Viviparity in lizards

A new paper by Pyron and Burbrink (2013) combines lizard viviparity and lizard phylogeny and finds multiple origins for viviparity…and, multiple reversals to oviparity. The paper also suggests that the basal condition in lizards was oviparity. Only living taxa were tested.

Figure 1. From Wang and Evans 2011, a gravid Cretaceous lizard with 2 embryos. Odd that they are located as high as the forelimb.

Figure 1. From Wang and Evans 2011, a gravid Cretaceous lizard with 2 embryos. Odd that they are located as high as the forelimb, but when you have 15, allowances have to be made.

Earlier, Wang and Evans (2011) produced fossils of a Cretaceous lizard and her embryos (Fig. 1), all 15 of them!

From the Wang and Evans abstract:
“Although viviparity is most often associated with mammals, roughly one fifth of extant squamate reptiles give birth to live young. Phylogenetic analyses indicate that the trait evolved more than 100 times within Squamata, a frequency greater than that of all other vertebrate clades combined. However, there is debate as to the antiquity of the trait and, until now, the only direct fossil evidence of squamate viviparity was in Late Cretaceous mosasauroids, specialised marine lizards without modern equivalents. Here, we document viviparity in a specimen of a more generalised lizard, Yabeinosaurus, from the Early Cretaceous of China. The gravid female contains more than 15 young at a level of skeletal development corresponding to that of late embryos of living viviparous lizards. This specimen documents the first occurrence of viviparity in a fossil reptile that was largely terrestrial in life, and extends the temporal distribution of the trait in squamates by at least 30 Ma. As Yabeinosaurus occupies a relatively basal position within crown-group squamates, it suggests that the anatomical and physiological preconditions for viviparity arose early within Squamata.”

I would hasten add: perhaps not early in pylogeny, but often. Note these yabeinosaurs (Fig. 1) are beneath the rib cage close to the humerus. Moreover, the orientation is not head first toward the cloaca. Evidently it all works out.

We’ve seen fossils of reptiles huddling together in Decuriasuchus, Heleosaurus and others.

We’ve also looked a possible viviparity in mesosaurs. Ichthyosaurs and plesiosaurs are also notable live-bearers. Pterosaurs maintained embryos within the mother until shortly before hatching took place, based on the extreme thinness of the leathery eggshells and the degree of development of known embryos.

References
Pyron RA and Burbrink FT 2013. Early origin of viviparity and multiple reversions to oviparity in squamate reptiles. Ecology letters. doi: 10.1111/ele.12168.
Wang Y and Evans SE 2011. A gravid lizard from the Cretaceous of China and the early history of squamate viviparity. Naturwissenschaften Sept 98(9):739-43.

An embryo Anurognathus

Anurognathus ammoni is a small pterosaur, about the size of the embryo pterosaur found in China (IVPP 13758). So it was with some surprise that I found tiny impressions in the matrix (Fig. 1) that looked like a miniature (6x smaller) version of Anurognathus, still enrolled, apparently expelled from the pelvis during taphonomy.

We know of very few embryo/adult relations in the fossil record and even fewer among pterosaurs.

The Pterodaustro embryo
We know from the Pterodaustro embryo that it was 8x smaller than an adult Pterodaustro. Like the adult, the jaws were very long, putting to rest any hypotheses that imagined a short rostrum on every embryo pterosaur.

The IVPP embryo
The IVPP embryo is not associated with an adult, but using the 8x formula we can surmise what it looked like here. It was originally imagined to be an ornithocheirid embryo with a very short snout. But pterosaurs grow isometrically, not like archosaurs. Moreover, phylogenetic analysis nests it with anurognathids. Detailed tracings of the embryo using DGS also make the case for an anurognathid affinity, even though it is a VERY big anurognathid.

The JZMP embryo
On the other hand, the JZMP embryo is a verifiable long-snouted ornithocheirid, also not associated with an adult. Details tracings using DGS support that nesting.

