Sperm whales have faces, too!

Figure 1. This image comes from a news story on whale strandings and the contents of their stomachs. But I see two distinct faces here, like humans, chimps and other mammals with distinctive coloration patterns and variations on morphology.

Figure 1. This image comes from a news story on whale strandings and the contents of their stomachs. But I see two distinct faces here, like humans, chimps and other mammals with distinctive coloration patterns and variations on morphology.

Humans have distinct faces.
So do chimps, dogs, cows, other mammals and animals in general. We just have to see two in close proximity (as in Fig. 1) to notice the slight variation that Nature puts on pod mates and/or family members. This minor variation, of course, is the engine by which large variation can add up in isolation to produce new species, whether larger or smaller, more robust or more gracile, shorter, longer, with longer or shorter limbs, longer or shorter faces. The variations are endless, but patterns can be gleaned in phylogenetic analysis.

Look closely
and you’ll see the profile of these two beached whales are slightly different, the flippers are slightly different, to say nothing of the variations on the white patches and scars that they are partly born with and then develop during their lifespan as white scars.

Just think,
this odontocete is derived from swimming tenrecs, derived from basal placentals, derived cynondonts, etc. etc. all due to subtle variations in family members like you see here, over vast stretches of time and millions of generations.

Trimerorhachis and kin to scale

Updated April 23 with a revision to the tabulars of Panderichthys. Thanks DM! My bad. 

Yesterday we took a revisionary look at Trimerorhachis insignis (Cope 1878, Case 1935, Schoch 2013; Early Permian; 1m in length; Fig. 1). Today we take a quick peek at the taxa that surround it in the large reptile tree (LRT, 980 taxa, Fig. 1) all presented to scale. Several of these interrelationships have gone previously unrecognized. Hopefully seeing related taxa together will help one focus on their similarities and differences.

Figure 1. Trimerorhachis and kin to scale. Here are Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

Figure 1. Trimerorhachis and kin to scale. Here are Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

And once again
phylogenetic miniaturization appears at the base of a tetrapod clade. Note: the small size of Trimerorhachis (Fig. 1) may be due to the tens of millions of years that separate it in the Early Permian from its initial radiation in the Late Devonian, at which time similar specimens might have been larger. Provisionallly, we have to go with available evidence.

We start with…

Panderichthys rhombolepis (Gross 1941; Frasnian, Late Devonian, 380 mya; 90-130cm long; Fig. 1). Distinct from basal taxa, like Osteolepis, Pandericthys had a wide low skull, a wide low torso, a short tail and five digits (or metacarpals). No interfrontal was present. The orbits were further back and higher on the skull. Dorsal ribs, a pelvis and large bones within the four limbs were present.

Tiktaalik roseae (Daeschler, Shubin and Jenkins 2006; Late Devonian, 375mya: Fig. 1) nests between Pandericthys and Trimerorhachis in the LRT. Distinct from Panderichthys the opercular bones were absent and the orbits were even further back on the skull.

Ossinodus pueri (Warren and Turner 2004; Viséan, Lower Carboniferous; Fig. 1) was orignally considered close to Whatcheeria. Here it nests between Trimerorhachis and Acanthostega. The presence of an intertemporal appears likely. Distinct from Acanthostega, the skull is flatter, the naris is larger. Distinct from sister taxa, the maxilla is deep and houses twin canine fangs. A third fang arises from the palatine.

Acanthostega gunnari (Jarvik 1952; Clack 2006; Famennian, Late Devonian, 365mya; 60cm in length; Fig. 1) was an early tetrapod documenting the transition from fins to fingers and toes. Based on its size and placement, the nearly circular bone surrounding the otic notch is here identified as a supratemporal, not a tabular, which appears to be lost or a vestige fused to the supratemporal. This taxon is derived from a sister to Ossinodus and appears to have been an evolutionary dead end.

Trimerorhachis insignis (Cope 1878, Case 1935, Schoch 2013; Early Permian; 1m in length; Fig. 1) was considered a temnospondyl close to Dvinosaurus, but here nests as a late surviving basal tetrapod from the Late Devonian fin to finger transition. It is close to Ossinodus and still basal to Dvinosaurus (Fig. 1) and the plagiosaurs. As a late survivor, Trimerorhachis evolved certain traits found in other more derived tetrapods by convergence, like a longer femur and open palate. The presence of a branchial apparatus indicates that Trimerorhachis had gills in life. Dorsally Trimerorhachis was covered with elongated scales, similar to fish scales.

