STL Science Center

STL Science Center

07 October 2020

Synonyms Everywhere

Hadrosaurs, the so-called "Duck-billed" dinosaurs have been compared to cows on a regular basis. I think the main idea with this comparison is in the idea that there were likely vast herds of these dinosaurs stretching as far as the eye could see in different regions of the world during the Cretaceous. One of those dinosaurs was Edmontosaurus. Possibly not the most exciting in appearance, we know that Edmontosaurus annectens was a well distributed genus across North America, consists of two recognized species (E. regalis, Type species, Lambe 1917 and E. annectens, Marsh 1892) has a number of synonyms associated with it including Anatotitan, Anatosaurus, and Trachodon. Appearing as a "typical" hadrosaur (probably only outshined by the familiar appearance of Iguanodon in this respect), Edmontosaurus is a large herbivore and some might remark that it is quite unremarkable. The type specimen is estimated to have been between 9 and 12 m (30 and 39 ft) long and may have weighed as much as 4 tons. A handful of both species have even been discovered as fossilized mummies or have been associated with skin impressions and even a rhamphotheca, the keratinous portion of the beak-like end of the mouth and snout.

Reconstruction by Nobu Tamura

01 October 2020

A Notable Find

Cryolophosaurus, as a holotype fossil, consists of a partial skeleton that was discovered with a complete skull (cranium and face plus lower jaw and teeth). As far as paleontology goes that is somewhat rare, but it does happen from time to time and the dinosaurs (or other fossil animals) we find as mostly whole are often bewildering and exciting. Obviously, I think, Cryolophosaurus was a very exciting find for a number of reasons: the state of preservation, as just noted; that it was discovered in Antarctica; and the always talked about head crest adorning its cranium. The head of Cryolophosaurus is amazing all by itself though, even without the crest. Approximately 65 cm (26 in) long, Cryolophosaurus' head was a little less than 1/3 the length of the largest Tyrannosaurus skull (1.52 m; 5 ft). The skull was also considerably narrow and tall, from top to bottom, as well. Running from the nasal area to the side-to-side crest is a ridge that runs courses along the midline until it widens and meets with the perpendicular crest that is so iconic of the dinosaur. The purpose of the crest has been discussed many different times in many different ways. It has been considered a fighting implement by some and a social ornamentation by others. Many have hypothesized that the crest was used for mating displays. Much consideration has been given to other characteristics of Cryolophosaurus as well. The crest as well as a large group of other characteristics have led to many different phylogenetic hypotheses for Cryolophosaurus. Rather than revisiting all of these, we can note that currently Cryolophosaurus appears to be nestled in a spot in the family tree that is a little more advanced than Dilophosaurs and Ceratosaurs, but more primitive than Allosaurs and Coelurosaurs (the group that includes Tyrannosaurus as well as ornithomimids and maniraptorans).

Photo by Jonathon Chen while Cryolophosaurus (reconstruction) was on display at the Field Museum, Chicago


Chan-gyu, Yun. (2019). "An enigmatic theropod Cryolophosaurus: Reviews and Comments on its paleobiology". Volumina Jurassica17: 1–8.

Hammer, W.R.; Hickerson, W.J. (1999). Tomida, Y.; Rich, T.H.; Vickers-Rich, Y. (eds.). "Gondwana Dinosaurs from the Jurassic of Antarctica". Proceedings of the Second Gondwana Dinosaur Symposium National Science Museum Monographs. 15: 211–217.

Hendrickx, C.; Hartman, S.A.; Mateus, O. (2015). "An Overview of Non- Avian Theropod Discoveries and Classification". PalArch's Journal of Vertebrate Palaeontology12 (1): 1–73.

28 September 2020

A Short Discussion

 Enjoy this video today from the Natural History Museum of Los Angeles County. This is a short tour of the Cryolophosaurus reconstruction that was at the museum late last year. Dr. Nate Smith talks about our friendly crested theropod. If you need more fun videos, check this out too (you know you want to).

27 September 2020

Antarctic Dinosaurs

©Dmitry Bogdanov
Over the years this site has covered a lot of different dinosaurs and fossil animals from a lot of different areas, regions, and continents (and of course some animals that were living between continents at one point or another). I have been digging back in the oldest of entries lately to try to re-cover some dinosaurs and fossil animals that we may not have discussed. Interspersed with new animals, I think this is useful for everyone interested in reading, and it lets us all revisit some old friends. One of the past favorites is a large theropod dinosaur from Antarctica with a rather interesting bony headpiece: Cryolophosaurus ellioti (Hammer and Hickerson 1994).

Cryolophosaurus roughly translates to "cold/frozen crest lizard" and for good reason. Being found in Antarctica explains the first part of the name, but the second part of this genus is attributed to the large crest on the top and back of this animal's head. An estimated size of 6.5 m (21.3 ft) and 465 kg (1,025 lb) make Cryolophosaurus the largest theropod of the Early Jurassic. It has been noted that the known remains are not even those of an adult, which means that these size estimates probably are not for a full grown animal. Molina-Pérez and Larramendi (2016) presented estimates of a fully grown adult at approximately 7.7 m (25.3 ft) and 780 kg (1,720 lbs). Regardless of its size, what people notice first is the crest. The dinosaur has been given the nickname "Elvisaurus" (I very much dislike Elvis, but to each their own) because the crest is oriented from one side to the other, making it appear, apparently, like Elvis' pompadour hairstyle. You be the judge on this one.


Hammer, W. R., and Hickerson, W. J., (1994). A Crested Theropod Dinosaur from Antarctica. Science. 264 (5160): 828–830. doi:10.1126/science.264.5160.828

Molina-Pérez, R. and Larramendi, A. (2016). Récords y curiosidades de los dinosaurios Terópodos y otros dinosauromorfos. Barcelona, Spain: Larousse. p. 254.

Smith, N. D.; Makovicky, P. J.; Hammer, W. R.; Currie, P. J. (2007). Osteology of Cryolophosaurus ellioti (Dinosauria: Theropoda) from the Early Jurassic of Antarctica and implications for early theropod evolution. Zoological Journal of the Linnean Society. 151 (2): 377–421. doi:10.1111/j.1096-3642.2007.00325.x

24 September 2020

Life at the End

It is not too often that we are able to say that we have a fossil that comes from an area of the fossil record that basically butts up against the K-Pg boundary (Cretaceous - Paleogene Boundary; the site in time of the extinction event that claimed the non-avian dinosaurs). However, Alamosaurus, is one of those animals that appears to have been one of the very last of the non-avian dinosaurs that roamed the earth. Known largely from the Ojo Alamo Formation (as previously noted) and the Javelina Formation, many of the fossil remains of Alamosaurus are between 69 and 66.5 million years old. There is at least one juvenile skeleton known from the Black Peaks Formation, an area of the fossil record that envelops (contains) the K-Pg boundary. We can infer from this that Alamosaurus was alive (and well, probably) near, if not right up to, the extinction of the non-avian dinosaurs.  

