Time for another entry in my Australian megafauna A-Z series. We’ve previously looked at Alkwertatherium and Barawertornis. Both these taxa have come from the north of the continent, so I think it’s only fair we give some attention to fossils from the southern end of the continent this time around. This fossil bird species was found in a cave in the south-eastern corner of South Australia. Ladies and Gentlemen, C is for Centropus colossus, better known as the giant coucal.
Coucals are closely related to cuckoos and roadrunners (it’s a real bird not just a cartoon). They are also related to the enigmatic South American bird the hoatzin, although exact relationships are still being debated. This makes the group one of the earlier diverging lineages of modern birds(Edit: thanks to David in the comments and also me going and doing some further reading, coucals are not closely related to the hoatzin. Moral of the story, check your sources! Thanks for the heads up David!). Today in Australia there is one living species of coucal, the pheasant coucal. However, this taxon only lives in the northern forests of Australia and when the fossil species was found in the late seventies, it came as a bit of a surprise to discover this group so far south.
Centropus colossus was described based on an almost complete left humerus by Robert Baird in 1985. Its reduced muscle attachment points on the pectoral crest of the humerus suggest that it was flightless. Modern coucals only fly when disturbed, but the giant coucal was a third larger in size than the pheasant coucal and may therefore have been completely flightless. The presence of the giant coucal in what is today relatively arid country suggests that in the past this region had much more plant cover.
A similar issue has arisen with the discovery of fossil coucal remains from the Thylacoleo Caves in the Nullarbor Plain, south-central Australia. These remains, which are from an undescribed species of coucal were discussed in a talk at CAVEPS 2013 (the conference I recently attended, see here for my quick round up of the week) by Flinders University PhD student Elen Shute (also see this article for further info). The presence of the coucal indicates that this region was thickly covered in vegetation in the past, despite it being desert at present.
The generic name, Centropus, comes from two Latin words; centro, meaning spine and pus, meaning foot. This is referring to the characteristic elongate nail on the hallux of other taxa in the genus. The specific name refers to the fact that this species is larger than other taxa of this genus.
Well that’s C done, D will be a slightly better known animal, if not the best known of all the Australian megafauna. All will be revealed in the near future…
Baird, Robert F., 1985. Avian fossils from Quaternary deposits in ‘Green Waterhole Cave’, south-eastern South Australia. Records of the Australian Museum 37(6): 353–370.
Baird, R.J.F. 1985. Centropus colossus Baird 1985, The Giant Coucal, Pp. 205–208 in Vickers-Rich P., and Van Tets, G.F. (eds), Kadimakara, Extinct Vertebrates of Australia. Princeton University Press: New Jersey. 284 pp.
Clode, D. 2009. Prehistoric giants, the megafauna of Australia. Museum Victoria Nature Series, Melbourne, 72 pp.
Other posts in the Australian Megafauna A-Z series:
It’s been a few weeks since I last posted anything, so I thought it’s about time to remedy that situation. The reason there’s been a lull in activity on the blog (other than the usual PhD and related research) is that all last week I was attending the 14th Conference of Australasian Vertebrate Evolution, Palaeontology and Systematics (better known as CAVEPS) in Adelaide.
The conference is held every two years (the previous meeting was in Perth) and it draws in almost every vertebrate palaeontologist in Australasia, as well as archaeologists, palynologists (fossil pollen people) and several palaeontologists from across the globe. It gives us fossil nerds a chance to catch up, discuss our research, perhaps plan some new collaborations and share a beer or two (or ten). It is exceptionally useful to students like me who have heard, seen or watched these big names in our field but have never met them. We can actually get the chance to put a face to the name and maybe even get to have a chat with them, in addition to meeting fellow students who may we not have even been aware of and make some connections. Palaeontology, like a lot of things in life, is all about who you know!
The conference started off with a series of workshops. There were drawing fossils, fossil casting, radiometric and luminescence dating and phylogenetic methods workshops to choose from. I went to the fossil casting workshop as this was something I had seen done but had never done myself. I had an attempt at casting the tooth row of a wombat which didn’t come out as horribly as I expected!
On the Tuesday the talks began. The first symposium was dedicated to Ruben Arthur Stirton, the man whose 1953 expedition is part of Australian palaeontological legend, and his subsequent researches have had a lasting and profound impact on palaeontology in this part of the world. The conference celebrated the 60th anniversary of the expedition. For the student poster session that evening, myself and Flinders University PhD student Sam Arman tried to quantify Stirton’s impact on Australasian palaeontology by tracing the academic ancestry of the attendees of CAVEPS 2013, finding that 26% of them could trace their lineage back to him. This poster stemmed from an earlier blog post of mine (see here), where I traced my own academic ancestry (Stirton is my great-great-great academic grandfather) and it was a very cool project to do that seemed to go down well with almost everyone at the conference.
Wednesday’s talks were very interesting, the main theme being phylogenetics. Colleagues of mine who aren’t particularly interested in the subject even found the talks interesting, so the speakers must have been doing something right! Wednesday night saw the conference auction, which saw several of my hard earned dollars depart from my wallet in exchange for a couple of books and some papers I look forward to reading.
I missed Thursday morning’s lectures due to being just a tad hungover from the night before (I was at a conference after all), but I heard from people who were there that they were very good! After enjoying the afternoon talks it was time for the conference dinner, where we were treated to a performance from Professor Flint and the Flintettes, an experience not to be missed! You can see an example of their work in the video below.