We’ve also learned that pterosaurs are sexually mature at half their final adult size (Chinsamy et al. 2008), but we’re not sure what that means with regards to egg and embryo relative size.

The Darwinopterus egg
In the Darwinopterus egg, (AMNH M8802, Lü et al. 2011) the eggshell  is clearly marked, but the embryo is immature and poorly ossified, invisible to the original workers, but traced here using DGS.

The Ornithocephalus egg
A possible even smaller embryo tucked into a compact egg shape is found here in one of the very smallest of all pterosaurs, Ornithocephalus.

Back to Anurognathus
So Anurognathus is a small, but not a tiny pterosaur. The embryo is 6x smaller than the adult and appears to be complete and enrolled with the wings and feet breaking out of the imaginary ellipse that surrounds the rest of the bones. None of these embryonic bones are ossified or differently colored. They are slight impressions and hard to see until you start colorizing them (Fig. 1). This poor ossification is not unexpected as many of the bones in the adult are likewise poorly ossified or incompletely prepared.

Figure 1. Possible Anurognathus embryo, isometrically one-sixth the size of the adult. Wing bones are not identified here.

Figure 1. Click to enlarge. Possible Anurognathus embryo, isometrically one-sixth the size of the adult. Wing bones are not identified here.

For an illusion,
the embryo Anurognathus matches the adult pretty well. It is curled up like an embryo would be and would have fit into a broad elliptical egg shape. The size, at one-sixth the size of the adult, is larger than the Pterodaustro embryo at one-eighth the size of the adults. The egg-shape could have passed through the pelvis identified here (Fig. 1). The displacement of the adult tail matches the displacement of the embryo from the pelvic opening.

I encourage anyone interested in details to click on figure 1 to see a larger rollover image that shows the in situ fossil on one level and the interpretation on the other. Perhaps this is the only way to see the bones that are, admittedly, barely visible.

Side note
I saw the embryo had a longer neck before I realized the adult had a long neck. That observation led to further study chronicled earlier. The small orbit, as in other pterosaurs, is in the back of the skull, behind the large antorbital fenestra. This is in contrast to the Bennett version of Anurognathus discussed here and here.

No egg shell that I can tell
There is no indication of an egg shell here, but then again, pterosaur egg shells of larger specimens are among the thinnest of all reptiles. In three prior larger pterosaurs (IVPP embryo, JZMP embryo, Pterodaustro embryo) the adult and the egg shell are both fairly well-ossified. In the Darwinopterus egg, the egg is clearly marked but the embryo is immature and poorly ossified, invisible to the original workers.

What this mother/embryo relationship tells us
Intent on avoiding any association with lizards, previous interpretations of pterosaur eggs and embryos had them laid under rotting heaps of vegetation (Lü et al. 2011), the way certain superprecocial birds do in the present*, rather than having the eggs retained by the mother until just before hatching, as many lizards do**. Darwinopterus was the first instance of a mother and embryo, even though the original workers did not see the embryo, only the eggshell. So Anurognathus is the second instance of same, but this time, with the embryo, not the eggshell. As lizards, mother pterosaurs kept their embryos with them, until just before hatching from the thinnest of egg shells, contra traditional thinking.

The Anurognathus embryo is further evidence that pterosaurs were able to fly out of their eggs, but they would have remained in their eggshells for only minutes or hours, like certain lizards, not weeks, like birds or crocs.

The Anurognathus embryo argues against pterosaurs producing clutches of eggs, but rather one at a time. No one has ever found two pterosaur eggs in the same area.

Independent? 
It is intriguing to wonder whether the mother pterosaur providing care for the new embryo by allowing it to cling to her for the first few days, or whether the embryo was on its own from the get-go. Evidence for the former would come from an adult/infant relationship preserved as a fossil without the enrolling that characterizes the embryo within the eggshell.

It would be worthwhile to keep looking for enrolled tiny skeletons near the pelves of other pterosaurs. This one was overlooked for 90 years.