In its fossil range, Alamosaurus is one of the most commonly found fossils, along with Quetzalcoatlus, the enormous pterosaur. It has been hypothesized that Alamosaurus represents a reintroduction of enormous sauropod dinosaurs to North America. Opposing hypotheses point to ancestral sauropods originating further inland in western North America (and ultimately from Asia) and that Alamosaurus was essentially endemic (restricted in range to a certain area) to the North American inland plains. Lucas et al. (2016) noted that the dispersal of Alamosaurus fossil across the western inland plains provides evidence that Alamosaurus was actually quite mobile and not endemic, having a large demonstrated range. Their review did concede that Alamosaurus has only been found within inland plain environments and that no fossils of Alamosaurus have yet been found that would indicate that the animal made its way to the coastline or coastal plains of the Western Interior Seaway.

©Dmitry Bogdanov


Lehman, T. M., (2001), Late Cretaceous dinosaur provinciality: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 310–328.

Lucas, S., Sullivan, R, Lichtig, A, Dalman, S., and Jasinski, S. (2016). Late Cretaceous dinosaur biogeography and endemism in the Western Interior basin, North America: A critical re-evaluation. New Mexico Museum of Natural History and Science Bulletin. 71. 195-213. 

Sullivan, R.M., and Lucas, S.G. (2006). "The Kirtlandian land-vertebrate "age" – faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America." New Mexico Museum of Natural History and Science, Bulletin 35:7-29.

21 September 2020

The Alamo

Reconstructed and restored skeletons of Tyrannosaurus rex harassing a Alamosaurus sanjuanensis. CC BY 2.0
Reconstructed and restored skeletons of
Tyrannosaurus rex harassing an
Alamosaurus sanjuanensis. CC BY 2.0
Alamosaurus sanjuanensis is estimated to have grown to approximately 30 m (98 ft) long and to have weighed up to 80 tons. That is a just a little more than 4 fully loaded school busses (we can call it 4 and a half just to be safe). If one were curious about length, that is about 10.6 m (8 ft) longer than two school busses; we can make that 2.2 school busses in length. Living immediately prior to the K-Pg (Cretaceous - Paleogene) boundary, notable because of the extinction event that eradicated the non-avian dinosaurs, Alamosaurus was one of the last of the sauropods and, more generically, the non-avian dinosaurs to have evolved and to have lived. Remains from multiple specimens represent differently aged animals. This means that we have, we think, a good representation of this dinosaur species across a large portion of its lifespan from juvenile to adult (also known as ontogeny). Named in 1922 by Charles Whitney Gilmore, a paleontologist at the United States National Museum, now the National Museum of Natural History (commonly referred to at large as "The Smithsonian"), Alamosaurus was originally discovered in New Mexico during 1921 in the Ojo Alamo Formation and not, as the name immediately may appear to suggest, in Texas. It was named by Gilmore for the formation, not for the more widely famous Alamo in San Antonio, TX. The specific name refers to the New Mexico county (San Juan County) where the holotype remains were originally discovered. Remains have been found, since that time, all across the southwestern United States of America and are housed in different institutions from California to Washington D.C. When considering its estimated size, we can say that Alamosaurus is a titanosaur (a group of large sauropod dinosaurs) that is nearly the same weight as the titanosaurs Argentinasaurus and Puertasaurus; both are large sauropods known mainly from Argentina.

17 September 2020

Pack or Loner?

 It is unclear exactly how most dinosaurs hunted prey. We have hypotheses, some more well supported and considered than others, but unfortunately we do not have any trail cameras or candid videos documenting behaviors of dinosaur hunting. As many of us may have heard, at least once in our lifetime, many dromaeosaurs are thought to have been pack hunters; tracking, ambushing, and taking down prey that was far larger than a single animal to feed a large group of predators. Not all of the species within this group of animals is thought to have exhibited this complex social system. Often, wolves are used in analogy to describe these pack hunting behaviors, and for good reason. Utahraptor, as the largest of the dromaeosaurs, is one of those dinosaurs thought to have hunted this way and is often compared to wolves. Recently Frederickson et al. (2020) analyzed the teeth of Deinonychus and concluded that pack hunting was not likely to have been a characteristic social interaction in these smaller dromaeosaurs, but no such study has been conducted with Utahraptor. For the moment at least the idea of Utahraptor as a pack hunter still exists. This behavior was central to the theme of the novel discussed the other day. Additionally, this behavior is hinted at by a number of studies of Utahraptor including one that looks at a hypothesized quicksand death trap that may have enveloped an entire pack including juvenile animals (Kirkland et al. 2016). It is possible that, in addition to a complex social system wherein pack mentalities developed, that Utahraptor may have been simply too large to chase fast prey and too small to take on large prey alone. The describing paper, Kirkland et al. (2001), concluded that Utahraptor was likely slower than its cousins Velociraptor and Deinonychus

Rather than post an image directly in here today, I refer you to Julius Csotonyi's interpretation of the quicksand event that may have led to the discovery of a pack of fossilized Utahraptors as described in Kirkland et al. (2016).

Literature Cited

Frederickson, J. A.; Engel, M. H.; Cifelli, R. L. (2020). "Ontogenetic dietary shifts in Deinonychus antirrhopus (Theropoda; Dromaeosauridae): Insights into the ecology and social behavior of raptorial dinosaurs through stable isotope analysis". Palaeogeography, Palaeoclimatology, Palaeoecology: 109780. doi:10.1016/j.palaeo.2020.109780

Kirkland, J.I.; Simpson, E.L.; DeBlieux, D.D.; Madsen, S.K.; Bogner, E.; Tibert, N.E. (2016). "Depositional constraints on the Lower Cretaceous stikes quarry dinosaur site: Upper yellow cat member, cedar mountain formation, Utah". PALAIOS31 (9): 421–439. doi:10.2110/palo.2016.041

15 September 2020

Popular Raptor

Proof that the author did once like a lot
of sports all at once, but more importantly, the
pretty great 1995 Toronto Raptors logo, mostly.