Friday was the last day of the conference, and also the day of my very own talk. This meant I got the unfortunate pleasure of having to wait all week before being able to finally relax! I presented on some fossil penguin research that I will hopefully be submitting soon. That night we relaxed with some dinner and a few celebratory drinks before departing off on the long drive back to Melbourne on Saturday morning. A great week; and I look forward to the next CAVEPS, which will apparently be held in Alice Springs of all places! Another road trip to look forward to then…
A massive thank you to the Flinders crew for putting on such a great conference, fantastic work!
Well, time certainly flies when you’re busy and before you know it, it’s been almost a month since you’ve last written a blog post. At least that’s what has just happened to me! I’ve been busy doing research on fossil whales, fossil penguins, talking fossil penguins at Museum Victoria’s latest SmartBar, giving a talk on Australian fossil seabirds as well as preparing and submitting abstracts for an upcoming conference, whew! But I haven’t been blogging and bringing you, dear readers, new and cool fossil discoveries. So let’s rectify that situation then shall we?
As you may have guessed from the title above, this post is about fossil dugongs, or more precisely, the lack of them in the Indopacific region. Whilst today the region is the centre of sirenian abundance and fossils are known from areas such as Madagascar, Somalia, India, Sri Lanka and Indonesia, fossil evidence from the Indopacific has been lacking with the only reported finds being a partial mandible from the Pliocene of South Australia, a partial rib from the Miocene-Pliocene boundary of Victoria and fossils of the extant Dugong dugon from the Quaternary of Papua New Guinea and Holocene of southeast Australia. There is no clear explanation for the scarcity of dugong fossils in the Indopacific region as the find from South Australia shows they were present in the area in the past. Furthermore, there are plenty of available outcrops of sediments of the correct age, the sediments also indicate the climate would have been suitable for dugongs to be present and the high densities of sirenian bones make them favourable for preservation. Therefore any new finds would be crucial to gaining a more detailed understanding of sirenian evolution in the Indopacific.
One such find was made but it was actually 30 years ago, with the fossils not being studied until only recently and published in the Journal of Vertebrate Paleontology this July by Erich Fitzgerald (who also happens to be one of my PhD supervisors) and colleagues from the Smithsonian, Howard University College of Medicine and Flinders University. The recovery of the fossils (consisting of three posterior vertebrae, one anterior caudal vertebra and seven partial ribs) is a story in itself. The fossils were found in a cave in the remote Hindenburg Range of the New Guinea Highlands, Papua New Guinea, but when the fossils were being recovered the cave suddenly flooded meaning the crew had to make a quick exit leaving some fossil material behind!
The fossils date to between 11.8–17.5 Ma, giving a minimum age of just before 12 Ma for sirenians being present in Australasian coastal marine ecosystems, and by implication their primary food source: seagrasses. As Dr. Fitzgerald explains, “Modern-day dugongs are major consumers of sea-grass, and, by doing so, have a tremendous impact on the structure of the ecosystem,” said Dr Fitzgerald. “They participate in a delicate balancing act: their feeding allows diversity in sea-grass and animal species that would otherwise be lacking. Previously, it was thought that sea cows were fairly new arrivals in Australasia, and that their relationship with sea-grass ecosystems here was a recent event. This new evidence suggests sea cows have been an important component of Australasia’s marine ecosystems for at least 12 million years and that their role in the long-term health of these environments may be substantial.”
So whilst we are still in the dark about an awful lot of the history of sirenians in Australasia, this new find does shed a little light their evolution and now we know that they were there around 12 Ma, researchers can start looking in shallow marine sediments of similar age to find the next illuminating discovery.
Dr. Fitzgerald’s comments are taken from the Museum Victoria media release.
Erich M. G. Fitzgerald, Jorge Velez-Juarbe & Roderick T. Wells (2013) Miocene sea cow (Sirenia) from Papua New Guinea sheds light on sirenian evolution in the Indo-Pacific. Journal of Vertebrate Paleontology 33: 956–963.
Just over a month ago I started a new series here on Blogozoic, the Australian megafauna A-Z, in order to show people the weird and wonderful products of the evolutionary and geographic isolation of Australia. In the first post of the series I wrote about Alkwertatherium, a large marsupial that roamed the Northern Territory in the Late Miocene. Now we move onto the letter B and this time the animal, somewhat appropriately, is a bird.
The group to which this bird belongs is the Dromornithids. Those of you with very good memories may remember I wrote a post giving an introduction to these giant, extinct, flightless birds back in February (click here to read it). This time however I will focus on one species of dromornithid in particular, the species in question is Barawertornis tedfordi.
Barawertornis tedfordi isamong the oldest known dromornithids, dating to the Late Oligocene to Early Miocene. Its generic name means ‘ground bird’ and it specific name is in honour of the vertebrate palaentologist Richard Tedford, who was one of the first people to collect dromornithid remains in Australia. The holotype, a partial left femur, was described along with other partial hind limb fragments and a dorsal vertebra by Vickers-Rich (1979). Little more was discovered of the taxon until 2010, when Nguyen and colleagues described multiple partial femora, tibiotarsi and tarsometatarsi from the Riversleigh World Heritage Area in north-western Queensland. This new material allowed the researchers to better understand the relationship of B. tedfordi to other dromornithids as well as make some inferences about how this animal may have lived.