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.

*NewScientist.com reports, “Unwin suggests that pterosaurs, like modern reptiles, may have buried many small eggs in ground where moisture could seep through their parchment-like eggs during incubation, nearly doubling their mass before hatching.” 

** Ovoviviparity: this is oviparity with retention of zygotes in the female’s body or in the male’s body, but without major trophic interactions between zygote and parents (there may be minor effects, such as maintenance of water and oxygen levels). Anguis fragilis is an example of ovo-viviparity. (from Wikipedia)

References
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Döderlain L 1923Anurognathus ammoni, ein neuer Flugsaurier. Sitzungsberichte der Königlich Bayerischen Akademie der Wissenschaten, zu München, Mathematischen-physikalischen Klasse: 117-164.
Lü J, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011a. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323

wiki/Anurognathus

Tetrapod Eggshell Trends

Amniotes (= reptiles) produce different sorts of eggs (soft, flexible, rigid). Some are laid shortly after fertilization (most lizards, turtles, birds and crocs). Others are retained within the mother until just before hatching. Still others “hatch” within the mother and give way to live birth (mammals, several distinct clades of lizards, enaliosaurs, choristoderes).

Most fossil taxa are not also known from their eggs. However phylogenetic bracketing (Fig. 1) permits us a good guess as to what sorts of eggs they produced if we know what sort of eggs some clade members produced.

Eggshell trends in reptiles.

Figure 1. Click to enlarge. Eggshell trends in reptiles. Here the different types of shells permit phylogenetic bracketing of taxa for which no eggs are known. Apologies, this is not the most recent reptile tree.

Eggshells
From the Introduction to eggshells (Berkeley): “Most lizards, snakes, and tuataras lay soft eggs composed of an organic framework and poorly organized calcite crystals. These eggs collapse and shrivel after the animal hatches, and are therefore unlikely to be identified or even preserved in the fossil record.

“Many amniotes, including some lizards, snakes, and turtles, lay eggs with flexible shells. These shells differ from soft shells because of their higher mineral content. Nevertheless, preservation of flexible eggs is also rare in the fossil record.

“Some turtles and geckos, and all crocodilians, dinosaurs, and birds lay eggs with rigid eggshell. The calcite crystals form a relatively thick eggshell of interlocking shell units. Fossilization is more likely to occur in rigid eggshell because the crystalline calcium carbonate (calcite or aragonite) layer is stronger, more durable, and does not shrivel upon hatching.”

In summary, turtles lay all three kinds of eggs. Mammals, lizards and snakes lay eggs with flexible shells or produce live young. Only the more derived reptiles (crocs and dinosaurs including birds) produce hard-shelled eggs. Extinct marine reptiles of all sorts (probably except prehistoric turtles) produced live young.

Reptilian eggs can be separated into three separate groups based on the structure of their shell structure:
1. flexible shells with no calcareous layer
2. flexible shells with a thick calcareous layer, and
3. rigid shells with a well-developed calcareous layer.

Squamates mainly lay eggs with flexible shells. All squamate eggs are flexible except for two subfamilies of gekkos. Some flexible shells contain no calcareous layer, only a shell membrane. This fibrous membrane is comprised of an irregular series of ridges and a finely woven mat of fibers on the outer surface.

Some turtles also have flexible eggs
with a calcareous layer as thick as the shell membrane. Sea turtle eggs have a poorly ordered, open matrix calcareous layer with undefined shell components and pores. In contrast, in emydids and chelydrids (turtle clades) the calcareous layer is highly structured with well-defined components and pores.

Rigid-shelled Eggs
produced by crocodilians, some turtles, dibamids and gekkonids have a well-developed calcareous layer that makes up most of the eggshell and a thin shell membrane. The shell units fit together tightly and interlock.