Utahraptor once inspired an arguably wonderful digital mashup on my part, more because I saw an opportunity than the fact that I like basketball. Despite my less-than-excited (as an adult; see photo) attitude toward basketball, Utahraptor was actually the inspiration, it is said, for one basketball team's mascot (and it was not the Utah Jazz, all of my artwork aside). Their logo is not nearly as cool as it used to be of course (dinosaurs playing basketball > basketball with claw marks), but the Toronto Raptors still exist. This has not been the Utahraptor's only foray into the sphere of popular culture. As we witnessed the other day there are many video game incarnations of this large and deadly dromaeosaur and, if we count the "velociraptors" of the Jurassic Park franchise as misnamed Utahraptors, they have appeared in 5 full length films; being highlighted in pretty much every film at some point (unlike the barely seen Dilophosaurus or Triceratops of Jurassic Park). Aside from reading what science I could as a pre-teen and teenager, my major introduction to Utahraptor was through a short novel titled Raptor Red. Whether you like the story or the writing or the science within Bakker's imagined dinosaur tragedy or not, it was, for me in 1995 (along with the Toronto Raptors t-shirt; see photo) one of the coolest dinosaur things I got my hands on. 

The story centered around a Utahraptor that lost her mate, found and joined her sister, was wooed, and, through many more trials and tribulations founds a large and successful pack of Utahraptors (and yes I skipped a lot of storyline). Some of the story may have even inspired later Jurassic World scenes, or are so highly similar that they could be accused of being questionable (we are all looking at you death of "Indominus rex" scene). The point is that this was one of the first books, post-Jurassic Park, that I really enjoyed that was about dinosaurs and it reignited my interest in the animals. Utahraptor, specifically, reignited my want to read more about dinosaurs. Jurassic Park had done a good job of that as well, but I have always been an imaginative person, and seeing that it was okay to speculate and drive a narrative like this resonated with and inspired me in a way that scientific literature never did and still does not. Of course scientific literature is extremely important and I value it. I do not find inspiration in it the way I do fiction though. Of course I have not read it in years and so I cannot say how I would feel about it today. Nor would I say that I recommend it, because I do not know how my adult self would view it. However, 13 year old me thinks it was a wonderful book and that it is worth reading. Maybe it has been long enough that I do not want to destroy the good memory of it, like when you watch a cartoon from your childhood and wonder what made you think that that show was so great. But I cannot say that because I do not know.

12 September 2020

Revisiting Utah

On this blog we have covered a lot of different dinosaurs over time. Nine years ago we discussed one of the largest of the dromaeosaurs, Utahraptor ostrommaysi (Kirkland, Gaston and Burge, 1993). I think it might be time to revisit this medium sized carnivorous dinosaur. It was discovered, originally, in 1975, and again in 1991 and named around the time of the release of Jurassic Park in 1993. This quieted (but of course did not silence) some discontent surrounding the "Velociraptors" in that movie. Their scientific name in the movie and the book was Velociraptor antirrhopus, which also appears in a Gregory S. Paul book (Predatory Dinosaurs of the World, 1988) as a synonym for Deinonychus antirrhopus. It appears as though Michael Crichton may have referenced this book in writing Jurassic Park. However, I digress from our main point, Utahraptor

Standing at a similar height to the movie 'raptors, Utahraptor, was approximately 7 m (23 ft) long and is estimated to have weighed somewhere between 230 and 500 kg (500 - 1100 lb). It is the largest dromaeosaur known at this time. Like other dromaeosaurs, Utahraptor did possess a large sickle-shaped "Killing Claw" on each foot. The claws known from Utahraptor are estimated to have been approximately 24 cm (9.4 in); actual fossilized unguals, or two claws, are slightly smaller than this.

Size chart of different well known dromaeosaurs: Microraptor gui, Velociraptor mongoliensis, Austroraptor cabazai, Dromaeosaurus albertensis, Utahraptor ostrommaysorum, and Deinonychus antirrhopus ©Fred Wierum, CC BY-SA 4.0

09 September 2020

Toothless in Death

Skull of Qianzhousaurus sinensis: top – lateral view, middle – dorsal view, bottom – ventral view. Scale bar – 5 cm. Image credit: Junchang Lü et al. Retrieved from Sci-News on 9-9-20

It is not weird to find fossil teeth. It is not weird to find fossil jaws (mandibles, lower jaws, or maxillae, upper jaws). It is not even strange to find fossil jaws with few teeth or many broken teeth. It may be a little alarming, then, to see the skull of Qianzhousaurus and notice it has no fossil teeth. The teeth The team describing the fossil describes long narrow and thin teeth. They based their conclusions off of comparisons of relatives of Qianzhousaurus (both Alioramus species). Some inferences may have been drawn from the shape of the alveoli (the "sockets" of the teeth in the bone) but this is not expressly stated in the paper that describes Qianzhousaurus. The number of alveoli, and therefore teeth, is described in the paper and constitutes some of the differences that the authors use to make the case for this dinosaur being a unique genus and species as opposed to a larger member of the Alioramus genus. There are 18 teeth in the mandible (Tyrannosaurus rex, the author's common comparison species, possesses up to 17 teeth here), 15 in the maxilla, and 4 in the premaxilla. That is a total of 37 teeth. But we have to remember there are two sides to the head, which means there are 74 teeth in the skull of Qianzhousaurus (providing my math checks out).

08 September 2020

Standing Tall

 Possessing a femur approximately 70 cm (27.6 in) long and a tibia approximately 76 cm (30 in) long, Qianzhousaurus was a long-legged, though not overall a very tall, dinosaur. These measurements come from the holotype leg that was discovered and may not fully represent the adult height of Qianzhousaurus. Characteristics of the vertebrae indicate that this was an immature animal and, because we do not have an adult specimen, the full length of these legs is not currently known. However, as an immature specimen, possessing a hip probably a little above 1.46 m (57.5 in; this assumes hip height at femoral contact with the pelvis, not at the iliac blade as is often reported and therefore represents a shorter estimation of hip height) above the ground any additional height (from the foot, erectness of posture, and additional growth into adulthood) may allow for Qianzhousaurus to be a taller dinosaur than the current estimate. Estimated to be a little over 7 m (23 ft) from nose to tail, Qianzhousaurus is around 3 m (9.8 ft) shorter than Tarbosaurus bataar, a Mongolian tyrannosaur that appears to have been a contemporary neighbor. However, Qianzhousaurus is longer than either described Alioramus species, sister taxa within the Alioramini tribe, as they are both approximately (A. altai) or just over (A. remotus) 5 m (16.4 ft) in length. Depending on the juvenile status of the Alioramus discoveries, these estimates may be low end estimates as well.

Alexander Vieira's life reconstruction of the tyrannosaurid species Qianzhousaurus sinensis based on a skeletal by Ashley Patch

Literature Cited

Brusatte, S. L., Carr, T. D., Erickson, G. M, Bever, G. S., Norell, M. A. (2009). A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. Proc Natl Acad Sci U S A 106:17261–66.

