July 2020 Frt page...PALEONTOGRAPH Volume 9 Issue 3 July 2020 Page 5 The most famous is the 2003...

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A newsletter for those interested in all aspects of PaleontologyVolume

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Edited by Tom Caggiano and distributed at no charge

From Your Editor

Welcome to our July edition.are, I've been staying home for the most parfossil workshop. I've been getting many large construction projects done with theextra time. I'm also continuing to work thru my backlog of fossils.

I'm lucky in that I've only lost one friend to the virus. It's weird tlucky to have lost a friend. This whole thing is scary. I don't know why it hasbecome so political. It's a virus. We are nearing 150,000 dead Americans. Whycan't we all just wear a mask for a few weeks? Even if that does not work, howmuch harm is done by beingfew weeks? What's the big deal?

I hope you enjoy the issue and stay healthy.

The Paleontograph was created in 2012 to continue what was originally the newsletterof The New Jersey Paleontological Society. The Paleontograph publishes articles, bookreviews, personal accounts, and anything else that relates toFeel free to submit both technical and nonrange of people interested in fossils. Articles about localities, specific types of fossils,fossil preparation, shows or events, museum displaywelcome.

This newsletter is meant to be onethere is enough content to fill an issue. I encourage all to submit contributions. It will beinteresting, informative and funcontributors want it to be, so it will be a work in progress. TC, January 2012

The Paleontograph________

A newsletter for those interested in all aspects of PaleontologyVolume 9 Issue 3 July, 2020

_________________________________________________________________


[email protected]

edition. I hope this issue finds you healthy and safe. As mostare, I've been staying home for the most part working around the house and myfossil workshop. I've been getting many large construction projects done with theextra time. I'm also continuing to work thru my backlog of fossils.

I'm lucky in that I've only lost one friend to the virus. It's weird to say that one islucky to have lost a friend. This whole thing is scary. I don't know why it hasbecome so political. It's a virus. We are nearing 150,000 dead Americans. Whycan't we all just wear a mask for a few weeks? Even if that does not work, how

uch harm is done by being a little uncomfortable, when being near othersWhat's the big deal?

I hope you enjoy the issue and stay healthy.

The Paleontograph was created in 2012 to continue what was originally the newsletterof The New Jersey Paleontological Society. The Paleontograph publishes articles, bookreviews, personal accounts, and anything else that relates to Paleontology and fossils.Feel free to submit both technical and non-technical work. We try to appeal to a widerange of people interested in fossils. Articles about localities, specific types of fossils,fossil preparation, shows or events, museum displays, field trips, websites are all

This newsletter is meant to be one, by and for the readers. Issues will come out whenthere is enough content to fill an issue. I encourage all to submit contributions. It will beinteresting, informative and fun to read. It can become whatever the readers andcontributors want it to be, so it will be a work in progress. TC, January 2012

________

A newsletter for those interested in all aspects of Paleontology

_________________________________________________________________


I hope this issue finds you healthy and safe. As mostt working around the house and my

fossil workshop. I've been getting many large construction projects done with theextra time. I'm also continuing to work thru my backlog of fossils.

o say that one islucky to have lost a friend. This whole thing is scary. I don't know why it hasbecome so political. It's a virus. We are nearing 150,000 dead Americans. Whycan't we all just wear a mask for a few weeks? Even if that does not work, how

when being near others, for a

The Paleontograph was created in 2012 to continue what was originally the newsletterof The New Jersey Paleontological Society. The Paleontograph publishes articles, book

Paleontology and fossils.technical work. We try to appeal to a wide

range of people interested in fossils. Articles about localities, specific types of fossils,s, field trips, websites are all

by and for the readers. Issues will come out whenthere is enough content to fill an issue. I encourage all to submit contributions. It will be

to read. It can become whatever the readers andcontributors want it to be, so it will be a work in progress. TC, January 2012

PALEONTOGRAPH Volume 9 Issue 3 July 2020 Page 2

Earliest Angiosperm Pollinatorin Amber

Bob Sheridan November 21, 2019

Angiosperms (flowering plants) originated sometimein the Early Cretaceous. Today, most angiospermpollen is transported by insects, and thepresumption is that this was always true. There aremany amber specimens from the same time periodcontaining insects, of which many show attachedpollen grains. However, the shape of the pollengrains suggests they are from gymnosperms orcycads. Unambiguous pollination of angiosperms byinsects from the Cretaceous is not seen until now.

