P. tubicen would have filled the Late Cretaceous with beautiful music while communicating with one another over large areas. P. tubicen belongs to the hadrosaurid clade, which is one of the most famous ornithischian clades as well.

 

Moving on to this month’s challenge:

 

(Taken at the New Mexico Museum of Natural History and Science)

 

Good luck to everyone that plays.

 

Religion, YouTube

Misunderstandings about Atheism

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I recently saw an interview with David Mitchell. One topic discussed was his stance on religion. Here’s what he said:

1) Up until 0:30, it’s rather uninteresting: So he’s not an atheist, he’s an agnostic. I’d point out that he’s wrong, he’s an agnostic atheist, but that’s not a huge problem.

 

I’ve mentioned it a few times all over the forum, so I won’t go into it here, but “agnostic” is a qualifier about the position of God, it’s not a position in and of itself. You can be an agnostic about everything, aka. claiming that nothing is knowable, but you can’t purely be an agnostic about whether or not God exists: If your life does not include a God or gods, then you’re an atheist.

 

2) The first real problem I have is between 0:30-0:40. David says that he “wants there to be an all-powerful, benevolent God”. That’s fine, lots of atheists want that. In fact, I’ll use two definitions now:

An atheist is a person whose world view does not contain a God or gods.

An anti-theist is a person whose world view does not contain a God or gods, sees both organized religion (i.e. churches) and religion itself (i.e. the belief in a God) as something detrimental. Such a person would not like for there to be a God.

 

Christopher Hitchens famously said:

Such a person [an atheist] might very well say that he wished it were true [the existence of a god]. I know some atheists who say, ‘Well, I wish I could believe it. I just can’t. There’s not enough evidence for it’ … I say I’m an anti-theist because I think it’d be rather awful if it was true … you would never have a waking or sleeping moment where you weren’t being watched, and controlled, and supervised by some celestial entity from conception until, well, not even until your death because it’s only after death when the real fun begins, isn’t it? It’d be like living in North Korea.”

This is what I understand anti-theism to be: Absolute opposition to both organized religion and the hope of an afterlife. I’ll be absolutely clear: I agree with Christopher Hitchens that any god yet proposed* would be ghastly and I seriously hope that there is no god.

 

*That needs extra clarification as well: The gods of ancient Greece do not count as gods in this context. They are basically humans with super-powers, not gods. If they do count as gods, then I’d have no problem with them, they’re awesome.
What I do have a problem with is the bogus claim that a god can be both all-powerful and all-loving and that there can be a heaven.

3) The next problem I have is between 0:45-1:01.

David says that there are, and I have to be fair here, “some atheists” who want to tear the comfort of religion away from people. While it may be true that there are some people who want to do that, that’s not the position of the vast majority of atheists. Or anti-theists, for that matter.

Instead, most atheists I have talked with are perfectly happy to let people pray in their own homes or even in churches as long as religious stupidity (genital mutilation, fanaticism, religiously motivated killings, opposition to homosexual marriage, etc.) stays within the confines of their own homes or even churches. Another famous Christopher Hitchens quote goes as follows: “What about the most important minority in the history of the world? … We have to be insulted and outraged every day by what we see and what we read. By slaughter and murder. Slaughter and murder and barbarism and insult and superstitious nonsense.”

If religion and the insanity associated with religion wasn’t shoved down our collective throats, I think few people would have a problem with religion. As it stands however, I see it as my duty to stand up to it.

 

4) The last problem I have is between 1:25-1:35. David says (roughly) that “the idea that you take away one of the excuses, that the killing will suddenly stop happening is absurd.”

Quite right, that is an absurd proposition. That’s why I’ve never seen anyone make it. There is one argument put forward by Christopher Hitchens that there would have been peace long ago in Northern Ireland if there had been no religion, but I think that’s wrong. There certainly might have been a better chance, but I think the struggle would have been largely political instead of largely religious.

However, things like 9/11 would undoubtedly never have happened. If not for the crazy idea that you get rewarded for your death in an afterlife, nobody would have strapped a bomb to themselves and blown themselves up. It’s ridiculous.

Much of the opposition against evolution would be gone, a good deal of anti-science would simply vanish. Genital mutilation would be gone almost entirely. Abortion clinics would be largely safe. And so on and so forth. A lot would definitely change and the way David explains it is a simple misrepresentation.