Lü, J., Yi, L., Brusatte, S. L., Yang, L., Li, H., Chen, L. (2014). A new clade of Asian late Cretaceous long-snouted tyrannosaurids. Nat Commun 5:3788.

06 September 2020

The Qianzhou lizard

 A young (in terms of how long we have known it) alioramid tyrannosaur from China was first described in 2014 and was immediately given a rather interesting nickname. Due to the narrow and elongate snout, rather different from most other tyrannosaurs that possess dorsoventrally deep (i.e. tall from the bottom of the lower jaw to the the top of the skull) skulls, this dinosaur was given the nickname "Pinocchio rex". The real name of this slender-faced giant is Qianzhousaurus sinensis (Lu et al. 2014). Found in the southern Chinese province of Jianxi near the city of Ganzhou in 2010 (in the middle of a construction project), the holotype fossil consisted of a full skull (minus the teeth; more on this later), 30 vertebrae from along the entirety of the spinal column, parts of the shoulder and pelvic regions, and the majority of the left leg (some elements of the foot are missing). These remains were given the museum designation GM F10004, but the name Qianzhousaurus honors the city of Ganzhou (it is an alternate and older spelling of the city's name). the specific name, sinensis, is a derivative of the Greek word meaning "of/from China" and is used as a species name in many dinosaurs from China. 

Size comparison of the three species in the tribe Alioramini, a group of tyrannosaurid theropod dinosaurs from the Upper Cretaceous of Asia. Composite image by User:Slate Weasel (public domain release) Theropods based on the following references:

Animatronic restoration of Qianzhousaurus sinensis at Jurassic Quest drive-thru attraction. Photo by the author.

04 September 2020

Megatherium Mysteries

Mounted Megatherium americanum. The first prehistoric animal skeleton mounted, in 1795.Photo by Xauxa Håkan Svensson.

As a disclaimer: The story below is simply me relaying the facts. I do not have any stake in how these facts are interpreted or the validity of the published or reported facts.

The Natural History Museum in London has a page on the internet dedicated to Megatherium and it asks, quite simply, at the top "What was Megatherium?" Rather than starting with an outright answer, the museum explains why it would even ask that question by telling you the connection between London and a South American fossil sloth: two of Darwin's fossils (the last two digitized in the NMH collection) are halves of a Megatherium skull. Megatherium was actually well known before Darwin took the fossil skull from its home continent. The first fossils had been unearthed in 1788 by Manuel Torres, who sent them from the Lujan area west of Buenos Aires near the Lujan River of Argentina to the Museo de Ciencias Naturales of Madrid. Modern day Lujan appears to be rather far from the river, though changes in the urban landscape around Buenos Aires may account for this (I have found no record of the river changing course in my, admittedly, short search). These actually made up, as seen here, what is considered the first mounted fossil skeleton in a museum (circa 1795).

Despite having been discovered in 1788 in Argentina, official description of this fossil mammal was not produced until 1796. This publication resulted from an interpretation by Georges Cuvier, a French naturalist, of a series of drawings and descriptions of the bones by Juan Bautista Bru, a museum employee and also a dissector of the Royal Laboratory, who had helped to assemble and mount the skeleton. It is argued by Lopez Piñero (1988) that Bru's work with Megatherium and the description for the museum should count as the original description. Lopez Piñero goes on to state that an agent of the French government, a man named Roume, obtained copies of Bru's plates while in Madrid, sent them back to France, and the Institut de France asked Cuvier to write a description of what Cuvier would later state were "a bad copy of the... whole skeleton." As insulting as this may have been (not to mention that a South American animal was sent to Spain to be described in the first place), when the description was sent to Spain to be translated into Spanish, it was discovered that Bru's highly detailed description already existed. It was published almost immediately, but in the years after, Cuvier incorporated a French translation of Bru's description into his 1812 opus (Recherches sur les ossements fossil) and Bru was all but forgotten in the history of Megatherium

Argot (2008) reports that Cuvier changed his hypotheses about the lifestyle of the sloth a number of times. First he stated that the animal, like modern sloths, must have climbed trees. Later he changed his assertions, claiming the animal was a fossorial, or underground, animal somewhat like a mole. Darwin sated the British museum's desire for some Megatherium fossil material in later years, but he never mentioned Bru. Even Tomas Villanova, a professor of zoology at the museum in Madrid, refused to refute Cuvier's description, or his hypotheses, and would not give Bru credit. Megatherium has had, as many fossils did, a very interesting history.

Literature Cited

Argot, C. (2008). "Changing Views in Paleontology: The Story of a Giant (Megatherium, Xenarthra)". In Sargis, E. J.; Dagosto, M. (eds.). Mammalian Evolutionary Morphology: A Tribute to Frederick S. Szalay. Springer. pp. 37–50. 

Cuvier, G. (1812). Recherches sur les ossements fossils, oú l'on rétablit les charactères de plusiers animaux dont les révolutions du globe ont dé-truit. Tome quatrieme. Troisiéme partie. Sur les ossemens fossils de ruminans. Chez G. Dufour et E. D'Ocagne, Libraries, Paris, France.

Lopez Piñero, J. M. (1988). Juan Bautista Bru (1740–1799) and the description of the genus Megatherium. J Hist Biol 21:147–63.

03 September 2020

Giant's Diet

 Despite discussing the bipedal implications of biomechanical analyses yesterday, it is important to note that many of the foods Megatherium was thought to have consumed are on the ground. Unlike present day sloths, Megatherium was more of a grasslands and edge of the forest dweller. The open ground of the entirety of the genus' range was different depending on where in the range we discuss. However, the generally accepted diet, aside from trees, consists of lower growing plants like yucca, agave, and grass. As we probably all recognize, herbivores are often large bodied animals. There are a number of reasons for this. The explanation in its simplest form is that plant matter needs to be consumed in large quantities and the digestive anatomy needed to extract the most nutritional value from this material needs to be extensive in these large herbivores (e.g. Westoby 1974; Danell et al. 2006, and Felton et al. 2016). 

Understanding the guts of extinct animals like Megatherium that relied on the leaves of trees and low-lying plants like those mentioned above can be accomplished using living examples that we know well to draw conclusions. The understanding of how Megatherium processed, and even ate, all of that material can also be inferred from living animals, but thankfully for us, we have the capability to model feeding and the technology to do so. This has led to biomechanical and morphometric analyses of Megatherium skulls. Bargo (2001) is one such study that analyzes both the biomechanical capabilities of these sloths and takes into account the morphometrics of the skull as well as dietary limits. What we learned from Bargo (2001) is that Megatherium teeth and jaws were well suited to biting and cutting medium to soft vegetation and do not appear to have been used for grinding tough and fibrous plant materials. 