Bao et al. (2019) describe a specimen of Burmeseamber with an age of ~99Myr. This specimen wasstudied with optical microscopy and also micro-CTscanning. The specimen contains a single beetlethat resembles a modern tumbling flowerbeetle. Tumbling flower beetles, (family Mordellidae)are named for their jumping, turning, and tumblingmotion when disturbed. Many have a backwardpointing abdominal spine that helps this motion.Many have a hump-backed shape as seen from theside, and a wedge-shape as seen from the top, withthe wide end at the head. Modern tumbling flowerbeetles are (not a surprise) found in flowers and arecommon pollinators. The specimen in amber, about0.5 centimeters long, is very round as seen from theside, and has very thick thighs. It is given the nameAngimordella burmitina (“Angiosperm Mordella-likebeetle from Burmese amber”).

The most interesting aspect of this specimen inamber is the pollen that is attached to it, and thereappears to be only one type of pollen. The pollengrains are 20-30 micrometers long and aredescribed as “tricolpate”, i.e. have three grooves oneach grain. Such pollen is associated with the

eudicot subclass of angiosperm. Tricolpate pollen isknown from the fossil records as old as ~125 Myr.,but this specimen represents the earliest knownassociation of angiosperm pollen and an insect.

Interestingly, pollen grains are often too hard to seein amber with regular optical microscopy, especiallyif it is caught in hairs on the insect. However, thisstudy uses confocal laser scanning microscopy.

Sources:

Bao, T.; Wang, B.; Li, J.; Dilcher, D.“Pollination of Cretaceous flowers.”Proc. Natl. Acad. Sci. USA 2019, 118, 24707-2474.

Stupendemys souzai,the Largest Turtle Ever

Bob Sheridan February 17, 2020

Among the very large animals from South America isStupendemys (“astonishing turtle”), which was firstdescribed in 1976 based on partial shells found inMiocene sediments from Venezuela and Columbia.Other shell specimens from large turtles wereassigned to the genus Caninemys and others putinto a loose category “Podoncnemidae indet.”Stupendemys is an example of a pleurodire, i.e. aturtle that withdraws its head by folding its necksideways. The shape of the shell is consistent withStupendemys being an fresh-water aquatic turtle.

Based on new specimens from the original localities,Cadena et al. (2020) give further context toStupendemys. The totality of specimens forStupendemys includes carapaces (the dorsal part ofthe shell), plastrons (the ventral part of the shell), afemur, a scapulocoracoid (part of the shouldergirdle), a cervical vertebrae and a lower jaw.

Cont'd


Big Turtle Cont'd

The largest carapace has a length of 2.4 meters,which would make Stupendemys larger than theprevious champion Archelon, a marine turtle fromthe Cretaceous with a carapace length of 2.2meters. (The largest living turtle has a shell 2.1meters long.) Stupendemys would be over metric tonin weight. The authors feel that other large turtlesfrom the same time and place are all specimens ofStupendemys. In particular, the skull from“Caninemys” probably belongs toStupendemys. Also, they feel the carapaces fall intotwo categories which either have or lack “horns”protruding forward from the front of the carapace oneach side. These the authors assign as “male” and“female”, and suggest the horns, which would becovered in keratin in life, are for combat. Other livingand extinct turtles have horns, but seldom in thefront of the carapace. However, other sexuallydimorphic characteristics in turtles, such as aconcavity in the plastron in males, is not seen inStupendemys.

Turtles have a toothless mandible with an up-curvingsharp beak. In some turtles, both upper and lowerjaws have “triturating” surface that contact eachother and act to crush food. In the case ofStupendemys the triturating surface in the mandibleis a deep groove. The anatomy of the jaw is notenough information to infer the diet of Stupendemys.The authors point out that many large turtles, somecurrently in the Amazon, eat fruit.

Phylogenetic analysis shows that Stupendemysrepresents a basal example of South Americanturtles, most of which are from the Miocene, but oneof which, Peltocephalus, is living. Peltocephalus isknown as the “big-headed Amazon river turtle,” andis not particularly large.

Stupendemys presumably lived in a large lake andriver environment called the Pembas system, whichexisted in the north of South America during theMiocene. Giant forms of rodents, snakes, andcrocodilians also inhabited that environment. Theauthors assign bite marks and an embedded toothon the carapace of one Stupendemys specimen asbeing from a 10-meter crocodilians like Purussaurus(a caiman) or Gryptosuchus (a gharial). It is not clearwhether such gigantism in Miocene Sourth Americais a result of high temperatures or an arms racebetween predator and prey.