Dvinosaurus primus (Amalitzky 1921; Late Permian; PIN2005/35; Fig. 1) Dvinosauria traditionally include Neldasaurus among tested taxa. Here Dvinosaurus nests basal to plagiosaurs like Batrachosuchus and Gerrothorax and was derived from a sister to Trimerorhachis.

Batrachosuchus browni (Broom 1903; Early Triassic, 250 mya; Fig. 1) nests with Gerrothorax, but does not have quite so wide a skull.

Gerrothorax pulcherrimus (Nilsson 1934, Jenkins et al. 2008; Late Triassic; Fig. 1) was originally considered a plagiosaurine temnospondyl. Here it nests with the Trimerorhachis clade some of which  share a lack of a supratemporal-tabular rim, straight lateral ribs and other traits.

This clade of flathead basal tetrapods
is convergent with the flat-headed Spathicephalus and Metoposaurus clades and several others.

References
Berman DS and Reisz RR 1980. A new species of Trimerorhachis (Amphibia, Temnospondyli) from the Lower Permian Abo Formation of New Mexico, with discussion of Permian faunal distributions in that state. Annals of the Carnegie Museum. 49: 455–485.
Broom R 1903. On a new Stegocephalian (Batrachosuchus browni) from the Karroo Beds of Aliwal North, South Africa. Geological Magazine, New Series, Decade IV 10(11):499-501
Case EC 1935. 
Description of a collection of associated skeletons of Trimerorhachis. University of Michigan Contributions from the Museum of Paleontology. 4 (13): 227–274.
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Clack JA 2009. The fin to limb transition: new data, interpretations, and hypotheses from paleontology and developmental biology. Annual Review of Earth and Planetary Sciences. 37: 163–179.
Coates MI 2014. The Devonian tetrapod Acanthostega gunnari Jarvik: Postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution. Earth and Environmental Science Transactions of the Royal Society of Edinburgh.
Coates MI and Clack JA 1990. Polydactly in the earliest known tetrapod limbs. Nature 347: 66-69.
Colbert EH 1955. Scales in the Permian amphibian. American Museum Novitates. 1740: 1–17.
Daeschler EB, Shubin NH and Jenkins FA, Jr 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature. 440 (7085): 757–763.
Gross W 1941. Über den Unterkiefer einiger devonischer Crossopterygier (About the lower jaw of some Devonian crossopterygians), Abhandlungen der preußischen Akademie der Wissenschaften Jahrgang.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
Jenkins FA Jr, Shubin NH, Gates SM and Warren A 2008. Gerrothorax pulcherrimus from the Upper Triassic Fleming Fjord Formation of East Greenland and a reassessment of head lifting in temnospondyl feeding. Journal of Vertebrate Paleontology. 28 (4): 935–950.
Nilsson T 1934. Vorläufige mitteilung über einen Stegocephalenfund aus dem Rhät Schonens. Geologiska Föreningens I Stockholm Förehandlingar 56:428-442.
Olson EC 1979. Aspects of the biology of Trimerorhachis (Amphibia: Temnospondyli). Journal of Paleontology. 53 (1): 1–17.
Pawley K 2007. The postcranial skeleton of Trimerorhachis insignis Cope, 1878 (Temnospondyli: Trimerorhachidae): a plesiomorphic temnospondyl from the Lower Permian of North America. Journal of Paleontology. 81 (5):
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
Williston SW 1915. 
Trimerorhachis, a Permian temnospondyl amphibian. The Journal of Geology. 23 (3): 246–255.
Williston SW 1916. The skeleton of Trimerorhachis. The Journal of Geology. 24 (3): 291–297.

 

wiki/Ossinodus
wiki/Acanthostega
wiki/Tiktaalik
wiki/Panderichthys
wiki/Trimerorhachis
wiki/Gerrothorax
wiki/Batrachosuchus

Carroll 1988

Figure 1. Vertebrate Paleontology by RL Carroll 1988.

Figure 1. Vertebrate Paleontology by RL Carroll 1988 is one of the starting points for this blog and ReptileEvolution.com

In 1988
Dr. Robert L. Carroll published a large work devoted to the study of fish and tetrapods: Vertebrate Paleontology and Evolution. Between its black covers and silver dust jacket there was – and is – an immense amount of data on just about every taxon known at the time… a time just before software driven phylogenetic analysis became de rigueur.