It has also been suggested that Megatherium was an opportunistic omnivore, and not a strict herbivore. These sorts of occasional carnivory have been documented, anecdotally, at least, in deer that have been reported eating mice. Hummingbirds have been known to feed arthropods to their offspring and augment their nectar diets with fruit flies (Young 1971, Brice 1992, Yanega and Rubega 2004). Occasional carnivory possibly (probably) occurs in other animals as well. Fariña and Blanco (1996) originally put forth the hypothesis that Megatherium may have engaged in scavenging of carcasses based on the muscle attachments of the elbow. This view remains controversial, but hinges (elbow pun intended) on the idea that the anatomical configuration of the Megatherium elbow is like that in other carnivores and accentuates speed of movement rather than force. The authors contended that this permitted Megatherium to use their large curved claws like daggers to stab prey or lash out at carnivores that they were attempting to wrest carcasses away from. Isotope data (and other studies) appears to completely refute this hypothesis, however (e.g. Bargo 2001, Bocherens et al. 2017)

"Skull of Megatherium americanum in dorsal (A) and ventral (B) views (MLP 2-64). Scale bar 10 cm." From Bargo 2001

Literature Cited

Bargo, M. S. (2001). "The ground sloth Megatherium americanum: Skull shape, bite forces, and diet"Acta Palaeontologica Polonica. 46 (2): 173–192.

Bocherens, H.; Cotte, M.; Bonini, R.A.; Straccia, P.; Scian, D.; Soibelzon, L.; Prevosti, F.J. (2017). "Isotopic insight on paleodiet of extinct Pleistocene megafaunal Xenarthrans from Argentina". Gondwana Research48: 7–14. doi:10.1016/

Brice, A. T. (1992). The essentiality of nectar and arthropods in the diet of the Anna's hummingbird (Calypte anna). Comparative Biochemistry and Physiology Part A: Physiology101(1), 151-155.

Danell, K., Bergström, R., Duncan, P., & Pastor, J. (Eds.). (2006). Large herbivore ecology, ecosystem dynamics and conservation (Vol. 11). Cambridge University Press.

Fariña, R. A.; R. E. Blanco (1996). Megatherium, the stabberProceedings of the Royal Society of London263. pp. 1725–1729. doi:10.1098/rspb.1996.0252

Felton, A. M., Felton, A., Raubenheimer, D., Simpson, S. J., Krizsan, S. J., Hedwall, P. O., & Stolter, C. (2016). The nutritional balancing act of a large herbivore: an experiment with captive moose (Alces alces L). PloS one11(3), e0150870.

Westoby, M. (1974). An analysis of diet selection by large generalist herbivores. The American Naturalist108(961), 290-304.

Yanega, G. M., & Rubega, M. A. (2004). Hummingbird jaw bends to aid insect capture. Nature428(6983), 615-615.

Young, A. M. (1971). Foraging for insects by a tropical hummingbird. The Condor73(1), 36-45.

02 September 2020

Tail for Balance

 In the more distant past, we are thinking about 100 years ago possibly, Megatherium was not considered a bipedal animal. It may have been described as facultatively bipedal, meaning it could occasionally walk on two feet, at most. The most often that Megatherium was thought to have engaged in such behavior was during its feeding forays. It was also hypothesized, in the past, that the tail was used as peg upon which Megatherium could balance its weight while foraging in trees on two legs. This impression was captured in the restoration of Robert Bruce Horsfall published in 1913. This tripod of legs and tail remains relatively popular. However, Casinos (1996) did conduct biomechanical analyses that suggested that the vertebral column and the femur were constructed and situated such that they would resist bending that they would be subjected to if Megatherium was an obligate quadruped, that is, if they were only able to walk on four legs. The analyses appear to indicate that Megatherium was adapted to a bipedal lifestyle and that the strong muscular tail, while it may have acted as an element in a tripod feeding stance, was not only strong and robust to resist this tripod feeding stance.

Two Megatherium americanum are seen eating from and around a tree. One is standing on two feet, feeding from a branch, and the other is walking in the foreground. Produced and published in 1913 by Robert Bruce Horsfall

Literature Cited

Casinos, A. (1996). "Bipedalism and quadrupedalism in Megatherium: An attempt at biomechanical reconstruction". Lethaia. 29: 87–96. doi:10.1111/j.1502-3931.1996.tb01842.x.

01 September 2020

Wrinkly Sloth

 In 2002 Richard Fariña published a paper in Ameghina that, while it did not forever change the view of Megatherium, did cast an interesting perspective on what the animal may have looked like. The popular outlets for prehistoric animal shows may not have adopted the look since that paper was released, but some artists, like Marcus Burkhardt ( have at least given the idea some thought an produced hypothetical reconstructions incorporating Fariña's hypothesis. What is that hypothesis, one might be asking at this point. The hypothesis contends that, due to their size, it may have been possible that Megatherium was not covered in fur, as it is often depicted, but may have been covered by loose wrinkly skin, more like a modern elephant than the fur covered "ice age" animals we think of. The premise is that, at their size, fur would have potentially maintained their body temperature such that it would have caused overheating that would have been potentially lethal. Taking away the heat retention of fur is thought to have made this less of a problem. However, as the artist makes clear in his description, this idea and the graphic representation of this idea are speculative at best because we do not have any preserved Megatherium integument and fur (or lack thereof). The interpretation is interesting either way. 

An unusual Megatherium reconstruction without fur because of the plausibility that it would have been overheated if it was covered with hair. ©Marcus Burkhardt