Sources:

Cadena, E.-A.; Scheyer, T.M.; Carrillo-Briceno, J.D.;Sanchez, R.; Aguilera-Socorro, O.A.; Vanegas, A.;Pardo M.; Hansen, D.M.; Sanchez-Villagra, M.R.“The anatomy, paleobiology, and evolutionaryrelationships of the largest extinct side-neckedturtle.”Science Advances 2020, 6, eaay459.


Oculudentavis—A Very TinyCretaceous BirdBob Sheridan March 14, 2020

There is a flood of papers in the literature aboutinclusions in Burmese amber, which is from theMiddle Cretaceous (~99 Myr.). Whereas amberinclusions have been studied with opticalmicroscopy, nowadays, micro-CT scanning isused. Although rare, one type of inclusion inBurmese amber, hardly seen anywhere else, is ofbirds and bird parts.

Xing et al. (2020) describe a new specimen HPG-15-3 which contains an isolated bird skull. The pieceof amber is 32 X 20 X 9 millimeters, and the skullitself less than 10 millimeters long. The authors givethis specimen the name Oculudentavis khaungraae(“eye-tooth-bird” and after Khuang Ra who donatedthe specimen). This is by far the smallest fossil birdskull known, and it is smaller than that of the beehummingbird, which is the smallest modern bird.This is significant if Oculudentavis represents anadult bird; the fused state of some of the skull bonessuggest this is true.

Oculudentavis has a slender beak and very largeeye sockets relative to the length of the skull. Thesesockets point clearly to the side, so thatOculudentavis had no stereo vision. Smaller birdstend to have large eye sockets relative to the lengthof the skull, and Oculudentavis is the extreme caseof a well-established trend. In birds and reptiles,scleral ring bones give rigidity to the eyes. InOculudentavis, the individual scleral bones makingup the ring are spoon-shaped instead of flat plates.Also, the scleral ring is quite wide in a radialdirection, implying the maximum pupil size is small,which might also imply that Oculudentavis was

active during the day. Oculudentavis has tinyconical teeth, which is not unusual for a Cretaceousbird. However, while the upper teeth of mostarchosaurs (including dinosaurs) do not extendfurther back than the front of the eye socket, theteeth of Oculudentavis extend back about one-thirdof the diameter of the eye socket. Also the teethseemed to be fused with the jaw rather than insockets, as with other archosaurs.

The skull of Oculudentavis seems to be extensivelyfused, much more like that of a dinosaur than that ofan early bird. Phylogenetic analysis places it amongthe most primitive birds, somewhere betweenArchaeopteryx and Jeholornis.

Modern birds cover a large size range, with the beehummingbird being the smallest (2 grams) and thethe ostrich (~100 kilograms) being the largest. MostCretaceous birds we know about are sparrow- toturkey-sized (10 grams to 10 kilograms), butOculudentavis gives us evidence that Cretaceousbirds could be very small also. We need to be awareof preservation biases. Only animals near the trunkof conifer trees and too weak to escape will betrapped by amber, so of course we would see onlyvery young or very small birds as amber inclusions.In contrast, we know most other Cretaceous birdsfrom limestone deposits in lakes, where very smallbirds are likely to be crushed during fossilization.

Sources:

Benson, R.B.J.“Tiny fossil sheds light on miniaturization of birds.”Nature 2020, 579, 199-200.

Xing, L.; O’Connoer, J.K.; Schmitz, L.; Chiappe,L.M.; McKellar, R.C.; Yi, Q.; Li, G.“Hummingbird-sized dinosaur from the Cretaceousperiod of Myanmar.”Nature 2020, 579, 245-250.

Oculudentavis—Not a Bird?Bob Sheridan March 29, 2020

When you follow paleontology as a hobby, every fewyears you come across an “identity crisis”. That is,one group of investigators interprets a fossil asbeing an unusual version of X. Another group looksat the same fossil and says the fossil is Y, where Ydoes not equal X. Off the top of my head I can thinkof several examples.Cont'd


The most famous is the 2003 dispute about Homofluorensiensis as either a primitive, but surprisinglyrecent, hominin or a modern human with a rarepathology (microcephaly).