As well as being one of the oldest dromornithid species, B. tedfordi is also the smallest known species of dromornithid and would have been similar in size to the extant southern cassowary (Casuarius casuarius), with an estimated mass of around 45 – 65 kg (Nguyen et al. 2010). Furthermore, the relative proportions of the hind limb bones in are also most similar to that of the southern cassowary, suggesting that it may have been capable of similar cursorial (walking and running) abilities. With Australia being mainly covered by forest during the Early Miocene it makes sense that B. tedfordi would have converged upon a similar physique to the cassowary that today still roams the forests of north-eastern Australia and Papua New Guinea.
The phylogenetic position of B. tedfordi is also still not certain. Previous analyses (e.g. Murray & Vickers-Rich 2004) and the strict consensus of the analysis by Nguyen et al. (2010) found that B. tedfordi was the sister taxon to all other dromornithids. However, Nguyen et al. (2010) also found weak support for B. tedfordi forming a clade with Ilbandornis sp., I. woodburnei, Dromornis planei, and D. stirtoni. The fragmentary nature of most dromornithid material however prevents more definitive statements being made about their phylogenetic relationships at present.
So that’s B done, I’ll leave it up to you clever people to figure out what letter is coming next. Stay tuned…
MURRAY, P.F. & VICKERS-RICH, P., 2004. Magnificent Mihirungs: the Colossal Flightless Birds of the Australian Dreamtime. Indiana University Press, Bloomington, 410 pp.
NGUYEN, J.M.T., BOLES, W.E. & HAND, S.J., 2010. New material of Barawertornis tedfordi, a dromornithid bird from the Oligo- Miocene of Australia, and its phylogenetic implications. Records of the Australian Museum 62, 45–60.
Other posts in the Australian Megafauna A-Z series:
In February this year I started my PhD thesis. As with all PhD’s there are various milestones where you must hand in some sort of report to make sure you aren’t just sitting around all day and drinking beer (tempting though it is). This week I finally completed my 3 month report (only 2 months late) and now I can finally worry about doing some research. This post came about after a brief twitter conversation with fellow PhD student Jon Tennant (aka @Protohedgehog, check out his awesome blog here) from Imperial College London, who suggested doing what he did and blogging his 3 month report so everyone can see what I’m getting up to (for better or for worse). So here you go! It’s a little bit drier than my usual blogging style and has a bit more jargon, but have at it anyway! If there’s anything you want to know more about drop me an email (see about section at top of the blog page for the email address).
Proposed thesis title
Investigations into hearing in toothed mysticetes (Cetacea: Mammalia) via studies of the periotic (petrosal) and mandible.
This thesis will investigate the auditory structures of the inner ear and mandible shape in toothed mysticetes. Whilst at first, these two subjects may at first appear to be disparate fields, consider that in modern cetaceans, auditory cues are the primary method by which they locate their prey, their olfactory abilities being the poorest of any mammal group (Pihlström 2008) and vision being limited due to low light levels in water at depth. Furthermore, in odontocetes (and most archaeocetes except for pakicetids) sound is/was received via the jawbone. Thus, the structure of the jaw must adequately serve the demands of two processes, namely sound reception and prey capture/feeding.
Whilst these aspects have been investigated to varying extent in archaeocetes, odontocetes and extant mysticetes (e.g. Nummela et al. 2004, 2007, Werth 2006, Yamato et al. 2012, Pyenson et al. 2013), there has been insufficient time devoted to them in toothed mysticetes. Toothed mysticetes are however, of particular interest in these aspects for several reasons. The transition from teeth to baleen is one of the key innovations in cetacean evolution, requiring major morphological changes in the skull and perhaps even led to the shift towards enormous body size (Werth 2000, Fitzgerald 2006, Slater et al. 2010). Some species of toothed mysticetes are reported to have possessed “proto-baleen” (Deméré & Berta 2008, Deméré et al. 2008, Berta 2012), but this is based on the inference of homologous structures in the palate and remains to be corroborated by the presence of fossilised baleen in association with one of these toothed mysticete taxa. Yet it is crucial to remember at this point that these toothed mysticetes were not just transitional species, an intermediate point on the path to baleen; rather they were successfully making a living and had carved their own ecological niche. This is the catalyst for this thesis. I plan to investigate how these animals were making a living, what sounds they could hear and how it affected how they fed. Modern mysticetes have low frequency hearing, very different to the high frequency, echolocating odontocetes. What is unknown is what frequencies the toothed mysticetes were capable of hearing. Did they share the low frequency hearing found in modern mysticetes or did the shift to lower frequencies and the attendant morphological changes of the inner ear coincide with the evolution of baleen? The hearing capabilities of the toothed mysticetes will also add further inferential morphological data to their hypothesised feeding ecologies. I plan to test these hypotheses in quantitative manner for the first time (in toothed mysticetes) using morphometric and finite element analyses (FEA). This will be set out in more detail in the research methodology section
Below I briefly review the evolution of early cetaceans and the mysticetes. I then summarise the current knowledge on the evolution of cetacean hearing and mandible shape before outlining my intended research methodology and estimated timeline for the project.
Cetacean Origins: The Archaeocetes
Cetaceans (whales and dolphins) are a poster child of evolution, their origins from land mammals and transition to the ocean giants of today being one of the most extraordinary tales of adaptation. Yet this has not always been the case, their extremely derived morphology causing earlier workers much frustration in their efforts to classify the “most peculiar and aberrant of mammals” (Simpson 1945), with the key revelations of their ancestry not coming to light until the past few decades (Uhen 2010). Now, the numerous fossils documenting almost every stage of this group’s evolution make it one of the most valuable case studies in macroevolution.