Monotreme (Mammal) Eggshells 
are soft, leathery and flexible, lacking mineralization and without a calcareous covering. Monotremes (and all mammals) are descendants of extremely tiny early mammals. Their tiny adult size affected the tiny size and composition of the eggs of subsequent mammals.

Unfortunately we know of no pelycosaur or therapsid eggs yet. We have no basal reptile eggs from the Lepidosauromoph branch either. Our best guess is they were leathery.

Enaliosaurs
The marine reptiles (ichthyosaurs, plesiosaurs, nothosaurs, thalattosaurs, placodonts, mesosaurs) were all  live-bearing. There was a recent discovery of Early Permian (280 mya) mesosaur embryos curled into egg shapes within the mother and almost the size of nearby hatchlings. Mesosaurs were either viviparous or they laid eggs ready to hatch in days or minutes, which would have been a precursor to live birth. There is no trace of any eggshell.

EarlyPermianReptileEgg

Figure 2. Size of the earliest known Permian Egg, full scale at 72 dpi. The mesosaur egg was similar in size.

Oldest Reptile Egg
Previous to the mesosaur egg, Romer and Price (1939) described the oldest reptile egg (Admiral Formation Artinskian, 280 mya) based on shape, size, shell-like cracks and chemistry. The internal structure of the nodes and the pattern of their distribution, the type of layering and the microstructure are not like those found in calcareous eggshells.  Hirsch (1979) determined that the reptilian egg was “soft-shelled.”

An Early Archosauriform Egg
Hyphalosaurus, an aquatic choristodere (within the Archosauriformes), had a thin, leathery egg and the embryo was retained within the mother until just prior to hatching. Note that choristoderes are sisters to Proterochampsids and Parasuchians within the Parachosauriformes. Does this mean hard-shelled eggs were restricted to the Archosauria (birds and crocs)? It could.

Pterosaur Eggs
The extreme thinness of pterosaur eggs is at odds with the thick calcareous eggshells of crocs and birds, their putative kin in traditional palaeontology. Their eggs alone tell us that pterosaurs are lizards, as their morphology and phylogeny confirms. The mother carried the egg until just prior to hatching as three full term embryos and one poorly ossified expelled egg from Darwinopterus.

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
Hirsch KF 1979. The oldest vertebrate egg? Journal of Palaeontology 53(5): 1068-1084.
Romer AS and Price Li 1939. The Oldest Vertebrate Egg, Peabody Museum, Yale University, New Haven, Connecticut, American Journal of Science, Volume 237:  826-82.
Hou LH, Li, PP, Ksepka DT, Gao K-Q and Norell MA 2010. Implications of flexible-shelled eggs in a Cretaceous choristoderan reptile. Proceedings of the Royal Society B, 277(1685): 1235-1239. 10.1098/rspb.2009.2035.
Ji Q, Ji S, Lü J, You H and Yuan C 2006. Embryos of Early Cretaceous Choristodera (Reptilia) from the Jehol Biota in western Liaoning, China. Journal of the Paleontological Society of Korea, 22(1): 111-118.
Ji Q, Wu X-C and Cheng Y-N 2010. Cretaceous choristoderan reptiles gave birth to live young. Naturwissenschaften, 97(4): 423-428.  doi:10.1007/s00114-010-0654-2.
Sander PM 2012. Reproduction in Early Amniotes.Science 337, 806.

More Modern Folklore on Pterosaur Growth Patterns

A recent post by blogger Dr. David Hone described pterosaur growth patterns (= ontogeny) considering those patterns to be very much like those of mammals and archosaurs (=birds and crocs) in which the embryo/hatchling skull is shorter with big eyes, the bone texture is grainy, the sutures are clearly visible, etc.  This is the traditional view upheld by nothing more than tradition.

We’ve covered the alternative view, but since this bit of false propaganda hit the ‘Net (again!) its worthwhile to show the data once again. I have abridged Hone’s remarks to save space. The complete text is linked above.

If you have a pterosaur specimen in front of you, just how do you know if it’s an adult or not?