My copy
has been used so much it has a broken binder, which makes every section lighter, easier for scanning.

For its time, and for a few decades later
Vertebrate Paleontology and Evolution was the ‘go-to’ textbook for students and artists of this science. (See below).

A few quotes from the Amazon.com website:

  1. “This book was my textbook for Vertebrate Paleontology and Evolution at the University of Rochester back in 1992.”
  2. “the only easily available work that goes to any depth on this intensely interesting subject.”
  3. “The book is very daunting to look at if you just flip through it. However, it does a nice job of introducing concepts and terms to the reader. Its organization is straightforward, starting with the simplest vertebrates and eventually finishing with mammals.:
  4. “Just realize that some of the information may not reflect our current understanding since the book is over 10 years old and many new finds have come to light, new ideas have been introduced, and old ideas reexamined.”
  5. “It’s an essential for anyone building a library of paleo textbooks.”
  6. “I’m a working fossil preparator and this is the primary reference text used in paleontology labs at the American Museum of Natural History, Yale Peabody Museum and others I’m sure.”
  7. “I had Romer’s Vertebrate Paleontology, which is an excellent book, until a paleontologist friend directed me to Carroll’s book. He acknowledges Romer’s work in the field but this is an updated version (for the time of publication).”
  8. “If you want to chart the course of evolution up to the present – read this book!”

Carroll 1988 updated
Romer’s Vertebrate Paleontology  (1933, 1945, 1966) which was the ‘go-to’ textbook of its day.

ReptileEvolution.com
updates portions of Carroll 1988. Likewise and in due course, someone someday may want to update ReptileEvolution.com. I hope they do so.

Every so often
it’s good to give credit to one’s mentors and resources. Sometimes you learn by doing. Other times you learn by reading. I suppose everyone who writes such a large gamut book knows he/she is doing something to help future students and enthusiasts who they will never meet. I feel the same way, butI imagine both Carroll and Romer were additionally warmed by a healthy royalty check once or twice a year.

References
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.
Romer AS 1966. Vertebrate Paleontology. University of Chicago Press, Chicago; 3rd edition

wiki/Vertebrate_Paleontology_and_Evolution

Zhenyuanlong: Dromaeosaur? No. Tyrannosaur with wings? Yes.

Lü and Brusatte 2015
described a short-armed, winged Early Cretaceous Liaoning theropod, Zhenyuanlong suni (Fig. 1, JPM-0008 Jinzhou Paleontological Museum), as a dromaeosaur. Their published phylogenetic analysis included only dromaeosaurs but their text indicates a large inclusion set.

Figure 1. Zhenyuanlong in situ with colors applied to bones and feathers. These colors are transferred to create the reconstruction in figure 3.

Figure 1. Zhenyuanlong in situ with colors applied to bones and feathers. These colors are transferred to create the reconstruction in figure 3. The pelvic elements are reconstructed at right. The manus and pes are reconstructed at left.  Scale bars are 10cm.

From the Lü and Brusatte text
“We included Zhenyuanlong in the phylogenetic dataset of Han et al., based on the earlier analysis of Turner et al, which is one of the latest versions of the Theropod Working Group dataset. This analysis includes 116 taxa (two outgroups, 114 ingroup coelurosaurs) scored for 474 active phenotypic characters. Following Han et al., characters 6, 50, and 52 in the full dataset were excluded, 50 multistates were treated as ordered, and Unenlagia was included as a single genus-level OTU. The analysis was conducted in TNT v1.142 with Allosaurus as the outgroup.”

I reconstructed this theropod,
from published photographs (Figs. 1, 2) using (DGS digital graphic segregation), added it to the large reptile tree and found that it nested between tiny Compsognathus and gigantic Tyrannosaurus rex. Of course, Zhenyuanlong had the opportunity to nest with several dromaeosaurs, but it did not do so.

Figure 2. Skull of Zhenyuanlong in situ, as originally traced, colorized with skull, palate and mandible segregated.

Figure 2. Skull of Zhenyuanlong in situ, as originally traced, colorized with skull, palate and mandible segregated. Original quadrate may be a quadratojugal.

When you look at the reconstruction,
(Fig. 3) the similarity to T. rex becomes immediately apparent… except for those long feathered wings, of course.

I’ll run through several of the traits that link
Zhenyuanlong to Tyrannosaurus to the exclusion of dromaeosaurs here. It’s a pretty long list. Even so, if you see any traits that should not be listed, let me know and why.