I hope this clears up some of the misunderstandings about what atheism is and what atheists believe. There will be, I hope, a fair amount of discussion on this issue.
Some people may disagree with the distinction of atheism and anti-theism, nor atheism and agnosticism.
Some people may disagree with the notion that a god is by definition a bad thing.
Some people may disagree that religion interferes too much.
Some people may disagree that the things I listed under 4) will go away if religion were to disappear.

Ah well…

Education, Geology, Paleontology, Science

Know Your Bones: November 2014

16 Comments

Last month’s challenge was more of a challenge than I actually thought it would be, but that was not because of the crazy looking critter. That was because of my poor photo quality. I just want to say that I am sorry about that. I guess, since I know what it was, I thought the photo was okay, but it obviously was not. I will do better in the future.

 

With all that said, who was able to identify the weird looking critter in the bad photo? It was Mugnuts and he was the only person to guess the correct answer.

 

Eobasileus cornutus!!! This is what He-Man should have rode.

 

This crazy looking critter is Eobasileus cornutus, and I agree that it would have been a much better companion for He-Man than Battle Cat.

 

(Wikimedia Commons Image)

 

E. cornutus is very rare in the fossil record and is only known from Colorado, Utah, and Wyoming. It lived during the Eocene 46.2 to 40.4 million years ago. During that time, E. cornutus would have been one of the largest animals on earth. It was ~4 meters from snout to tail, ~2 meters at the shoulder, and weighed in at ~4000 kg. Like all large animals, E. cornutus was an herbivore. It is not known if E. cornutus lived a terrestrial life like a rhinoceros or a semi-aquatic life like a hippopotamus.

 

E. cornutus belonged to the Uintatheriidae clade. This is a family of large herbivorous mammals with unique skulls. E. cornutus had saber tusks that extended from their top jaw. Their bottom jaw had a bony protrusion that protected the tusk when the mouth was closed. E. cornutus also had three pairs of blunt horns. The first pair was behind the eyes, the second was between the eyes and nostrils, and the third was atop the nostrils. This assortment of horns and tusks is shared among most of the Uintatheriidae. Judging from related animals in the Uintatheriidae clade, the horns and tusks were most likely sexually dimorphic, wherein males would have used them to joust for females. However, there are not enough fossil remains to determine any use of the horns and tusks at this time.

 

Moving on to this month’s challenge:

 

(Taken at the New Mexico Museum of Natural History and Science)

 

Once again, sorry about the quality of last months photo and I hope this month’s is better. Good luck.

Education, Geology, Paleontology, Science

Know Your Bones: October 2014

13 Comments

Last month’s challenge took some backbone in order to make a guess. The correct answer was given a few times within the first day of the blog going up. However, Isotelus was the first person to answer correctly.

 

Genus Eurypterus. I’m obviously not a Eurypterid worker, so the only species I know are remipes and lacustris, and I don’t know the differences between the two. Um. E. remipes, because remipes is everywhere lol. Science! :lol:

 

This critter is Eurypterus remipes, the first invertebrate to appear on Know Your Bones.

 

 

(Wikimedia Commons Image)

 

E. remipes lived during the late Silurian 432 to 418 million years ago and are found in North America, parts of Asia and Europe, which made up one continent. They had an average length of 13 to 23 cm. E. remipes were a shallow marine animal that lived near the coast and unable to travel the open ocean. E. remipes had one pair of swimming appendages that they would have used to travel longer distances, but would have spent most of their time walking across the sea floor. They most likely were generalists, feeding on whatever they could find.

 

E. remipes is one of the most abundant fossils in the world. They are mostly found as disarticulated exoskeleton remains. Whole specimens are extremely rare. E. remipes appears to have been able to walk on land based on a few anatomical structures. However, if that is the case, it probably only spent a limited amount of time out of water, much like modern horseshoe crabs.

 

Moving on to this month’s challenge:

 

 

(Taken at the Denver Museum of Natural History and Science)

 

Good luck.

Education, Geology, Paleontology, Science

Know Your Bones: September 2014

10 Comments

Once again, last month’s challenge brought in a few good guesses and several participants guessed the correct answer. However, Dragan Glas was the first to give the correct answer.

 

 It’s likely to be one of the following three:

a) M. Comlumbi – most common in NM;
b) M, Meridionalis;
c) M. Imperator.

Since I don’t know how to tell the difference between them, I’ll go with the numbers – M. Columbi.

 

This is indeed Mammuthus columbi (Columbian mammoth). I also want to say that I loved seeing the deductive reasoning that brought Dragan Glas to this answer.