1. if you have a pterodactyloid with a 20 cm wingspan then it’s going to be a juvenile, and likewise if you have a rhamphorhynchoid coming in close to the 2 m mark it’s very unlikely to be anything but a big adult.

We’ve seen that embryo and juvenile pterosaurs are virtual copies of adults. And this growth trait goes back to the ancestral tritosaur lizard, Huehuecuetzpalli. Moreover, tiny (non-embyro) pterosaurs don’t nest with adults but with other small pterosaurs at the bases of major clades. No one has phylogenetically matched small pterodactyloids to 2x-8x larger adults yet (other than Pterodaustro and Ptweety). Every time that happens, then you’ll have ontogeny demonstrated – and its isometric rather than the traditional allometric.

2. Young animals (and especially very young animals) tend to have big heads compared to their body and especially very big eyes compared to the size of the head.

Pterodaustro embryo

Figure 1. Pterodaustro embryo. There certainly is no short snout/large eye here!

But you don’t find that in juvenile pterosaurs like Pterodaustro (Fig. 1) and the JZMP specimen. Only the IVPP specimen had a short snout, but so did all of its sister taxa among the anurognathids (not the ornithocheirids as originally reported). Only small adults related to other not so small adults with big eyes, especially in the Scaphognathus clade.

3. A bunch of fusions are absent in young pterosaurs that are present in adults too, just as you’d expect for most animals. The sutures between the centrum and neural arch of the vertebrae will be open in juveniles and closed in adults, and similarly the elements of the pelvis and sacrum, and the scapula and coracoid will be separate in young animals and fused together in adults.

a) Haven’t seen anything yet on the centrum/neural arch suture in tiny pterosaurs. If you have, please send references and we’ll cover that later. b) The fusion of the pelvis/sacrum and the scapula/coracoid follows phylogenetic patterns, not ontogenetic ones. Maisano (2002) covers this very well as in lizards fusion may occur long before growth ceases or it may never occur, as in certain clades of large adult ornithocheirids and ctenochasmatids. One of the largest pteranodontids, YPM 2501, lacks fusion in the extensor tendon process.

4.  Very young pterosaurs also tend to have a very grainy texture to the surfaces of their longbones, despite the fact that even embryonic pterosaurs have a pretty ossified set of bones (unlike many young animals).

Interesting conundrum. Embryos well-ossified, but tiny pterosaurs grainy? We earlier discussed the probable short, fast-growth life of tiny pterosaurs, as in other tiny animals (live fast, die young). These tiny adults were rarely smaller than known embryos, all of which would grow to become 8x larger adults that likely lived longer multi-year lives. The grainy texture reflects this fast growth. Tiny pterosaurs, with such little mass as adults, and even less growing up, don’t require the same ossification that larger pteros do.

5. Smaller pterosaurs also tend to have various parts of the skeleton being less ossified and rather amorphous compared to those of adults. The tarsals are often not well ossified and can be missing (well don’t preserve) and if present may be very simple shapes. The carpals tend to look more ‘blobby’ and lack the detailed morphology seen in adults and will be separated into multiple elements whereas in adults the wrist will primarily be formed of just two massive elements (plus the pteroid).

There’s no doubt certain clades of tiny pterosaurs have embryonic proportions, but other clades of tiny pterosaurs don’t. Wellnhofer (1970) described missing tiny pedal elements (p3.2, p4.2 and p4.3) in certain tiny pterosaurs, but these were displaced. I found them off to the side. With regard to the carpals, I wonder if some of this lack of detail could be due to geological factors as the joints tend to accumulate obscuring minerals? Often the proximal carpals will not fuse. This can be seen in the presumed adult Germanodactylus cristatus. The distal carpals are not fused in the rather large ornithocheirid Zhenyuanopterus (I drew them fused, my mistake). On the other hand the tiny pterosaur TM 10341 has syncarpals.