This difficulty with identity is due to two things. First,fossils are usually incomplete and there may not beenough information to decide identityunambiguously. Second, the living world is messyand unexpected exceptions are not unheard of.

Just two weeks ago, I wrote up the story about thepaper by Xing et al. (2020) in Nature aboutOculudentavis. This new genus is described froman isolated skull found in Burmese amber (~99 Myr).The original data was generated by CT-scanningand exists as a 3D virtual model. The authorsidentified Oculudentavis as a primitive bird, but onethat has at least a few unusual characteristics:

1. It is smaller than the smallest known bird,fossil or living.

2. It has a tooth row that extends under the eyesocket. This has never before been seen inarchosaurs (the reptile group that containsdinosaurs, birds, pterosaurs, and moderncrocodilians).

3. The teeth are attached to the bone insteadof being in sockets.

4. The bones of the scleral ring have anunusual shape.

However, despite these anomalies, phylogeneticanalysis places Oculudentavis among the mostprimitive birds.

Tom forwarded me a short preprint from Li et al.,who reexamined the original CT-scan ofOculudentavis. A “preprint” is a version of amanuscript that has not yet undergone peer review,i.e. has not been vetted by other scientists beforeformal publication in a journal. Nowadays, eachscientific field has its own “preprint server” to whichanyone can post a paper. On the one hand, thisfacilitates timely exchange of data betweenscientists. On the other hand, the amount of junkthat gets through is much higher than articlespublished in journals.

In the opinion of Li et al., Oculudentavis is a not abird, but a lizard, or at least some type of squamate(the group of reptiles that includes lizards andsnakes). Some points:

1. The attached teeth are characteristic ofsquamates, as are teeth that extend furtherback as the front of the eye socket.

2. Oculudentavis does not have an antorbitalfenestra like archosaurs, but it does have alower temporal fenestra, which occurs insquamates.

3. Oculudentavis has a few teeth on its palate,again characteristics of squamates.

Against this, I have to note that Oculudentavis has avery long and narrow snout, something you don’toften see in lizards, but do often see in birds.

Despite this being only a preprint, I think Li et al.’sview will carry the day. It is obviously easier tobelieve Oculudentavis is a slightly unusual lizardthan a very weird bird. However, you never reallyknow what will happen. For example, upon analysisof postcranial material, Homo fluorensiensis turnedout to be more like primitive hominins than expected,i.e. the “weirder” interpretation won the day.

Some comments:1. It is very good in this case that the original

data was made available to other scientists.2. Interpretation of anatomy is tricky, especially

if bones are missing or crushed, or variationin anatomy is found. In particular, in regardto fenestra (holes in the skull), modern birdshave very open and light skulls, and havelost some bones that demarcate thefenestrae.

3. Phylogenetic analysis is only as good as theorganisms the original specimen iscompared to and what characteristics areselected. Li et al. suggest thatOculudentavis would nest within thesquamates if it were compared withsquamates instead of dinosaurs and birds.

4. Dealing only with cranial anatomy is tricky.Certainly, if more than the skull had beenavailable, it would have been obviouswhether Oculudentavis was a bird or alizard.

Cont'd


Bird? Cont'd5. By nomenclatural rules, we are stuck with thename “Oculudentavis” for this animal even

though the “avis” part is probably false. The mostfamous “misleading name” is “Basilosaurus”, whichwas named “king lizard” even though it later turnedout to be a primitive whale.Sources:

Xing, L.; O’Connoer, J.K.; Schmitz, L.; Chiappe,L.M.; McKellar, R.C.; Yi, Q.; Li, G.“Hummingbird-sized dinosaur from the Cretaceousperiod of Myanmar.”Nature 2020, 579, 245-250.

Li, Z.; Wang, W.; Hu, H.; Wang, M.; Yi, H.; Lu, J.

“Is Oculudentavis a bird or even archosaur?”

bioRxiv preprintdoi: https://doi.org/10.1101/2020.03.16.993949.