 

 

(Taken at The New Mexico Museum of Natural History and Science)

 

The Columbian mammoth roamed across the southern half of North America during the Pleistocene (2,588,000 to 11,700 years ago), also known as the Ice Age. The northern and southern boundary of their range would have changed with the movement of the ice sheets, but their fossils have been found in southern Canada thru Nicaragua. The Columbian mammoth inhabited the grasslands of North America and was an herbivore.

 

The Columbian mammoth was larger then its more famous cousin to the north, the woolly mammoth (M. primigenius). The Columbian mammoth could reach a height of 4 meters and weighed in at 9 tons. The males had tusks that could reach a length of over 4 meters. This makes the Columbian mammoth the largest animal to have inhabited North America since the Mesozoic and one of the largest land mammals to ever walk the earth. It is believed that the Columbian mammoth would have little body hair, much like modern elephants, because it inhabited warmer areas of North America during the Pleistocene and had such a large body size.

 

Moving on to this month’s challenge.

 

 

(Taken at The Dinosaur Museum and Natural Science Laboratory)

 

Good luck to all those that read this.

Education, Geology, Paleontology, Science

Know Your Bones: August 2014

4 Comments

Last month’s challenge brought in a few good guesses, but only one correct answer. Once again, Isotelus guessed correctly, and within hours of the posting of the challenge.

 

 While it looks like a Wiener dog T-rex, I’m going to guess: 

 

This prehistoric wiener-dog is indeed Postosuchus.

 

(Taken at the Dinosaur Museum and National Science Laboratory)

 

During the late Triassic (288 to 202 million years ago), Postosuchus roamed across most of what is now North America. During this time, it was one of the largest predators on earth. It could have hunted and killed any of the dinosaurs that were alive at the same time. Postosuchus could reach a length of ~4 meters, stood ~2 meters tall, and could have weighed 250 to 300 kilograms. Postosuchus possessed a skull that was 55 cm long and 21 cm across.

 

Postosuchus had protective plates that covered their back, neck, and tail. These plates are something they share with their closest living relative the crocodilians. However, Postosuchus was a terrestrial predator and walked with its legs directly under the body. This armor probably protected them from other Postosuchus. During the late Triassic, there were not many other critters in the world that could challenge a full-grown and healthy Postosuchus, except another Postosuchus.

 

Moving on to this month’s challenge:

 

 

(Taken at the New Mexico Museum of Natural History and Science)

 

Good luck and thanks to everyone that reads and guesses.

Education, Geology, Paleontology, Science

Know Your Bones: July 2014

5 Comments

Last month’s challenge is a true titan. It held the record for being the largest dinosaur for several decades. So, who was able to name this giant? Isotelus once again named this critter.

 

 Brachiosaurus. I would guess the species name starts with an ‘a’ :P

 

This is indeed Brachiosaurus altithorax.

 

 

(Taken at the New Mexico Museum of Natural History and Science)

 

Brachiosaurus roamed 145 to 150 million years ago, during the Jurassic (and possibly the early Cretaceous) across the Western U.S. Brachiosaurus shared its range with several other sauropods and an earlier Know Your Bones critter. Brachiosaurus was ~25 meters in length, ~13 meters tall, and it had an estimated weight of ~28 tons, making it a true giant by any standard. Unlike most other dinosaurs, Brachiosaurus had longer forelegs than their hind legs. This curious trait is where it gets its genus name from (Brachiosaurus literally means, “arm lizard”).

 

Brachiosaurus was an herbivore, most likely feeding off the tops of fern trees that the other sauropods could not reach. Its large body would have been more than enough protection from predators that lived at the same time. It probably took a Brachiosaurus ten years to reach full size and could eat up to (if not more) ~182 kg of plant matter a day as an adult.

 

Moving on to this month’s challenge:

 

 

(Taken at the Dinosaur Museum and National Science Lab)

 

Good luck to all.

Paleontology, Science

The Transitional Tiktaalik Tirade

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The objection to the importance and/or existence of transitional fossils is based on a fundamental misunderstanding of the definition, implications, and usage of the term in the scientific literature. This post is therefore an attempt to accurately describe the significance of transitional fossils and their correct application as is evident in science articles. Popular science and media publications are in a sense obligated to use hyperbole when describing new fossil finds, and I personally consider them as on-the-go summaries of larger, more important bodies of work. While not always the case, such news reports often construe and overemphasize the results published by the paper in question. Granted, they are writing to the general public and must use language palatable to the lay person, although the use of certain terms in a news article may not accurately reflect their use within a scientific setting. That being the case, news publications are not and should never be viewed as a viable source or taken as such over the primary literature, even when said literature is not available to the public. When possible (unfortunately often not the case), always read the paper itself.