Cetaceans are first found in the fossil record in the Early Eocene of Pakistan; these animals were semiaquatic “walking whales” belonging to the Pakicetidae and Ambulocetidae (Gingerich et al. 1983, Thewissen et al. 1996, Madar 2007). They still retained the ability to move around on land although their skeleton was denser than their wholly terrestrial relatives (Madar 2007). By the Middle Eocene, the Remingtonocetidae and Protocetidae had appeared (Kumar & Sahni 1986, Hulbert Jr 1998, Gingerich et al. 2001, 2009, Bajpai et al. 2011) and evolved into nearshore marine (Gingerich et al. 1995, Clementz et al. 2006) animals that possessed a crocodile-like morphology, with long bodies, short limbs and a narrow, elongated rostrum (Thewissen & Bajpai 2009). The protocetids rapidly diversified and spread across the globe, reaching as far as North and South America (Hulbert et al. 1998, Uhen et al. 2011). The transition to an obligately aquatic lifestyle was completed with the appearance of the basilosaurids in the late Middle Eocene. Their remains are known from every continent except for Antarctica (Uhen 2009). Features that indicate a fully marine lifestyle include: acoustic isolation of the auditory structures within the skull via enlarged sinuses; shortened neck; forelimb dorsoventrally flattened to form a flipper; greatly reduced hind limbs that were incapable of supporting the animal’s weight on land and the presence of a tail fluke (Gingerich et al. 1990, Uhen 1998, 2004, 2009). The basilosaurids (and especially the Dorudontinae) are thought to have given rise to Neoceti (the group comprised of Mysticeti and Odontoceti) (Uhen 2004).
Modern mysticetes, also known as the baleen whales due to the keratinous plates attached to the gum of the upper jaw that are used to sieve small marine organisms from seawater, are some of the most awe-inspiring creatures ever to grace this planet. They include the largest animal to have ever evolved, the blue whale (Balaenoptera musculus), which can reach lengths of 30 m and weigh more than 170 tons (Bannister 2009). There are four extant mysticete families: Balaenidae (right, bowhead whales); Balaenopteridae (rorquals); Eschrichtiidae (gray whale) and the enigmatic pygmy right whale (Caperea marginata) which has traditionally been placed in its own family, the Neobalaenidae, but a recent analysis by Fordyce & Marx (2012) found it to belong to the Cetotheriidae, making it the sole living taxon of this family.
Whilst all living mysticetes possess baleen, not all fossil mysticetes share this feature, with early taxa possessing teeth instead. The transition from these toothed mysticetes into the baleen bearing behemoths of today is one area in cetacean evolution that is still subject to vigorous debate and research (Deméré et al. 2008, Esperante et al. 2008, Fitzgerald 2012, Armfield et al. 2013). There are three families of toothed mysticete, the Llanocetidae, Mammalodontidae and the Aetiocetidae, in addition to the still undescribed “archaeomysticete” taxa from the Oligocene of South Carolina (Barnes & Sanders 1996a, b). The Llanocetidae is represented by Llanocetus denticrenatus, from Seymour Island, Antarctica. The holotype consists of a partial mandible and an endocranial cast (Mitchell 1989), although an almost compete skull and partial skeleton, believed to be from the same individual is currently under study by Ewan Fordyce (Berta & Deméré 2009, Fitzgerald 2010). Llanocetus represents the oldest known mysticete and has been interpreted as a filter feeder that is intermediate in form between basilosaurids and crown mysticetes (Fordyce 2003), with the large diastemata between its teeth allowing the teeth to act as a food sieve (Fordyce 2003), or even potentially being filled by baleen (Berta 2012). The mammalodontids consist of the taxa Mammalodon colliveri and Janjucetus hunderi from the Late Oligocene of Australia (Pritchard 1939, Fitzgerald 2006, 2010). These species were small-bodied with short rostra and large orbits. They have been most recently interpreted as a suction-feeder and a macrophagous predator respectively (Fitzgerald 2006, 2010). The aetiocetids were the most numerous toothed mysticete family with a total of five genera. They too were small bodied (estimated length of 2-3 m) but differed from the mammalodontids in that they had a relatively elongate, broad rostrum and mandible that bowed outwards, similar to modern mysticetes (Berta & Deméré 2009). One species of aetiocetid (Aetiocetus weltoni) reportedly possesses palatal foramina and sulci that may homologous with similar structures found in modern mysticetes. These structures house the blood vessels that supply the epithelia from which baleen develops, meaning that it too may have possessed baleen, if even in only an incipient form (Deméré & Berta 2008). The earliest edentulous (toothless) mysticetes are the eomysticetids, known from North America, New Zealand and Mexico (Sanders & Barnes 2002, Berta & Deméré 2009). However the majority of these specimens are yet to be described, therefore the full diversity of this group remains unknown.
The Evolution of Cetacean Hearing
As cetaceans evolved from wholly terrestrial animals into obligately marine mammals during the course of the Eocene, their organ systems underwent major restructuring and modification. Perhaps no other organ system underwent as profound and wholesale a series of changes as that of the ear region which had to evolve the ability to detect sound in a much denser medium.