The Inner Ear of PelagicCrocodylomorphs

Bob Sheridan May 18, 2020

This story concerns the inner ear of vertebrates, andterminology would be helpful at this point. There aretwo parts to the inner ear: the cochlea (whichtranslates the vibration of the eardrum into asensation of sound), and the vestibule, a sack-likepocket with the semi-circular canals (threeperpendicular loops) at the top. The vestibuledetects the orientation of the head relative to gravityand detects acceleration. Fortunately forpaleontologists, although inner ears are small, theyare hollow spaces surrounded by a bony sheathcalled the “labyrinth”, and so their shape, size, andorientation relative to the rest of the skull can bediscerned by CT-scanning skulls, both of living andfossil animals. One application to fossil animals isdue to the expectation that the lateral semi-circularcanal should be horizontal; therefore one can guessthe habitual orientation of the heads.

It is well-accepted that the inner ears of mammalsthat are obligatory swimmers are quite different fromthose of their land-dwelling ancestors. In particularthe inner ears of whales, manatees and seals are upto three times smaller relative to the size of the skull.One thought is that large accelerations though thewater would overwhelm the senses of these aquaticanimals if they had inner ears the size of land-dwelling animals. However, so far this explanation ishard to prove.

Schwab et al. (2020) test the idea that the inner earchanges due to a transition to an aquatic lifestyle,this time in a line of crocodylomorphs calledthalattosuchians. These lived from the Early Jurassicinto the Early Cretaceous and had a world-widedistribution. Some of these appear fully pelagic (i.e.they swam in open ocean and never walked onland); they are often restored as looking like slenderichthyosaurs. Some thalattosuchians aresemiaquatic, and resemble living crocodylians likegharials. There is a related group, supposedlyancestral to thalattosuchians that appear to be land-dwelling; they have long weight-bearing legs. Thus,it appears that some thalattosuchians made alifestyle transition to marine environments, much likewhales did. Modern crocodilians can also becompared to these fossil animals.

The authors produced CT-scans of the labyrinths ofa few dozen specimens of fossil and livingcrocodylomorphs and analyzed their shapes usingprincipal component analysis. Crocodylomorphs withthe same lifestyle seem to clustered in this labyrinthshape-space, and well-separated from the otherlifestyles. One can also see the difference by eye.The pelagic labyrinths appear shorter from top tobottom and have thicker semi-circular canals, whilethe terrestrial labyrinths appear taller and havethinner semi-circular canals. The semi-aquaticlabyrinths are somewhere between. Size, apart fromshape, does not help predict lifestyle.

Thus, thalattosuchians appear to show modificationsof the inner ear associated with a change to apelagic lifestyle, but these are different from themodifications seen in whales and other cetaceans,where only the size changed and not the shape. Itshould also be noted that thalattosuchians made thetransition over a period of tens of millions of years,whereas cetaceans made the transition in only a fewmillion years. Whatever the goal of changing theinner ear to adapt to a pelagic lifestyle, there are atleast two different ways of achieving it.

Sources:Schwab, J.A; Young, M.T.; Neenan, J.M.; Walsh,S.A.; Witwer, L.M.; Herrera, Y.; Allain, R.; Brochu,C.A.; Choineiere, J.N.; Clark, J.M.; Dollman, K.N.;Etches, S.; Fritsch, G.; Gignac, P.M.; Reubenstahl,A.; Sachs, S.; Turner, A.H.; Vignaud, P.; Wilberg,E.W.; Xu, X.; Zanno, L.E.; Brusatte, S.L.“Inner ear sensory system changes as extinctcrocodylomorphs transitioned from land to water.”Proc. Natl. Acad. Sci. 2020, 117, 10422-10428.


Turtles as Hopeful Monsters—A ReviewBob Sheridan May 17, 2020

Turtles are very unique among living and fossilreptiles. First, they always have a toothless beak.Second, they are covered in a bony box, made of a“carapace” above and a “plastron” below, with thetwo fused at several points. The shoulder blades ofturtles are inside rather than outside their ribs, whichis totally unlike any other tetrapod. Most modernturtles can withdraw their heads, and sometimestheir limbs, inside the shell. Turtles also tend to havevery short tails for reptiles. As with many interestingfossil groups, modern-looking turtles seem to appearvery suddenly in the fossil record, in this case induring the Triassic. Trying to figure out which type ofreptile is ancestral to turtles based on anatomy isvery difficult because there are not manyintermediate forms. At one time, it was thought thatturtles were descended from very primitive reptilesbased on the fact that their skulls have no openings.However, molecular evidence from living turtlessuggests they are most closely related to advanceddiapsid reptiles like lizards, and perhaps even morespecialized diapsid reptiles like archosaurs.