An example of these misconceptions at work is evident in any discussion with creationists regarding the elpistostegid fish Tiktaalik roseae and the discovery of an older set of Polish footprints, rendering Tiktaalik wholly “un-transitional”. To define the term, a transitional fossil is one that demonstrates characteristics in common both with its ancestral and descendent groups. As such, it may serve as a suitable representative of how morphological changes proceeded over time, and may have been closely related in some way to a true direct ancestor. The definition does not imply or assume that any such fossil represents direct relation or ancestry to any other, although it is implied that this is how it is viewed by some creationists who state that no evidence exists showing one organisms evolving into another. This misconception is intrinsically linked to the notion that evolution posits progression in a linear fashion. This view may or may not result from misinterpretation of diagrams that evidently depict primitive forms proceeding linearly into advanced forms in a series of strictly anagenetic events, but it must be noted that such graphical representations are often directed at non-scientists and simplified accordingly under the assumption that the viewer can place the image in the context of the correct version of evolution, which postulates the arrangement of organisms in a branching, tree-like structure. Transitional fossils do not imply a linear sequence of evolution and are instead imposed upon and considered within the context of branching lineages typical of phylogenetics (see figure below). The notion of one organism progressing into another also fails in that it makes an incorrect and rather tenuous assumption that any given form must die out and be succeeded by different, more advanced organisms. Recalling that transitional fossils are to be viewed in the context of lineages branching into consequent subsets, members of the branching lineages in question can coexist both in space and time, as each progresses following separate and independent trajectories. Any accumulation of changes would be similarly isolated. As a result, derived or advanced forms do not necessarily replace those that are basal or primitive, which can themselves persist unconstrained through time. This fact is directly observable in modern taxa and is both applicable to and demonstrable in fossil assemblages. Lastly and also crucial to understanding transitional fossils, a particular specimen such as Tiktaalik, is not considered to have been in the process of evolving itself, considering that evolution occurs at the population level, and not directly on individuals. Instead, the lineage containing Tiktaalik may show evolution, as it represents a collection of breeding populations progressing over a given length of time.

This and other arguments along a similar line tend to be incorrectly labeled by creationists as ad hoc rationalizations that feebly attempt to explain away major discrepancies in the fossil record. The description provided above is not however an after-the-fact justification, as it is the correct interpretation of transitional fossils that has always been applied in the scientific literature. The creationist argument that the finding of the older Zachelmie tracks necessarily implies Tiktaalik is out of stratigraphical order would only be true if  it was considered to be ancestral to later forms. The distinct possibility that Tiktaalik represents a member of a perserverant primitive lineage alone invalidates this view, and even more so given that transitional fossils are not presupposed to fall perfectly into a neat, simple progression of morphological forms over time, or fit into an ordered, designated time-slot in geological history. As such, finding fossils seemingly out of temporal order in no way disagrees with the definition of a transitional fossil, or the theory of evolution as a whole. In agreement with the notion of branching lineages, Tiktaalik was first described as the sister taxon to Tetrapoda (represented in the article by Acanthostega and Ichthyostega) and therefore not directly ancestral to later, more derived forms (see figure below) (Daeschler et al., 2006). Tiktaalik remained as a sister group to its derived relatives in spite of the age of the Zachelmie tracks, and as such the particular pattern of branching and progression of morphological traits within this specific group was likewise unaffected (Niedzwiedzki et al., 2010). This serves to demonstrate that the discovery of new specimens cannot and do not diminish or alter the intermediate status of other fossils, in spite of what the popular media may proclaim. The changes that do occur result from the necessary revision of previous hypotheses in accordance with the discovery of new lines of evidence.


A phylogenetic tree showing Tiktaalik as the sister group to Tetrapoda and its relative position based on a set of morphological characteristics. Note that Glyptolepis is representing the sister group to Tetrapodamorpha. The addition of Zachelmie prints did not affect the topology (branching pattern) of this tree.
Modified from “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan”, by E.B. Daeschler, N.H. Shubin, F.A. Jenkins, 2006, Nature 440: 757-763.