  1. skull not < cervical series length
  2. skull not < half the presacral length
  3. premaxilla oriented up
  4. lacrimal not deeper than maxilla
  5. naris dorsolateral
  6. naris at snout tip, not displaced dorsally
  7. orbit length < postorbital skull
  8. orbit not > antorbital fenestra
  9. orbit no > lateral temporal fenestra
  10. orbit taller than wide
  11. frontal with posterior processes
  12. posterior parietal inverted ‘B’ shape
  13. jugal posterior process not < anterior
  14. parietal strongly constricted
  15. quadratojugal right angle
  16. majority of quadrate covered by qj and sq
  17. postorbital extends to minimum parietal rim
  18. maxillary teeth at least 2x longer than wide
  19. mandible tip rises
  20. angular not a third of mandible depth
  21. retroarticular process expands dorsally and ventrally
  22. cervicals taller than long
  23. cervicals decrease cranially
  24. mid cervical length < mid dorsal
  25. caudal transverse processes present beyond the 8th caudal
  26. humerus/femur ratio < 0.55
  27. metacarpals 2 & 3 do not align with manual one joints
  28. pubis angles ventrally – not posteriorly
  29. 4th trochanter of femur sharp
  30. metatarsals 2 & 3 align with p1.1
Figure 3. Zhenyuanlong reconstructed in lateral view. Something behind the pelvis could be the remains of an egg, but needs further study. Both sets of wing feathers are superimposed here. Click to enlarge.

Figure 3. Zhenyuanlong reconstructed in lateral view. Something behind the pelvis could be the remains of an egg, but needs further study. Both sets of wing feathers are superimposed here. Click to enlarge. Note the pubis is not oriented posteriorly. Note the longer legs of Zhenyuanlong compared to tested dromaeosaurs.

Shifting
Zhenyuanlong to the dromaeosaurs adds a minimum of 127 steps to the large reptile tree. There is one clade of theropods that nests between the current tyrannosaur and dromaeosaur clades.

Figure 3. Cladogram subset of the large reptile tree showing the strong nesting of Zhenyuanlong as the current sister to Tyrannosaurus. Obviously many more theropod taxa are missing here. They have not been tested yet.

Figure 4. Cladogram subset of the large reptile tree showing the strong nesting of Zhenyuanlong as the current sister to Tyrannosaurus. Obviously many more theropod taxa are missing here. They have not been tested yet.

Note
I have not tested as many theropods as there are in several theropod cladograms.

The possible faults with the Lü and Brusatte study were

  1. a lack of reconstructions to work with, rather than just a score sheet that others had created and they trusted. Reconstructions test identifications by making sure the puzzle pieces actually fit, both morphologically and cladisitically.
  2. I think they were fooled by the apparent posterior orientation of the pubis in situ when in vivo it was not oriented posteriorly
  3. I’m guessing that the traits they used could be used on in situ fossils without making reconstructions. The traits I use require reconstructions.

With this nesting
the origin of long pennaceous wing feathers is evidently more primitive than earlier considered, developed twice. And perhaps this is why T. rex had such tiny arms. They were former wings, not grasping appendages.

References
Lü J and Brusatte SL 2015. A large, short-armed, winged dromaeosaurid (Dinosauria: Theropoda) from the Early Cretaceous of China and its implications for feather evolution. Scientific Reports 5, 11775; doi: 10.1038/srep11775.

Evolution basics – starring Jon Stewart and Babe Ruth

Evolution does not work in mysterious ways.
The basics (small variations leading over dozens of generations to larger changes) are simple:

GIF movie 1. Skull width as a variable demonstrated by Babe Ruth and John Stewart in this animated GIF file.

GIF movie 1. Skull width as a variable demonstrated by Babe Ruth and John Stewart in this animated GIF file.

  1. wider / narrower (skull, body, feet, etc.)
  2. taller-larger / smaller-shorter
  3. longer (more ribs) / shorter (fewer ribs)
  4. longer limbs / shorter limbs
  5. larger skull / smaller skull
  6. longer preorbital region / longer postorbital region
  7. longer neck / shorter neck
  8. sharp claws / rounded claws
  9. etc. / etc.

At left 
are extinct baseball star, Babe Ruth, and extant comedian/commentator, Jon Stewart, graphically demonstrating #1 on the above list, wider / narrower in the skull shape. Both are male members of the species Homo sapiens.