As an amateur paleontologist, I have been followingthe turtle story in the scientific literature for a whilenow. Every so often, a new putative turtle ancestoris described, often with a great deal of controversy.When I saw that there was a book “Turtles AsHopeful Monsters”, from a few years ago, Iendeavored to read it. This book is in the “Life of thePast” series edited by James O. Farlow. With a fewexceptions, but the books in this series areexcellent.

The author Olivier Rieppel is the Rowe FamilyCurator of Evolutionary Biology at the Chicago FieldMuseum, specializing in reptiles. He has published afew hundred technical articles, many of which are onturtles.

Chapters are:1. Misplaced turtles.

This chapter summarizes the author’s early career,including his embrace of cladistic analysis, which isaccepted now, but was controversial in the 1970s.Cladistic analysis places turtles close to crownreptiles and not the most primitive reptiles, contraryto traditional thought, but consistent with molecularevidence. Hence turtles were originally “misplaced.”

2. Reptile Classification and Evolution

This chapter summarizes the history of thought ofhow living and fossil reptiles are related to eachother. There is an especially interesting story of howturtles were associated with primitive reptiles likepareiasaurs or placodonts, which had no postorbitalskull openings and in some cases were heavilyarmored. Especially interesting is a discussion abouthow anatomical comparisons are at least a littlesubjective.

3. Levels of EvolutionThis summarizes the history of evolutionary thoughtas applied to the study of fossils, includingdiscussions about whether Darwin’s mechanism ofnatural selection was sufficient, whether there is“orthogenesis”, and whether “ontogeny” reallyreflects “phylogeny.”

4. Hopeful MonstersRichard Goldschmidt (1878-1958) first used thephrase “hopeful monster” to the idea that evolutioncould sometimes proceed by “macromutations”, i.e.an animal could be born that was very different fromits parents. This sort of mechanism is not acceptedby modern evolutionary theory. However, we stillhave no mechanism to explain how large changes inanatomy occur in seemingly very short periods ofgeological time. The appearance of jaws is a verygood example. Obviously, the appearance of turtlesis another.

5. The Turtle ShellHow much of the turtle shell is formed from thebones of the internal vertebrate skeleton, and howmuch from bony plates (osteoderms) in the skin?This has been debated for many decades; the mostusual way of addressing the question is to examineturtle embryos. However, different types of turtlesmay have different developmental mechanisms.There is a “polka dot turtle” hypothesis that turtleancestors might be covered by separateosteoderms.

6. Fossil Hunting in ChinaThis chapter discusses Triassic reptile fossils fromsouthwestern China, some of which might beputative turtle ancestors. Sinosaurosphargis andOdontochelys are examples. Sinosaurosphargis iscovered with dorsal osteoderms (a proto carapace)and has broad ribs that touch each other (not quiteforming a plastron). Odontochelys has a fullyossified plastron, but no dorsal armor whatever. Arethey both turtle ancestors, or is one or bothconvergent on a turtle-like plan? Interestingly bothseem to had skull openings consistent with beingdiapsids.

Cont'd


Turtle Monsters Cont'd

Overall, I am somewhat disappointed with this book.While there is some nice discussion of evolutionarymechanisms and the history of evolutionary theory,there is very little specific about turtles until Chapter5. Fossils turtles are not treated in any detail untilChapter 6. Compared to most “Life of the Past”books, there are very few illustrations, only a few perchapter. This is a shame because some conceptsare easier to understand through diagrams thanthrough words.

Sources:

Rieppel, O.

“Turtles as Hopeful Monsters. Origins and Evolution”2017, Indiana Press, Bloomington and Indianapolis,206 pages, $45 (hardcover)

The Tail of SpinosaurusBob Sheridan May 8, 2020

Spinosaurus was a very large theropod with a largedorsal sail that lived in the Middle Cretaceous ofAfrica. There are a number of lines of evidence thatSpinosaurus was aquatic, very unusual for adinosaur. Presumably the evidence of an aquaticlifestyle also applies to related theropods(Suchomimus and Baryonyx), which are smaller andlack the sail.1. Spinosaurus has a long snout with conical teeth,and there is a notch near the tip the maxilla,somewhat like the snout of a crocodile2. The oxygen isotope ratio in its bones areconsistent with a semi-aquatic diet.3. Its bones tend to be denser than that of othertheropods.4. Based on a new specimen described in 2014, anda composite skeletal reconstruction from severalspecimens (Ibrahim et al., 2014), Spinosaurus had avery short legs. This makes the bipedal lifestyle ofmost theropods very unlikely for Spinosaurus;instead a swimming lifestyle makes much moresense.