30 August 2020

Man or Nature

 Megatherium went extinct approximately 12,000 years ago. They were endemic to the southern continent of the Americas (South America). We know that the extinction events at the end of the Pleistocene era helped the giant ground sloths hasten their extinction. However, we also know that Megatherium, in particular the species M. americanum, was likely hunted in substantial numbers which helped them to become extinct. We mentioned yesterday that an adult Megatherium had few, if any, predators. One predator it did face had the social cooperation and tools necessary to kill these near-mammoth sized animals without extreme risks to themselves. Everyone thinking about humans is very correct. Human beings migrated across the isthmus of Panama somewhere around the time that Megatherium was beginning to severely decline, a little before 12,000 years ago. The migrating peoples of what we now know as South America were right on the heels of the extinction events that were driving down giant ground sloth numbers. Their hunting of Megatherium likely pushed an on the brink group of animals over the edge indefinitely. We know that the earliest human settlers of South America killed and ate giant ground sloths thanks to the discovery of a site known as Campo Laborde in the Argentine Pampas. In the Buenos Aires province near the east coast, a site was discovered and analyzed by Politis et al. (2019) that held evidence of the killing and butchering of the giant ground sloths by early human settlers of the region. The site is dated to around 12,600 years ago (BP: before present in the paper). Large numbers of stone tools have been found and Megatherium ribs with cutting marks match the overall striking characteristics of the stone tools. Coincidentally, North American ground sloths (Megalonychdidae) were on the menu as well for early humans. Fossil trackways from New Mexico somewhere between11,000 and 13,000 years ago (an estimated window of time is reported in this paper) appear to show a Megalonychdid sloth defending against human attackers. The interpretation by Bustos et al. (2018) is not about our Megatherium sloths, but if it is from slightly earlier (circa 13,000 BP), it may show that the migrating human groups encountered and knew how to hunt sloths long before they encountered Megatherium and hastened its extinction. Either way, the interpretive figure from the articles accompanying the Bustos et al. (2018) publication look a lot like dancing with giant ground sloths, and I will fully endorse that.

Credit: Alex McClelland, Bournemouth University

Literature Cited

Bustos, D., Jakeway, J., Urban, T. M., Holliday, V. T., Fenerty, B., Raichlen, D. A., Budka, M., Reynolds, S. C., Allen, B. D., Love, D. W., Santucci, V. L., Odess, D., Willey, P., McDonald, H. G., Bennett, M. R. (2018). Footprints preserve terminal Pleistocene hunt? Human-sloth interactions in North America. Science Advances 4:eaar7621. DOI: 10.1126/sciadv.aar7621

Politis, G. G.; Messineo, P. G.; Stafford, T. W.; Lindsey, E. L. (2019). "Campo Laborde: A Late Pleistocene giant ground sloth kill and butchering site in the Pampas". Science Advances. 5 (3): eaau4546. doi:10.1126/sciadv.aau4546.

29 August 2020

 During the early Pleistocene the South American landscape was filled with giant animals. Out birds from last week were (specifically Phorusrhacos) not a significant feature of the landscape anymore. Not that they were waiting specifically for the terror birds to become less important fauna of this landscape, but mammals were becoming more dominant and larger during this time. This coincided with stabilization of the animal movements across the isthmus of Panama, but the animal we will look at this week was not necessarily interested in that migration; though some of the descendants of the migration into South America likely impacted the evolution of this animal from its ancestors over the years since the Great American Biotic Interchange had started. The image below, by Dmitry Bogdanov, shows three of Argentina's large land animals of the early Pleistocene. From left to right they are Argentavis, a large flying bird, Glyptodon, a relative of the extant armadillos, and Megatherium, otherwise known as the Megathere, or Giant Ground Sloth. As a genus, Megatherium has been divided into two recognized subgenera (more on this later) and 6 recognized species. Ultimately, Megatherium and the Glyptodon are related, as both are in the Superorder Xenarthra, which also includes modern armadillos, tree sloths, and South American anteaters (other animals called anteaters, like the aardvark actually belong to different groups of animals). One of the largest ever known land mammals, Megatherium was approximately 6 m (20 ft) from nose to tail and 4 tons. These estimations make Megatherium as large as modern elephants and second, in history, to very few mammals; a few species of contemporaneous mammoths were larger. Possibly at least partially bipedal, Megatherium was capable of reaching vegetation that other herbivorous mammals, like Glyptodon, would not have been able to reach. Their shear size would have made them vulnerable to very few predators as full grown adults.

Argentavis, Glyptodon, and Megtherium on a hillside ©Dmitry Bogdanov

28 August 2020

This image shows the bones that were studied by Degrange et al. (2019) in white and the remainder of the reconstructed skeleton (Paul 2002) in grey. As we mentioned very briefly yesterday, the skull is massive. We can appreciate that by taking into account the scale bar from the paper, shown at the bottom, which equals 1 m. However, one part of Phorusrhacos we have not discussed really are the enormous legs that this animal possessed. The femur (and therefore thigh) is shorter than the leg (tibiotarsus) and the bone (tarsometatarsus) that is made up of bones that we possess in our ankle and the superior (upper) portion of the foot. This leg and foot are immense, like the skull, and have led many to consider the hypothesis that in addition to being able to run down prey and strike with their beaks, Phorusrhacos (and related birds) were able to stomp and kick their prey to death in a similar manner to the way that cassowary or secretary birds can kick attackers or prey (in the case of the secretary bird). The secretary bird (on a casual stroll in this video) looks a lot like what we might picture a theropod dinosaur looking like, as it skulks around the savannah. Add in the last portion of that video, where the bird kicks and stomps some lizards into submission, and then extrapolate that to a bird that is nearly 100 kg and 2.4 m (7.9 ft) tall. Then picture the animal it is trampling being a small mammal that looks something like a weasel. Those are actually pretty scary, and awe inspiring, legs on a flightless bird.
Literature Cited:
Degrange, F. J., Eddy, D., Puerta, P. Clarke, J. (2019). New skull remains of Phorusrhacos longissimus (Aves, Cariamiformes) from the Miocene of Argentina: implications for the morphology of Phorusrhacidae. Journal of Paleontology. 93 (6): 1221-1233. DOI: 
Scale bar = 1 m, reconstruction of Phorusrhacos is modified from Paul (2002). Adapted from Degrange et al. 2019.


26 August 2020

What Does it Eat?

 The simple answer to the question "What does Phorusrhacos eat?" is pretty much anything smaller than itself that it could chase down or surprise. However, we could instead scour the literature and time scales and create a much more definitive and accurate list as well, if one were so inclined. Knowing that Phorusrhacos is well represented in the Santacrucian South American Land Mammal Age, I think that looking at the group of animals that are known and of a size that they could have been prey is a little more informative than making a giant pile of South American fauna. In popular venues (like BBC's Walking with Beasts) Phorusrhacos is shown attacking Smilodon cubs, Macrauchenia (as a scavenger), and smaller animals like Diadiaphorous. The majority of the animals that Phorusrhacos attacks in popular culture shows are pure imagination because they were not contemporaneous with the large bird. Of the three mentioned, Diadiaphorous is the only likely prey item because it did live at the same time. Necrolestes (possibly a mole-like creature), Cladosictis (a small marsupial carnivore), and Peltephilus (a genus of canine-sized armadillos) could have also been on the menu for Phorusrhacos. "Anything smaller" could have included other terror birds, like Patagornis and Psilopterus, other birds in general (Liptornis and Thegornis, for example), and the young of large ungulates (like Nesodon), ground sloths (like Pelecyodon), and Astrapotheres (large animals that looked like elephants or tapirs). While images of Phorusrhacos like the one show here are really intriguing, Glyptodon would not have been on the menu either, as it was a Plesitocene mammal, and Phorusrhacos was long gone by the time it came around.