The trackway does bring into question the relative timing of tetrapod divergence, which was previously thought to occur at some point in the Late Devonian. Incorporating this new data with known body fossils pushed this divergence prior to the Eifelian age and into the Early Devonian. This resulted in the designation of Tiktaalik and other close relatives as ghost lineages, as phylogenetic analysis implied their existence in spite of no fossil specimens. In effect, the relative lengths of the branches signifying time scale changed in accordance, recalling however that the branch arrangement remains the same. This may be viewed as an excuse to explain away an obvious discrepancy, the above explanations as to why this is untrue notwithstanding. However, other studies have corroborated an older divergence date for Tetrapoda and therefore support the ghost lineage label. An earlier publication described a fragmentary Middle Devonian stem-tetrapod, Livoniana, predating Tiktaalik and coeval with Panderichthys, but more derived than both based on discernible morphological characteristics (Ahlberg et al., 2003; Niedzwiedzki et al., 2010). In contrast to what most fossil evidence at the time suggested, the authors predicted that the divergence of Tetrapoda occurred prior to the Late Devonian. As such, the discovery of the Zachelmie tracks simply confirmed their earlier findings. In addition, one phylogenetics paper using Bayesian credible and maximum-likelihood confidence intervals found tetrapod divergence spanned the Early Devonian and potentially extended into the very Late Silurian, a possibility briefly acknowledged by Niedzwiedski et al., (2010), the authors reporting on the Polish footprints (Friedman and Brazeau, 2010). The recent discovery of the fossil fish Tungsenia (409 MYA), now the oldest and most basal tetrapodomorph known, lends further support (Lu et al., 2012). This finding likewise pushed the divergence of early stem tetrapods back roughly 10 million years into Pragian of the mid-Early Devonian, and resulted in an Eifelian time frame for divergence of Tetrapoda; a later date than previous predictions but nevertheless close to the estimates given by Niedzwiedzki et al., (2010) and Friedman and Brazeau (2010). As it is, ghost lineages do not always remain as such, and new fossil material may effectively fill in the gaps over time.

While the dissenting viewpoint is all too eager to tout the trackway as proof against the theory or evolution as a whole, this standpoint foregoes the fact that the authors describing the Zachelmie prints cautioned against drawing hasty conclusions without a definitive body fossil, and that the implications of this finding on the timing of the appearance of tetrapods is suggestive, but nevertheless unresolved. Consideration must also be given to the general paucity of fossil record and other problematic factors such as the geographical sampling bias resulting from collection focused within more developed nations. These points taken together, the creationist’s incriminating position on the implications of the Polish trackways is duplicitous at best. As more relevant fossil specimens are uncovered, including trackways, other factors must be considered. A recent study by King et al., (2011) studying fin movements in the African lungfish (Subclass: Dipnoi, a separate but closely related lineage to Tetrapodamorpha (see figure: Glyptolepis is a Porolepiform and closely related to the Dipnoi)),  P. annectans, found that a tetrapod-like gait potentially originated in primitive and fully aquatic sarcopterygian fish prior to the split of Tetrapodomorpha, and as a result those fossil footprints and trackways lacking definitive digit impressions may in fact not belong to terrestrial tetrapods at all, but to primitive stem-tetrapods or related sarcopterygian fish. As per usual, more evidence is required, and new fossil discoveries will no doubt shed further light on the issue.

My hopes are that this post offers a brief but comprehensive review of the literature reporting on Tiktaalik and tetrapods as a whole, as well as describes clearly what a transitional is and isn’t, and what it means, and what it doesn’t.

END.

 

References

Ahlberg, P.E., Lukševičs, E., Mark-Kurik, E. 2003. A near-tetrapod from the Baltic Middle Devonian. Palaeontology 43 (3): 533–548.

Daeschler, E.B., Shubin, N.H., Jenkins, F.A. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440: 757-763.

Friedman, M., Brazeau, M.D. 2000. Sequences, Stratigraphy, and Scenarios: what can we say about the fossil record of early tetrapods? Proc Bio Sci B 278 (1704): 432-439.

King, H.M., Shubin N.H., Coates, M.I., Hale, M.E. 2011. Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes. Proc Natl Acad Sci USA 108(52): 21146–21151.

Lu, J., Zhu, M., Long, J. A., Zhao, W., Senden, T. J., Jia, L.,  Qiao, T. 2012. The earliest known stem-tetrapod from the Lower Devonian of China. Nature Communications

Niedzwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., Ahlberg, P.E. 2010. Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463(7277): 43-48.