Ibrahim et al. (2020) describe the tail ofSpinosaurus. The caudal vertebrae are from thesame quarry in Morocco from which a number ofSpinosaurus bones have been excavated for anumber of years. Also useful in the description arecasts of caudal vertebrae from the holotypespecimen discovered by Ernst Stromer in 1912, theoriginals of which were destroyed in World War II.

The next bit will require some nomenclatureconcerning the anatomy of vertebra. The centralcylinders of dense bone that stack up to form thespinal column are called “centra.” There is a bridge-like “neural arch” that is attached to the dorsalsurface of each centrum. The spinal cord runsthrough the tunnel formed by all the neural arches. A“neural spine” may stick up dorsally from eachneural arch. The “sail” of Spinosaurus is formed byvery tall neural spines on the thoracic vertebrae.“Transverse processes” stick out sideways from theventral part of the neural arch. These are mostly forthe attachment of muscles and ligaments. There arealso “pre-“ and “post-zygapophyses” at the front andrear of the dorsal part of the neural arch. These formjoints between adjacent vertebrae in addition to thejoints (cushioned by vertebral disks) between thecentra. In the caudal vertebrae of reptiles,“chevrons” are attached to the ventral surface of thecentra by joints and point downward.

The neural spines on the caudal vertebrae ofSpinosaurus are tall, making the tail very deep fromtop to bottom. Chevrons are not as long, but alsocontribute significantly to the depth of the tail.

Cont'd


Whereas in most dinosaur tails, the neural spinesand chevrons are wide front-to-back and narrowside-to-side, in Spinosaurus the opposite is true. Inmost dinosaur tails, the zygapophyses interlock withthe zygapophyses of adjacent vertebrae; this makesthe tail rigid. In contrast, in Spinosaurus, thetransverse zygapophyses are reduced, leaving thetail flexible from side-to-side.

The top-to-bottom depth of the Spinosaurus tailsuggests to the authors that it would be specializedfor propulsion in swimming. To test this, theyconstructed an artificial Spinosaurus tail from flatplastic and moved it through water by moving thebase side-to-side. They measured the thrust of thetail (force propelling forward), and the powerefficiency. They compared this to artificial tails of twomodern aquatic animals: the crested newt and acrocodile. They also compared two tails of non-aquatic dinosaurs: Coelophysis and Allosaurus. Allartificial tails were scaled to have the same surfacearea. The artificial Spinosaurus tail is clearly betterthan the other dinosaur tails in thrust, but not asgood as the crocodile and newt. This is enough toconvince the authors that propulsion duringswimming is at least a plausible use for the tail ofSpinosaurus, and therefore a swimming lifestyleseems more probable. In my opinion, the artificial tailexperiment is a drastic simplification, in that theauthors are assuming that the shape of the tail asseen from the side is the only thing that is important.

The artificial tails had a very small and uniformthickness (1 millimeter) and the rigidity of theartificial tails were determined by the plastic fromwhich they were made, so it is not clear how thiswould apply to real animals with three-dimensionaltails, where the rigidity of the tail is determined bymuscles and bone.

Also, it occurs to me that the authors did not includea relevant negative control. Long ago it was noticedthat hadrosaurs had deep tails from top to bottom,and this was once taken as evidence thathadrosaurs were aquatic, something we now knowis not true.Sources:

Ibrahim, N.; Sereno, P.C.; Dal Sasso, C.; Maganuco,S.; Fabbri, M.; Martill, D.M.; Zouhri, S.; Myhrvold, N.;Iurino, D.A."Semiaquatic adaptations in a giant predatorydinosaur."Science 2014, 345, 1613-1616.

Ibrahim, N; Maganuco, S.; Dal Sasso, C.; Fabbri, M.;Audiore, M.; Bindellini, G.; Martill, D.M.; Zouhri, S.;Mattarelli, D.A.; Unwin, D.M.; Wiemann, J.;Bonadonna, D.; Amane, A.; Jakubczak, J.; Joger,U.; Lauder, G.V.; Pierce, S.E.“Tail-propelled aquatic locomotion in a theropoddinosaur.”Nature 2020, 581, 67-70.

PALEONTOGRAPH

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