Other traits
one can add to this list include various perforations or fenestrae (which have several and often convergent origins and disappearances:

  1. fenestra between the naris and orbit (antorbital fenestra)
  2. fossa surrounding antorbital fenestra
  3. one or more fenestrae between the orbit and occiput
  4. fenestra in the mandible
  5. occipital fenestrae expand over braincase
  6. acetabulum perforated or not

And once fenestrae are formed:

  1. Loss of lower temporal arch
  2. Loss of upper temporal arch
  3. Loss of both

Then add
the size and shape of various bones and their processes compared to other bones and you have yourself a long character list. Enough of these (150+) provide a good matrix of characters and character states that can produce the menagerie of reptiles found in the large reptile tree, now numbering 566 taxa for 228 characters.

The wider / narrower and smaller / larger dichotomies 
can also be seen in the variety of specimens attributed to Proterosuchus and Chasmatosaurus (Fig. 2, Broom 1903). Some paleontologists (Welman 1998, Ezcurra  and Butler 2015) consider these taxa congeneric. They think this variety constitutes an ontogenetic series. On the other hand, the large reptile tree recovered these taxa in distinct nodes and clades. Narrower-skulled forms nest together. So do wider-skulled forms and they lead to other even more distinct taxa, including some once again tiny forms. The tall-skulled proterosuchids do not lead to more derived taxa.

Figure 3. The many faces of Proterosuchus to scale and in phylogenetic order, among with their closest known relatives. Note the phylogenetic miniaturization, reduction of the drooping premaxilla and loss of the antorbital fenestra after the TM 201 specimen of Chasmatosaurus. Click to enlarge.

Figure 2. The many faces of Proterosuchus to scale and in phylogenetic order, among with their closest known relatives. Note the phylogenetic miniaturization, reduction of the drooping premaxilla and loss of the antorbital fenestra after the TM 201 specimen of Chasmatosaurus. Click to enlarge.

The smallest taxon
shown here (Fig. 2), Youngoides romeri, leads to euparkeriids and then to a long list of archosauriforms including dinosaurs, crocs and birds. This last common ancestor of proterosuchids and euparkeriids (all archosauriforms) also had a small antorbital fenestra.

Have a great weekend!
Keep those cards and letters coming.

References
Broom R. 1903. On a new reptile (Proterosuchus fergusi) from the Karroo beds of Tarkastad, South Africa. Annals of the South African Museum 4: 159–164.
Ezcurra MD and Butler RJ 2015. Post-hatchling cranial ontogeny in the Early Triassic diapsid reptile Proterosuchus fergusi. Journal of Anatomy. Article first published online: 24 APR 2015. DOI: 10.1111/joa.12300
Welman J 1998. The taxonomy of the South African proterosuchids (Reptilia, Archosauromorpha). Journal of Vertebrate Paleontology 18 (2): 340–347.

You Must See: Your Inner Fish with host Neil Shubin

PBS presented part 1 of 3 last night of Your Inner Fish, a new TV series hosted by and based on Neil Shubin’s book of the same name. I recommend this highly. It’s wonderfully done and, it goes without saying, this subject is close to my heart. Shubin is an excellent host and his presentation is clear, true and entertaining.

Figure 1. Click to go to the website. Your Inner Fish is Neil Shubin's 3-part series based on his book of the same name. This the best presentation on human evolution I have seen on TV.

Figure 1. Click to go to the website. Your Inner Fish is Neil Shubin’s 3-part series based on his book of the same name. This the best presentation on human evolution I have seen on TV.

The book, Your Inner Fish, came out in 2008. Here’s the Amazon.com synopsis.
Why do we look the way we do? Neil Shubin, the paleontologist and professor of anatomy who co-discovered Tiktaalik, the “fish with hands,” tells the story of our bodies as you’ve never heard it before. By examining fossils and DNA, he shows us that our hands actually resemble fish fins, our heads are organized like long-extinct jawless fish, and major parts of our genomes look and function like those of worms and bacteria. Your Inner Fish makes us look at ourselves and our world in an illuminating new light. This is science writing at its finest—enlightening, accessible and told with irresistible enthusiasm.”

 

Author Neil Shubin along with this discovery, Tiktaalik.

Figure 2. Author Neil Shubin along with his discovery, Tiktaalik.