The image shows a Phorusrhacos standing on the skeletal remains of a Glyptodon, and animal which existed after the extinction of Phorusrhacos. ©H. Santiago Druetta

24 August 2020

Flightless Tradition

The ancestors of Phorusrhacos were mainly flightless animals. Studies like Alvarenga et al. (2003) placed these giant birds in the family of rails (Ralliformes), indicating that their closest living relatives are flighted birds. Garcia et al. (2020) published a tree of the rail family that did not include Phorusrhacos; their data was genetic though, and (to my knowledge) no genetic studies have been possible within the Phorusrhacidae. 

Ignoring rails for the moment, we turn to the immense height of Phorusrhacos. At 2.4 m (7.9 ft) tall, Phorusrhacos would have towered over modern human beings. These birds were barely above the middle height of their family though. Other members of the family Phorusrhacidae were over 3.2 m (10 ft) tall. Phorusrhacos was not the smallest of these birds either though; that honor currently belongs to two genera currently: Procariama simplex (Rovereto, 1914) and Psilopterus bachmanni (Moreno and Mercerat 1891) measuring in at 70 cm (2.3 ft) tall. Psilopterus genera are often estimated to be slightly taller than this, but the lower end estimate is the same as for Proceriama. A nearly 1 m or 2.5 foot tall terror bird is no laughing matter of course, but we can all agree that a bird the size of Phorusrhacos is very awe inspiring bird. I think we should appreciate that these birds existed and that they were all very large and dominant birds. I also think we might want to heave a sigh of relief that they have not persisted in history to the present day.

Though, I admit I would love to keep one as a pet, maybe get a saddle...

From Alvarengo and Hofling, 2003, showing the height comparison of a number of Phorusrhacid birds compares to a 1.75 m tall human

Sources to Consider

Alvarenga, H. M.F.; Höfling, E. (2003). "Systematic revision of the Phorusrhacidae (Aves: Ralliformes)". Papéis Avulsos de Zoologia. 43 (4): 55–91. doi:10.1590/S0031-10492003000400001

Degrange, F. J.; Tambussi, C. P.; Taglioretti, M. L.; Dondas, A.; Scaglia, F. (2015). "A new Mesembriornithinae (Aves, Phorusrhacidae) provides new insights into the phylogeny and sensory capabilities of terror birds". Journal of Vertebrate Paleontology. 35 (2): e912656. doi:10.1080/02724634.2014.912656

Garcia-R, J.C.; Lemmon, E.M.; Lemmon, A.R.; French, N. (2020). "Phylogenomic reconstruction sheds light on new relationships and timescale of rails (Aves: Rallidae) evolution". Diversity. 12 (2): 70. doi:10.3390/d12020070.

22 August 2020

The Name of Many Terrors

The holotype is a mandible in the Museo de La Plata in Argentina and it is labeled MLP-118. The name that the scientific community knows this mandible by is Phorusrhacos longissimus (Ameghino 1887). In more colloquial terms we refer to it as a "Terror Bird". Today remains of this bird are known from the Santa Cruz and Monte Leon Formations from the Miocene epoch (23 MYA to 5.3 MYA). Originally described as a toothless mammal, Florentino Ameghino appears to have named the animal for a wrinkled appearance of bone on the mandible; the name means "Wrinkle bearer" (for more on other possible translations see Lydekker, below). In 1889 Ameghino offered a change to the name, for grammatical reasons, to Phororhacos, but the original maintains seniority and is in use because of this, though the 1889 spelling is recognized by some as an alternative spelling. In 1891 Ameghino again published on remains of the bird, then recognizing it as a bird rather than a mammal and adjusting the initial description. 

Standing at approximately 2.4 m (7.9 ft) and with a skull 65 cm (26 in) long, Phorusrhacos was a giant in the Miocene of South America and was very capable of hunting and preying upon most other forms of life. Weighing around what a male ostrich weighs (130 kg, 290 lb), sporting a raptorial hooked beak, and strong legs that appear to have been capable of providing a long stride and a high speed, Pharusrhacos was an apex predator capable of running down, pinning, and dispatching prey with a swift stroke of its head and beak. Flightless and fierce, this was indeed a terror bird.

A drawing of the terror bird Phorusrhacos by Charles R. Knight published in Animals of the Past, 1901.

Sources to Consider

Ameghino, F. (1889). Contribución al conocimiento de los mamíferos fósiles de la República Argentina. Actas de la Academia Nacional de Ciencias de la República Argentina en Córdoba 6:xxxii-1027

Ameghino, F. (1891). "Mamíferos y aves fósiles argentinas. Especies nuevas, adiciones y correcciones". Revista Argentina de Historia Natural. 1: 240–259.

Fernicola, J. C., Cuitiño, J. I., Vizcaíno, S. F., Bargo, M. S., & Kay, R. F. (2014). Fossil localities of the Santa Cruz Formation (Early Miocene, Patagonia, Argentina) prospected by Carlos Ameghino in 1887 revisited and the location of the Notohippidian. Journal of South American Earth Sciences, 52, 94-107.

Lydekker, R. (1893). "On the extinct giant birds of Argentina". Ibis series 6 (5): 40–47.

19 August 2020

Awesome... Antennae? or Forks?

Walliserops trifurcatus, Houston Museum of Natural Science, Houston, Texas, USA Photo by Daderot

 Walliserops, a genus consisting of four recognized species, is a really interesting group of trilobites. Known from Devonian rocks of Morocco, there are a number of interesting anatomical features in these trilobites. All species of Walliserops exhibit some asymmetry, that is, the halves of their bodies are not identical on both sides, due to the curvature of the spine on the occipital lobe of the cephalon. Three spines in total originate from the cephalon. The occipital spine in the middle and farthest back (most caudal) and two more lateral spines located near or on the palpebral lobes, caudal to the compound eyes. Many trilobites have some kinds of spines on the cephalon or on the thorax (or both) and these are not what make this genus so interesting. What makes them truly interesting is the trident shaped fork protruding from the forward-most ridge of the cephalon. Each species has a distinctly shaped and different length trident. The exact purpose these tridents is not understood and, at one point, it was thought that they signalled differences between the sexes. This has been considered an interesting hypothesis, but is not considered a fact, as there is not enough evidence to support this claim. A plausible hypothesis states that the tridents were likely similar to the horns of the rhinoceros beetle, which are used for sparing against rivals during mating season and occasionally for digging. Of course, whatever the reason for the trident, it is a very interesting anatomical structure and makes the members of this genus appear very well protected (considering all of their spines as well).