Paleontology, Science

A Brief Structural and Developmental Comparison of Trilobites and Modern Arthropods

13 Comments

Hello all! I decided that it was about time to break into the World of Blog! I have a number of topics in mind for the future, which will focus on primarily on paleontology, geology, biology, zoology, etc. Many of you know very well that I tend towards the vertebrate paleontology side of things, however; I do love me some invertebrates! For those of you who weren’t aware, the username “Isotelus” is actually a genus of Asaphid trilobite, the main inspiration being Isotelus rex, the largest trilobite species currently known (My profile picture is as it is because the bird amuses me). In light of this astonishing revelation, my very first post will be a response to the myriad of creationist websites I have come across that seek to disprove evolution using trilobites as an example. As such, this post is not a direct response to any one particular article I’ve read, but a general statement on their overall position.

Trilobites represent one of the earliest arthropod groups in the fossil record, appearing in the lower Cambrian and persisting until the Permian-Triassic extinction event (520 MYA – 252 MYA). Comprising of roughly 20,000 species and 5000 genera, they were among the most successful group of arthropods to date. While an overall highly complex and sophisticated animal, a number of features identify trilobites as a basal member in relation to living relatives. Despite the often spectacular levels of preservation of trilobites (due primarily to their hard exoskeletons), the fact of their extinction can impose great difficulty in attempting to examine and interpret components of the body in relation to function or behaviour, especially when it involves microscopic structures, such as lenses of the eye or layers of the exoskeleton. Nevertheless, the trilobite eye, body plan and associated structures, as well as certain physiological processes including moulting and development, can be successfully compared with those of modern arthropods, which functions to demonstrate the set of primitive and derived characteristics that set the groups apart. Insects, crustaceans, and xiphosurans (horseshoe crabs) will be included here primarily, as most evaluations center on these groups in particular. This brief assessment also serves as a response to certain pseudoscience proponents who falsely claim the complexity of trilobites and modern arthropods is evidence against evolutionary processes. Advocates of creationism and intelligent design tend to focus only on the eyes of trilobite and neglect the rest of the animal, justifying the need to cover other aspects of the body and exoskeleton.

Eye Structure and Function

From personal experiences/encounters, the trilobite eye in particular is sometimes used by creationists to claim evolution has not occurred, primarily because they and modern representatives have functionally the same complex, perfected visual design. According to this view, trilobites were not primitive creatures, keeping in mind that “primitive” here is defined incorrectly. Taxonomically speaking, primitive or basal indicates a character that is closer to the ancestral condition, and relative degrees of complexity or superiority do not apply. In this context, the highly developed trilobite eyes were not necessarily inferior in their visual capacities relative to modern derived forms; however, there are many aspects of the overall structure that are nevertheless primitive in comparison to crown arthropods.

The majority of all arthropods have compound eyes that are comprised of a series of optical units called ommatidia, which themselves contain a variety of structures and function overall as photoreceptors. Trilobites are no exception, and are some of the earliest animals in the fossil record to show a more complicated version of the visual complex. While varying wildly in terms of general shape and placement, three types of eye are present in trilobites (see this site for a more detailed introduction to trilobite eyes, as well as some great pictures http://www.trilobites.info/eyes.htm). Holochroal eyes were the simplest form and by far the most prevalent type characterized by numerous closely packed lenses with a single outer corneal surface. This type is considered to be basal to the trilobite group and was likely associated with a benthic lifestyle (Clarkson et al., 2005). Schizochroal eyes occur only in one suborder and are distinguished from the holochroal type by a fewer number of lenses separated by sclera and each with an individual corneal surface. Both schizochroal and abathochroal eyes were derived from a holochroal ancestor and likely resulted from paedomorphosis (http://en.wikipedia.org/wiki/Neoteny), as fossils of juvenile holochroal species have been found with schizochroal and abathochroal eye types (The abathochroal eye appears only in a certain group for a short time, and so will not be dealt with here)(Thomas, 2005). Trilobite eyes are typically compared to those of horseshoe crabs, which retain a number of primitive traits, including potentially the most basic eye characteristics relative to other extant arthropods (Schoenemann and Clarkson, 2013). This comparison is not always well-supported, as schizochroal eyes are so unique that optical theories typical of many modern taxa cannot always be successfully applied (Fordyce and Cronan, 1993). However, detailed preservation of microscopic structures in a number of trilobite fossils has since reinstated xiphosurans as a useful structural analogue (Schoenemann and Clarkson, 2013). In terms of structural composition, the lenses of trilobite eyes were uniquely made of a single immobile calcite crystal formed in a biconvex shape to counteract aberration, rather than a proteinaceous crystalline cone typical of all insects and most crustacean groups (Nilsson and Kelber 2007). Additionally, trilobite ommatidia likely connected to a single photoreceptor, indicating both holochroal and schizochroal species had simple apposition eyes, which is considered primitive for Athropoda, and present in Limulus (horseshoe crab) and some members of other groups (Fordyce and Cronan, 1993; Nilsson and Kelber 2007; Schoenemann and Clarkson, 2013). Both trilobites and Limulus ommatidia also potentially show a star-shaped arrangement of tubular, light-collecting rhabdomeres associated with a modified sensory receptor (eccentric cell). The above similarities, as well as the similar size and number of ommatidial elements, suggest Limulus has inherited and maintained a comparatively primitive visual system (Schoenemann and Clarkson, 2013). The structure of the trilobite eye, while complex and highly functional, is clearly not as advanced as most modern arthropods.