How the book came to be as told by author Neil Shubin
“This book grew out of an extraordinary circumstance in my life. On account of faculty departures, I ended up directing the human anatomy course at the University of Chicago medical school. Anatomy is the course during which nervous first-year medical students dissect human cadavers while learning the names and organization of most of the organs, holes, nerves, and vessels in the body. This is their grand entrance to the world of medicine, a formative experience on their path to becoming physicians. At first glance, you couldn’t have imagined a worse candidate for the job of training the next generation of doctors: I’m a fish paleontologist.

It turns out that being a paleontologist is a huge advantage in teaching human anatomy. Why? The best roadmaps to human bodies lie in the bodies of other animals. The simplest way to teach students the nerves in the human head is to show them the state of affairs in sharks. The easiest roadmap to their limbs lies in fish. Reptiles are a real help with the structure of the brain. The reason is that the bodies of these creatures are simpler versions of ours.

During the summer of my second year leading the course, working in the Arctic, my colleagues and I discovered fossil fish that gave us powerful new insights into the invasion of land by fish over 375 million years ago. That discovery and my foray into teaching human anatomy led me to a profound connection. That connection became this book.”

Figure 1. From the Beginning - The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

Figure 3. From the Beginning – The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

If you are interested in human evolution and want to see more details on the development of human body parts and — when — they came to be, see “From the Beginning, the Story of Human Evolution” free online pdf here.

Only a few updates to this 1991 book are needed based on more recent discoveries. Updates can be found at reptileevolution.com where you can also read about the evolution of any reptile, from snakes to pterosaurs to whales, dinosaurs and bats, from their fishy genesis through all their transitional taxa.

Congratulations
to Neil Shubin for work well done!

 

 

Freed from their primitive function: posterior jaw bones become ear ossicles

 series of jaw bones demonstrating the gradual accumulation of traits that changed them into ear ossicles and an eardrum frame.

Figure 1. Click to enlarge. A series of jaw bones demonstrating the gradual accumulation of traits that changed them into ear ossicles and an eardrum frame. Largely from Allin 1975.

Today we’ll start a new series: Freed from their primitive function” in which we’ll look at various parts of the vertebrate anatomy that started off doing one thing, but ended up doing another. These are excellent examples of evolution.

Today we’ll look at the posterior jaw bones that started off articulating with the skull and ended up transmitting sound in mammals. It was first recognized as early as Reichert (1837) 20 years before Darwin’s “On the Origin of Species’ (1859) and subsequently confirmed and elaborated by several workers (e.g. Allin 1975, Kemp 2007). Reichert employed embryology and comparative anatomy as he reported in the developing mammalian embryo that the incus and malleus arise from the same first pharyngeal arch as the mandible and maxilla. (Which reminds us that before these were jaw bones they were gill rims!)

Allin (1975) drew on the several paleontological examples then known to more fully fill in the story (Fig. 1) demonstrating graphically the gradual rise and expansion of the coronoid process along with the reduction of the postdentary jaw bones. This series also demonstrates the evolution of the canine and molar teeth. The evolution of the angular (lower postdentary jaw bone) into a thin flange that encircled an open space then continued to reduce until it ultimately framed the eardrum. Allin (1975) suggested at an early stage the angular framed a very large eardrum. Later, Kemp (2007) thought the postdentary bones were too large and still functioning as jaw bones to allow this, suggesting instead that the eardrum formed in the earliest mammals when the articular became very much more gracile and smaller. The unusual anatomy of the postdentary bones would have supported ventral jaw muscles acting to open the jaws.

The moment of direct contact between the dentary and squamosal marks the origin of the Mammalia and permitted the postdentary bones to continue their evolution to become ear ossicles. Note that even in primitive mammals, like Asioryctes (Fig. 1), the postdentary bones continued a tenuous connection to the dentary that was finally disconnected in eutherian mammals.

It’s interesting to note that even as late as Homo sapiens the mandible was still evolving a chin, a feature not seen in prior taxa, including pre-human anthropoids.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Reichert KB 1837. Über die Visceralbogen der Wirbelthiere im Allgemeinen und deren Metamorphosen bei den Vögeln und Säugethieren. Archiv für Anatomie, Physiologie und wissenschaftliche Medicin: 120-222.

Allin EF 1975. Evolution of the mammalian middle ear. Journal of Morphology 147 (4): 403–437. 

Kemp TS 2007. Accoustic transformer function of the postdentary bones and quadrate of a nonmammalian cynodont. Journal of Vertebrate Paleontology 27(2): 431-441.