Sources to Consider:

Brett, K. and Chatterton, N. (2001). Parabolops, a new asteropygine trilobite from southern Morocco with an unusual trident-like anterior cephalic frontal process. 3rd International Conference on Trilobites and their Relatives. University of Oxford.

Knell, R. J. and Fortey, R. A. (2005). "Trilobite spines and beetle horns: sexual selection in the Palaeozoic?". Biology Letters. 1: 196–199. doi:10.1098/rsbl.2005.0304

Whittington, H. B. (1997). "Mode of Life, Habits and Occurrence". In R. L. Kaesler (ed.). Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida. Boulder, Colorado & Lawrence, Kansas: The Geological Society of America, Inc. & The University of Kansas.

18 August 2020

Triarthrus - Preserved Legs and Antennae

Pyritized Triarthrus Trilobite

Often when we imagine a "typical" trilobite we picture an animal something like this genus, Triarthrus. Known from the Upper Ordivician soils of North America (midwestern states as well as New York and regions of Canada), China, and Scandinavia, Triarthrus is one of the final lineages of Olenid trilobites which was highly successful and very diverse during the Cambrian. Triarthrus is so well represented in the fossil record and considered so highly "typical" of trilobites that it is often used as the textbook example of what a trilobite looks like. One of the best preserved species within this genus is T. eatoni, is known from the state of New York and Canada (multiple sites across the provinces of Ontario and Quebec). Shale and iron pyrite deposits in these areas, especially the Franklin Shale in the state of New York, have produced specimens so exceptionally preserved that legs and associated appendages, gills, and antennae are perfectly retained in the fossils. A single location in New York, the Beecher's Trilobite  Bed, is world known; Triarthrus makes up approximately 85% of all of the material that has been recovered in that location. An entire community, including the shed and discarded remains of younger life stages, of T. eatoni are known from the Franklin Shale. 

This community graveyard of Triarthrus has allowed for a great deal of interpretation concerning the life cycle of these animals. The discarded exoskeletons of members that could float about with plankton were interspersed with the exoskeletons of 2mm sea floor dwelling individuals and, of course, a large number of larger exoskeletons of adults. Though 2mm may appear to be minuscule, adults of T. eatoni were not very large overall either, at approximately 5cm (50mm). Triarthrus individuals are preserved in a variety of configurations; lateral dorsal, and ventral views are all well represented. 

Other discoveries of T. eatoni have led to the discovery of trilobite eggs as well. Because of all of these exquisitely preserved Triarthrus specimens, we know much about its life from birth to death. The image below shows a ventral view of a well preserved T. eatoni with eggs preserved near the cephalon shield. The eggs were photographed in the study of this fossil by Thomas A. Hegna using a scanning electron microscope. 

Pyritised trilobite (Triarthrus eatoni) w/ eggs from a quarry in ...
Image credit: Thomas A. Hegna et al, doi: 10.1130/G38773.1.

17 August 2020

Short Time

Rather than rush a post today (the first day of the semester was rather busy), I want to encourage you all to listen to Dr. Melanie J. Hopkins discuss trilobite fossils. She is an authority on trilobite evolution and paleobiology and knows a lot of valuable information about these intriguing arthropods. Take a 20 minute break from whatever you might be doing and listen to someone that really knows their science share their knowledge with you!

16 August 2020

General Anatomical Survey of Trilobites

Calling trilobite anatomy simple is not fair to these highly diverse creatures. Some of their fossils may make their anatomy appear to be highly simplified, but many of those fossils are lacking imprints and other evidence of the complex soft structures, fine details, and even fragile hard structures that also make up the bodies and appendages of trilobites. There are many great finds that do preserve these types of structures, so we do have information about them. These structures are variable across species, of course, and will be better described in individual circumstances. There are abundant excellently preserved core exoskeletons of these animals, and this generalized regional anatomy is our focus today. Discussing them in the simplest terms, however, we are able to break down trilobites into six general regions. 
We can break them down from one side to the other (4 - 6 below) and describe the right pleural, axial, and left pleural lobes. These three lobes are where the name trilobite originates from. Pleura is a word of medieval Latin and Greek origin that means "to the side of the body" and is often used in zoology to describe the sidewalls of arthropods. The pleural lobes are literally the "side lobes". The axial lobe, then, is the lobe that makes up the center of the body. 

We can also break down trilobite anatomy by discussing it from head to tail (1 - 3 below). The head region is known as the cephalon (1), the middle region is the thorax (2), and the tail region is the pygidium (3). The cephalon can be broken down into many different smaller regions based on facial sutures (natural fractures separating components of the head) that include preocular and postocular (before and after the eyes, respectively), the rostrum (the "nose" or front end part), and the hypostome ("mouth" part). The cephalon includes the mouth area of the trilobite, houses the compound eyes, and would have had attachments for antennae. The large lateral (side) flanges or fringes are also a part of the cephalon, regardless of how long they may have been; some trilobite species had lateral fringes that extended to being nearly the length of their body. 
The thorax is the middle region of the body and is made up of multiple articulated segments. The number was variable between species and could be as few as two and as many as 103. The thorax helped to protect vital organs of the trilobite, including the gills, and served as the attachment site for the limbs; the thorax  therefore protected the limbs as well. The articulation of these segments is such that they allow for "rolling up" as can be seen in modern pill bugs (I always called them "roly-pollies" when I was a kid). Fossils of this activity have been discovered and it is thought to have been a protective measure that trilobites employed against predators. The pygidium is the most posterior portion of the trilobite. It is formed by the fusing of a number of smaller segments, like those making up the thorax, and the most posterior portion of the animal, the telson.

1 – cephalon, 2 – thorax, 3 – pygidium, 4 – right pleural lobe, 5 – axial lobe, 6 – left pleural lobe

 Sources to consider:

Bruton, D. L.; Nakrem, H. A. (2005), "Enrollment in a Middle Ordovician agnostoid trilobite", Acta Palaeontologica Polonica (3 ed.), 50: 441–448, retrieved June 22, 2009

Paterson, J.R.; Edgecombe, G.D. (2006). "The Early Cambrian trilobite Family Emuellidae Popock, 1970: Systematic position and revision of Australian Species". Journal of Paleontology. 85 (3): 496–513.

Whittington, H. B. (1997), "Morphology of the Exoskeleton", in Kaesler, R. L. (ed.), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida, Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 1–85.