Schizochroal eye typical of phacopid trilobites, from http://www.trilobites.info/eyes.htm

Body Plan

Trilobites were so named because of the three lobes consisting of the cephalon (head), thorax, and pygidium (tail) (see image below) (Hughes, 2003). This basic pattern of segmentation is also reflected transversely in the axial and pleural lobes of the main body. This particular configuration is highly conserved across the group, with high levels of variation occurring typically in ornamentation (e.g. spines), and relative size, number, and shape of the segments and associated structures, such as the mouthparts. Unlike the majority of arthropods today, such as crustaceans and insects that occupy a great diversity of habitats, trilobites were restricted to a marine setting and evidently never invaded fresh water or terrestrial environments. This constraint is reflected in the comparatively static trilobite bauplan, which never diverged greatly from the basic roughly ovoid shape or formed highly complex structures such as wings, or showed the higher degree of morphological divergence achieved by comparable aquatic crustaceans (lobsters, crabs, ostracods, barnacles, etc). The latter may seem counter-intuitive considering how speciose and widespread the trilobites were as a whole. One potential interpretation considers their success over a long time period in correlation with their conserved and relatively simplistic body plan, which evidentially allowed them to function perfectly well in a range of marine environments, such that any significant changes or divergences were not favoured by natural selection. Evidence from trace fossils, taphonomy, and general morphology suggests the majority of trilobite species were likely feeding on particulate matter in a seafloor environment, which may at least partially explain both the relatively unchanging body plan and holochroal eye morphology (Hughes, 2003; Clarkson et al., 2005).

Soft-tissue preservation and ventral morphology of Triarthrus, from http://www.fossilmuseum.net/trilobites/ptychopariida/Triarthrus/Triarthrus.htm;

In addition to the body segments themselves, trilobite legs were biramous, consisting of two separate branches on a single limb (Hughes, 2003). Only the antennae were uniramous, and this trait along with biramous appendages seems to be the most basal arrangement among arthropods according to studies in homology and development (Boxshall, 2004).The same arrangement is also typical of crustaceans, while insect limbs are uniramous and lack a second branch. However, unlike both insects and crustaceans, the limbs of trilobites occur in identical pairs that repeated sequentially on the head, tail, and each thoracic segment, with divergence in form appearing in the uniramous antennae, or between species as a result of diet. Morphologically diverse and modified appendages typical of many modern arthropods, such as those for prey capture and consumption, reproduction, swimming, etc., are entirely absent in all known trilobite species; even those with soft tissue preservation. Although it may correlate with the relatively inflexible trilobite bauplan in association with a benthic, particulate-feeding lifestyle as discussed previously, this explanation may not necessarily account for the invariable limb morphology across all trilobite groups, including those that are interpreted as having been planktic, pelagic, or burrowing, based on other specific aspects of their morphology (such as eye size and placement) (Hughes, 2003).