The ancestors of cetaceans possessed the generalised form of the majority of land mammals. Sound is transmitted via an air-filled external auditory meatus to the tympanic membrane, where the differential pressures cause it to vibrate and pass the sound onto the three middle ear bones, the malleus, incus and stapes. These bones serve to amplify the sound pressure by decreasing the area through which the sound is passed (Nummela et al. 2007). The stapes, in turn, transmits the sound to the fluid-filled cochlea via a piston-like action (Nummela et al. 2004). The vibrations of the cochlear fluid (perilymph) are transmitted to the hair cells of the cochlea and converted into a neural impulse and sent to the brain. This pathway is not effective underwater however as the external auditory meatus becomes filled with water, reducing the pressure differential at the tympanic membrane and therefore greatly diminishing its ability to transmit sounds to the middle ear bones. Furthermore, when underwater, the similar density of animal tissue to water means that sounds will simply travel through the tissues and bones, a phenomenon known as bone conduction (Nummela et al. 2007) and will lessen the animal’s ability to distinguish which direction the sound originally came from.
The earliest whales, the pakicetids, displayed very few adaptations for underwater hearing. They possessed a thickened tympanic bone that lacked a rostromedial connection to the periotic, allowing the tympanic to vibrate independently of the periotic (Nummela et al. 2004). This enhanced their ability to use bone conduction, which would have been the method by which sound was transmitted when underwater, although directional hearing underwater would still have been poor (Nummela et al. 2004, 2007). In the ambulocetids, we see the first use of the mandible and the development of the tympanic plate for use in a sound conduction pathway in cetaceans. These animals possessed a large mandibular foramen, which would have housed a mandibular fat pad. It has been hypothesised that Ambulocetus may have used bone conduction on land, by placing its lower jaw to the ground and hearing the vibrations, in a similar manner to some extant crocodiles (Thewissen et al. 1996). The remingtonocetids and protocetids also possessed large mandibular foramen, but the lateral wall of the mandible is thinner than that seen in ambulocetids, meaning that the sensitivity of their underwater hearing had increased. The land mammal hearing mechanism is still present, indicating that these animals could also still hear in air, albeit poorly. The contact between the tympanic and periotic is also further reduced and the morphology of the middle ear ossicles are beginning to approach that of modern cetaceans (e.g. the malleus now lacks a manubrium for the attachment of the tympanic membrane (Nummela et al. 2007). In protocetids, the periotic has become more detached from the skull via the development of air sinuses, allowing for improved directional underwater hearing (Nummela et al. 2007). In basilosaurids, the ear is functionally the same as modern cetaceans, with the lateral wall of the mandible almost as thin as that in odontocetes, the middle ear bones are similar to those of modern delphinids and the petrotympanic complex is acoustically isolated via air sinuses, implying that underwater hearing was now the primary function of the air. Despite this the basilosaurids still retained an external auditory meatus, although it was most likely hardly ever used (Nummela et al. 2007). This transition from an exclusively terrestrial hearing system to a sensitive underwater hearing system took place in less than 10 million years (Nummela et al. 2004).
The focus of research on the hearing of modern whales has been toothed echolocating whales (odontocetes). The hearing of mysticetes, especially the sound reception pathway, on the other hand, remains virtually unknown. A recent study by Yamato et al. (2012) proposed a potential sound reception pathway via fat body that contacts the petrotympanic complex laterally in the minke whale (Balaenoptera acutorostrata). The sound reception in modern mysticetes is patently different to that in odontocetes as the periotic is attached to the skull via the squamosal and exoccipital bones, whereas it is acoustically isolated in odontocetes. The external auditory meatus is vestigial, and remains debatable whether it is still functional (Yamato et al. 2012).
The Evolution of Cetacean Mandible Shape
The evolution of the cetacean mandible represents the incorporation of this element into the auditory system as the group became progressively more aquatic. The resulting changes in mandibular shape are the product of a trade-off between the animal’s joint need to use the mandible to secure and manipulate prey as well as receiving the sounds of its environment.
The pakicetids, as noted above, did not possess the ability to hear underwater and this is reflected by the fact that they retained the land mammal morphology of the mandible, with a small mandibular foramen (Nummela et al. 2004, 2007). Ambulocetids show the first sign of the development of underwater hearing in cetaceans, possessing a slightly enlarged mandibular foramen, although the lateral wall of the mandible is still relatively thick, meaning that higher frequency sound would be unable to pass through (Nummela et al. 2007). Remingtonocetids and protocetids on the other hand, have evolved a large mandibular foramen, indicating the presence of the mandibular fat pad. The lateral wall of the mandible also became thinner, implying an improved ability to hear underwater. In basilosaurids, both the size of the mandibular foramen and the thickness of the lateral wall approach those seen in odontocetes.
In the toothed mysticetes however, we see further changes to the shape of the mandible. The mandibular symphysis, which in all archaeocetes had been long (Fitzgerald 2012), is shortened. This symphysis also underwent further evolution within the toothed mysticetes. In Janjucetus the symphysis is rigid and sutured, but in the aetiocetids Chonecetus and Aetiocetus the symphysis is elastic and smooth, allowing the mandibles to rotate which is thought to be an adaptation to bulk feeding. The mandibular coronoid process is also relatively larger in the mammalodontids compared to the aetiocetids, although the coronoid process is still relatively larger compared to later diverging mysticetes such as the eomysticetids. The mandibular foramen is large in all toothed mysticetes, suggesting that this method sound reception pathway was still employed by the toothed mysticetes. Aetiocetid mandibles also differ from mammalodontid mandibles in that they are more tubular (Fitzgerald 2010), whereas mammalodontid mandibles more closely approach the shape of archaeocete mandibles.