Exoskeletal Structure and Development

The trilobite exoskeleton was highly mineralized in comparison to modern arthropods, which tend to rely more on organic secretions (Miller and Clarkson, 1980). In addition, the calcitic exoskeleton was fairly typical of an arthropod cuticle, with structural similarities occurring variably with modern arthropods, but was composed of only two simple, distinct main layers, the exocuticle and endocuticle (Mutvei, 1981). This same study found that a lack of a thinning endocuticle and the pores and cavities near ducts used to transport/dissolve old calcitic and organic material for reuse indicated trilobites could not reabsorb their molted cuticle. Miller and Clarkson (1980) also showed that the calcium carbonate contained in the cuticle is shed in its entirety during ecdysis. In contrast, absorption of the old cuticle in modern arthropods potentially aids in speeding the hardening process after molting, and some groups will also consume the old cuticle for nutrients and/or later incorporation. Due to the high degree of mineralization of their cuticle, trilobites were not likely able to eat their exoskeleton given the strength of their mouthparts (Brandt, 2008). The inability of trilobites to reabsorb their exuviae (shed exoskeletons) potentially left them at a particular disadvantage during moulting, as regrowth of new calcium carbonate requires a substantial amount of energy and incurs a high metabolic cost. The soft stage directly following molting may have been protracted, making them particularly vulnerable to injury and/or predation directly following their moult.

Fossil evidence shows that trilobite development is hemimetabolous, with gradual change occurring over three stages from the egg, instar, and adult phases (Hughes, 2003; Thomas, 2005). This particular form of ontogeny is primitive among arthropods, with many modern insects being homometabolous. Insects and crustaceans tend to undergo metamorphosis and grow through a series of very distinct stages. Trilobite developmental patterns in contrast are less complex in that they grow by gradually adding thoracic and tail segments with each successive molt until they reach a certain point nearing adulthood at which they continue only to increase in size. The modification across instar stages was also comparatively slight and referred to as hemianamorphic, which is seen currently in primitive crustaceans and fossil xiphosurans, and as a result is considered to be a primitive trait (Hughes et al., 2006). This process, like the eyes and body segments, also remained unchanged over the length of trilobite evolution, possibly due to compromises of selection pressures acting on other aspects of the body (That’s the best explanation I’ve found thus far, as few studies tackle this particular question) (Brandt, 2006).

Concluding Remarks

From a scientific standpoint, comparisons between fossil and modern taxa serve as a means to potentially shed light on the functional significance of certain structures and morphology across various groups. This approach has an alternate function in that it confronts factual misinterpretations of creationists seeking to find faults in the theory of evolution by claiming it evidently did not occur in perfectly-designed fossil trilobites. Trilobite eyes were well-developed and similar in many aspects to modern arthropods, and they shared a number of archaic characteristics with modern horseshoe crabs, which are generally considered to be among the most basal extant arthropods. Conversely, the trilobite bauplan, exoskeleton, and limbs were comparatively basic in contrast to morphologically diverse crustaceans and insects, and remained relatively static over time, possibly due to habitat and lifestyle. Growth and development also seems to follow a primitive pattern both in types of stages and relative degrees of change. Trilobites evidently represent a highly successful group of primitive arthropods that maintained a basic platform over time relative to modern crown taxa.

End :). I highly recommend taking a look at this website: http://www.trilobites.info/, for everything trilobite. I also apologize if the eye section of this post is a tad confusing; it’s a rather complicated topic that deserves its own blog, which I plan on addressing in the future.

Also, special thanks to Prolescum for allowing me to write this blog in the first place, and he_who_is_nobody for helping with editing.

 

References

Boxshall, G. A. 2004. The evolution of arthropod limbs. Biological Reviews. 79 (2): 243-300.

Brandt, D. S. 2002. Ecdysial efficiency and evolutionary efficacy among marine arthropods. Alcheringa. 26 (): 399-421.

Fordyce, D., and Cronin, T. 1993. Trilobite Vision: A Comparison of Schizochroal and Holochroal Eyes with the Compound Eyes of Modern Arthropods. Paleobiology. 19 (3): 288-303.

Hughes, N.C. 2003. Trilobite tagmosis and body patterning from morphological and developmental perspectives. Integrative and Comparative Biology. 43 (1): 185–206.

Nilsson, D.E, Kelber, A. 2007. A functional analysis of compound eye evolution. Arthropod Structure & Development. 36 (4): 373-385.

Miller, J., and Clarkson, E.N.K. 1980. The Post-Ecdysial Development of the Cuticle and the Eye of the Devonian Trilobite Phacops rana milleri Stewart 1927. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 288 (1030): 461-480.

Mutvei, H. 1981. Exoskeletal structure in the Ordovician trilobite Flexicalymene. Lethaia 14 (3): 225-234.

Schoenemann B., Clarkson E.N. 2012. Discovery of some 400 million year-old sensory structures in the compound eyes of trilobites. Scientific Reports. 3 (1429).

Thomas, A.T. 2005. Developmental palaeobiology of trilobite eyes and its evolutionary significance. Earth-Science Reviews. 71, (1–2): 77-93.