This thesis aims to investigate the internal structure of the periotic in toothed mysticetes and attempt to discover any intragroup differences by comparing their morphological features. Furthermore, by comparing these structures to that of living taxa whom I plan to use as proxies, I hope to be able to infer what frequency range toothed mysticete hearing fell into. I then plan to study mandible shape in the group in order to determine how the mandibles of the mammalodontids and aetiocetids differ in morphospace relative to other marine tetrapods and also how they cope with applied loads and relate this to any differences in hearing abilities and hypothesised feeding ecologies.
This conceptual framework provides a foundation for the following core questions of the thesis:
What were the hearing capabilities of the mammalodontids?
What were the hearing capabilities of the aetiocetids?
Do toothed mysticete ears scale to body size?
Did the shift to lower frequency hearing in mysticetes occur before or after the acquisition of a bulk filter feeding lifestyle?
Where does toothed mysticete mandible shape fall in morphospace relative to other marine tetrapods?
Do mammalodontids and aetiocetids differ in their ability to cope with applied loads to their mandibles?
Do the differences in hearing (if any) between mammalodontids and aetiocetids correlate with differences in mandible shape and ability to cope with applied loads?
Does this relate to hypothesised feeding abilities of the clades?
These core questions will test the following hypotheses:
That mammalodontids required the ability to hear higher frequencies than that found in extant mysticetes.
That aetiocetids possessed the ability to hear lower frequencies than the mammalodontids but could still hear higher frequencies than extant mysticetes.
That the petrotympanic complex in toothed mysticetes will scale isometrically with body size.
That toothed mysticete mandible shape will fall somewhere in-between that of extant mysticetes and odontocetes in morphospace in a morphometric analysis.
That the mandibles of mammalodontids (in particular Janjucetus) will be better at coping with shaking and twisting loads than the mandibles of aetiocetids.
These hypotheses will be tested using a variety of methods and technologies. Earlier studies looking at internal structure of the periotic did so using the destructive method of thin sectioning (e.g. Kasuya 1973, Fleischer 1976). This thesis however shall employ the non-destructive method of CT scanning. Extant taxa will be scanned first to establish a proxy for various frequency ranges. Then the fossil taxa will also be scanned and compared to the living taxa to infer hearing capabilities. Whilst I will also research the possibility of defining new characteristics to use, the following list of morphological characters of the petrotympanic complex, both qualitative and quantitative, will be used to help determine hearing capacities in the fossil taxa:
Mass of the tympanic, periotic, malleus, incus and stapes (where available).
Volume of the tympanic.
Area of the tympanic plate and oval window.
Thickness of tympanic plate.
Density of malleus, incus and stapes (where available).
– This is the ratio of the radius of the cochlea at its base to the radius of the cochlea at its apex. This ratio is strongly correlated with low frequency hearing limits (Manoussaki et al. 2008).
Position of areas of high cochlear foramina density.
– The cochlear foramina housed the cochlear nerve fibres that transmitted the converted sound waves to the brain. Areas of high foramina density therefore represent areas of high nerve density. The position of these high density areas will therefore indicate what frequencies the animal was sensitive to as lower frequency sounds tend to travel further along the cochlear canal due to their longer wavelengths, whereas high frequency sounds will attenuate rapidly and be detected near the base of the cochlear canal (Geisler & Luo 1996).
Diameter of spiral ganglionic canal.
– This feature is related to the previous one as Geisler & Luo (1996) found that the highest diameter of the spiral ganglionic canal coincided with the position of the area of high foramina density in the fossil mysticete Herpetocetus.
Width of the laminar gap.
– The width of the gap between the two spiral lamina of the cochlea roughly approximates the width of the basilar membrane. A wider laminar gap at the same point in two different cochleae can indicate better sensitivity to lower frequencies. Ketten (2000) however cautions against using the laminar gap as synonymous with basilar membrane width, noting that membrane width can be overestimated by up to 110% in modern mysticetes. However, as a comparison between two cochleae the width of the gap of one relative to the other may indicate sensitivity to higher or lower frequencies.
Extent of inner and outer lamina.
– The length of the outer bony lamina of the cochlea can be used a generic indicator of high or low frequency hearing. In odontocetes the outer lamina is present for a greater length of the cochlea, whereas in mysticetes it is reduced or absent (Ketten 2000). This is believed to affect the stiffness of the basilar membrane, the stiff, rigid membrane of odontocetes being better at detecting high frequencies than the more flexible membrane in modern mysticetes.
– This is the axial height of the cochlea divided by the basal diameter of the cochlea. This is found to be generally negatively proportional to frequency (Ketten & Wartzok 1990). Geisler & Luo (1996) note that values for the basal ratio may vary across different studies, depending on how the diameter is measured.
– This is the axial height of the cochlea divided by the number of turns of the cochlea. This is found to be generally negatively proportional to frequency.
Ratio of scala tympani to scala vestibuli.
– The cochlea is divided into three chambers by membranes. These are the scala tympani, scala vestibule and scala media. This ratio was proposed by Fleischer (1976) as method for estimating frequency ranges where a large scala tympani relative to a small scala vestibuli implies high frequency hearing. Geisler & Luo (1996) found however that this condition occurs in mysticetes also and stated that it remains to be seen how effective this ratio is in distinguishing between low and high frequency hearing. This feature may or may not yet be used in this thesis.
The morphometric analysis section of this thesis will take a landmark based approach. This will involve obtaining scan data on both extant and fossil specimens. These will be gathered by myself from Museum Victoria specimens, specimens from other museum collections (via travelling to them or engaging the services of researches at those institutions) and scans already collected by colleagues. The specimens will include as wide an array of marine tetrapods (e.g. mysticetes, odontocetes, and pinnipeds) as possible to frame the following FEA study in a broader context. Using this dataset, landmarks on each specimen will be selected using an appropriate software package (e.g. Landmark (O’Higgins & Jones 2006)). Exact landmarks used in the morphometric analysis will be determined at a later point as homology of locations and functional significance must be ensured; however, the following list gives an example of some that will likely be employed:
Anterior of jaw (origin) – midline.
Posterior apex of mandible – left and right.
Posterior apex of symphysis – midline.
Dorsal apex of coronoid process – left and right.
Dorsal apex of symphysis at widest point – midline.
Ventral apex of symphysis at widest point – midline.
Once landmarks have been selected the data will be analyses using morphometric analysis software (e.g. Morphologika). Statistical techniques will be employed to eliminate as many confounding factors (e.g. size) as possible, before a multivariate (principle component analysis will be run.
The morphometric analysis component serves to frame the FEA in a broad phylogenetic context. In order to execute the FEA, scan data of toothed mysticete mandibles will used to create meshes for each taxon. Pending any new data on the effectiveness of low versus high order elements to construct meshes, low order elements will be used for the FEA. In order to ensure that results of the FEA are as consistent as possible with reality, boundary conditions will be set to constrain the model in space (i.e. simulations of muscle attachments, force vectors, etc.). FEA will be undertaken using specific software such as Strand7. Mandible models will be subjected to loading in various directions to compare effects of different hypothesised feeding styles on strain patterns. Statistical analysis of strain values will be undertaken using R.
Thesis Structure & Estimated Timeline
The following sections will form chapter/papers in my thesis. This is a preliminary structure only as these sections may yet be separated or combined into papers as my research progresses:
Review of petrotympanic morphology of toothed mysticetes.
– This review paper will complement other works such as Ekdale et al. (2011) and Luo & Gingerich (1999) that have detailed the morphology of the petrotympanic complex in modern mysticetes and archaeocetes respectively.
Study of mammalodontid hearing.
Study of aetiocetid hearing.
Comparison of toothed mysticete hearing, both intragroup and against extant cetaceans.
Toothed mysticete scaling study.
– This study may yet be removed from the thesis as I have concerns over the availability of middle ear ossicle data from fossil taxa, preventing all of the necessary data from being compiled. This will be decided upon at a future date and other ideas may be pursued instead.
Morphometric analysis of marine tetrapod mandible shape.
FEA of toothed mysticete mandibles.
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The tragedy of extinction (necessary evil that it is) is that we’ll never (barring the physicists getting their act together and building a time machine – sort it out guys) get to see all those incredible animals in the flesh, moving around, eating, sleeping, mating, fighting and just doing whatever it was that they did. One of the groups that I feel most dismayed about never being able to see is the extinct marine reptiles. This group consisted of animals such as the ichthyosaurs, mosasaurs, nothosaurs, thalattosaurs, thalattosuchians and the group which is the focus of this post, the plesiosaurs. All these reptile groups made the transition from being terrestrial animals to secondarily aquatic reptiles at various points during the Mesozoic era but from the reptiles, only turtles and snakes and iguanas have modern representatives in the seas today.
The plesiosaurs first appeared in the Late Triassic (Taylor & Cruickshank 1993) and had become widespread by the Jurassic. Plesiosaurs are distinguished by having a unique body plan which consists of a short, stiff trunk, short tail, all four limbs modified into flippers and highly variable neck length and skull size (Benson et al. 2013). Although there were also intermediate body shapes, the group tend to be fall into either the long necked, small skulled plesiosaurs or the short necked, large skulled pliosaurs. In addition to being adaptable morphologically, they were also adaptive ecologically too, as they are also known from freshwater localities including some from Australia.
Freshwater plesiosaurs are known from two localities in Victoria, Australia. These are the Otways and Flat Rocks localities, both of which are from the same formation (Eumarella Formation) and represent a time in the Early Cretaceous when the two localities were part of the same depositional basin. Southeastern Australia in the Early Cretaceous was joined to Australia (although they were in the process of rifting apart) and situated within the Antarctic Circle. The area was a flood plain that was filled with braided river channels that would have burst their banks when the spring thaw floods passed through each year. These flood waters carried with them the remains of animals that had perished in the waters and been carried downstream and it’s their fossils that we search for at these localities today (see here and here for previous posts on each locality).
In a new paper to be published in the next issue of the journal Alcheringa, Roger Benson and colleagues describe a freshwater plesiosaurian tooth (NMV P198945) from the Eumarella Formation in Victoria, Australia. The interesting thing about the tooth is that it is different to any other plesiosaur teeth that have been found at these localities previously. It is much larger than all other plesiosaur teeth discovered; it has a length of 45.1 mm, whereas the other teeth from that locality are between 10-30 mm in length. It has ridges on its enamel which are spaced apart from each other and stop short of the crown apex (the top of the tooth), differing from the smaller teeth from the locality, which possess ridges that are closer together and continue closer to the apex of the crown.
So what does this new tooth mean for our interpretation of the palaeoecosystem at this locality? Well, this is the first unambiguous evidence of a multitaxic (multiple species) freshwater plesiosaur assemblage known anywhere in the world. In freshwater river systems today there are equivalents in the form of river dolphins from China and the Amazon. Whilst the Chinese dolphins might actually be extinct, the Amazon dolphins partition food and habitat to avoid directly competing with each other. A similar thing may have been happening in the Early Cretaceous of Victoria with these freshwater plesiosaurs. The estimated size of the animal that this tooth belonged to is a seriously impressive 4-5 m, a true river monster if there ever was one! If those physicists ever do get round to that time machine, I hope they let Jeremy Wade use to do a palaeo special episode for this beastie. Jeremy, you might need a bigger boat…
(For those of you who don’t know who Jeremy Wade is, he is the host of a show on Nat Geo called River Monsters, definitely worth checking out!)
Roger B.J. Benson , Erich M.G. Fitzgerald , Thomas H. Rich & Patricia Vickers-Rich (2013): Large freshwater plesiosaurian from the Cretaceous (Aptian) of Australia, Alcheringa: An Australasian Journal of Palaeontology, DOI:10.1080/03115518.2013.772825
Taylor, M. A. and Cruickshank, A. R. I. 1993. A plesiosaur from the Linksfield erratic (Rhaetian, Upper Triassic) near Elgin, Morayshire. Scottish Journal of Geology, 29, 191-196.
This blog has been going for just over four months now and a couple of weeks ago I broke through the 2,000 hits barrier. Whilst that may be a drop in the ocean for some of the more popular blogs on the internet, the fact that my inane ramblings about palaeontology have been viewed over 2,000 times feels pretty cool and definitely spurs me on to keep writing the blog. So if you’re someone who’s been here before, thank you, I hope you’ve enjoyed my articles and will continue to do. If this is your first time here, welcome!
Now that there are least a trickle of people who read the blog on a semi-regular basis, I wanted to start the first regular feature on Blogozoic. Since I’m a palaeontologist (in training) working in Australia I thought that I should write a series of posts showing off some of Australia’s extinct beasties. So this is the first post in the Australian Megafauna A – Z. Every now and again there will be a new post covering an extinct megafauna taxon whose name begins with a different letter. I do also have an ulterior motive for doing this as it means I will become more familiar with the Australian megafauna whilst I write the series; having not grown up in Australia (I’m originally from Belfast, Northern Ireland and have lived in Melbourne for the past six years), I’m not as clued up on this group of animals as I would like to be.
Before I introduce the first species of the series, I’ll quickly define what I mean when I say Australian megafauna. They were a of group large animal species that existed in Australia until sometime in the Pleistocene. They weighed at least 30 kg, with the largest taxa weighing up to an estimated 2,000 kg! What exactly caused the Australian megafauna to go extinct is still the subject of very intense debate here in Australia, with research continually providing new perspectives on the matter (e.g. a PLOS One paper that came out last week showed via isotope analysis of kangaroo and diprotodontid teeth that southeastern Queensland in the late Miocene and Pliocene was less arid than previously thought, with tropical forests, wetlands and grasslands all present (Montanari et al 2013)). The Australian megafauna were comprised mostly of marsupials, but also contained reptiles and birds, some of which we’ll meet in subsequent posts. Let’s meet the first species in this series then shall we? The first taxon under the spotlight is Alkwertatherium webbi.
Alkwertathrium webbi is a zygomaturine diprotodontid known only from the Alcoota Local Fauna in Northern Territory. The diprotodontids were medium to large sized herbivorous marsupials that included the largest known marsupial, the rhino sized Diprotodon opatum (you may hear about it in a few posts time). There are two currently recognised sub-families of diprotodontids, the Diprotodontinae and the Zygomaturinae, of which A. webbi belongs to the latter. The two sub-families are distinguished mainly on the basis of differences in the structure of their third premolar.
Alkwertatherium webbi was a small zygomaturine, around the size of a horse. The holotype specimen, found at the famous fossil bearing site at Alcoota Station, includes a skull with lower jaws and measured just over 40 cm in length. The features of Alkwertatherium’s skull show similarities to several other zygomaturine taxa, causing confusion to those who have attempted to decipher its phylogenetic position. Some workers (Black and Archer, 1997) have placed it within the Zygomaturinae, whereas Murray (1990), in the paper where he described the holotype specimen, suggested it was in fact the sister taxa to all other zygomaturines, a claim that would mean it was the sole survivor of this ancestral group due to its Late Miocene age. Mackness (2010) follows the placement of Black and Archer (1997), in addition to stating that the archaic features of Alkwertatherium support the view that the Zygomaturinae evolved from basal Diprotodontinae.
Despite not being as well-known as other extinct megafauna taxa, Alkwertatherium gives a glimpse into past diversity in Australia. That’s the first in the Australian Megafauna A – Z Series; stay tuned for more to come in the following months!
Black, K and Archer, M (1997) Silvabestius gen. nov., a primitive zygomaturine (Marsupialia, Diprotodontidae) from Riversleigh, northwestern Queensland. Memoirs of the Queensland Museum 41: 193-208.
Mackness, B (2010) On the identity of Euowenia robusta De Vis, 1891 with a description of a new zygomaturine genus. Alcheringa 34:455-469.
Murray, PF (1990) Alkwertatherium webbi, a new zygomaturine genus and species from the late Miocene Alcoota Fauna, Northern Territory (Marsupialia, Diprotodontidae). The Beagle, Records of the Northern Territory Museum of Arts and Sciences 7: 53-80.