6. Rather like birds, in adult pterosaurs the sutures all but disappear, or even go entirely, such that the skull looks like a single smooth piece of bone.

This happens in some clades. Not others. Crushing makes it difficult to determine sutures from cracks. And pterosaurologists are famous for ignoring sutures in their reconstructions! You’ll see more sutures at reptileevolution.com.

7. Also as in some birds, bigger pterodactyloids have a notarium and this only fuses up and fully develops in adults.

A notarium develops in certain clades and only in the very largest pterosaurs within those clades. So a notarium is both a size and a phylogenetic trait, not an ontogenetic trait. No medium-sized Triassic and Jurassic adult pterosaurs have a notarium.

8. Similar to the point above about absolute size, the presence and development of some form of head crest is indicative, but not a great indicator of age. Yes a massive and elaborate crest in an animal is indicative that it’s an adult, but there could be a fairly well developed crest in an animal that is close to becoming and adult and of course there are taxa without crests and in at least once case it appears that females don’t have crests.

Only one embryo/juvenile crested pterosaur is known in which the crest is preserved and its crest (what we know of it) is just as large as the adult. So, at present there’s no indication that long crested pteros produced long-crested embryos and its difficult to imagine this in an elongated egg. Likely crests developed rather quickly after hatching with present data. We await the data on this interesting question. It’s certainly not set in stone yet.

7. As in mammals, but unlike dinosaurs and birds, pterosaur also have epiphyses. The growing plates at the ends of the long bones physically separate the main shaft of the bone from the proximal and distal ends, so things like the femur can appear to be in three pieces. Obviously as growth slows towards maturity these epiphyses slowly disappear as they fuse into the single element that you would expect to see.

Lizards also have epiphyses. Hmmm. Wonder why this was not mentioned? In any case, I have not seen epiphyses in any pterosaurs nor have I seen epiphyses in the three well-known embryos. Please send references if available. The epiphyses are seen in living lizards.

Summary: As ever with such things these are not absolutes, but merely guides. Good guides, certainly – you simply won’t see a notarium in a very young pterosaur, or open neurocentral arches in a big, old adult. However, in terms of determining more subtle difference in age it will be tricky – one animal may have fused up the notarium, but may have incompletely ossified tarsals and another could have the reverse. Although at least some characters do seem to have a bit of a pattern (the scapulocoracoid seems to fuse pretty early in most things) a general lack of numerous specimens of different ages makes it hard to do any more detailed analysis. Still, in terms of gross age (hatchling – young – adolescent – adult) even for a specimen of a previously unknown species with no obvious close relatives, it should be relatively easy to determine the approximate age of the animal.

Yes, there are exceptions. And when you put enough pterosaurs into a phylogenetic analysis, then the exceptions start to form phylogenetic patterns. That needs to be done by someone other than yours truly, and without excluding the tiny ones. Let’s find out if they are indeed juveniles, as Dr. Hone reports without a corresponding analysis, or if they are tiny adults, as the phylogenetic analysis here indicates. Pterosaurs are more interesting, it turns out, than we thought.

There’s also a difference between ‘show’ and ‘tell.’ Next time someone tells you the tiny Solnhofen pterosaurs are juveniles, ask him to match them up with their putative adults, if possible. Perhaps predicting frustration may be why nearly all pterosaurologists keep away from the tiny ones. There’s a PhD thesis here for the lucky grad student.

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
Maisano JA 2002. Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrae Paleontology 22: 268–275.

Mesosaurus Embryos!

Good friend, Dr. Graciela Piñeiro et al. (2012) just described the oldest known amniotic embryos. They belong to mesosaurs (Gervais 1865). From their abstract: “The earliest undisputed crown-group amniotes date back to the Late Carboniferous, but the fossil record of amniotic eggs and embryos is very sparse, with the oldest described examples being from the Triassic. Here, we report exceptional, well preserved amniotic mesosaur embryos from the Early Permian of Uruguay and Brazil. These embryos provide the earliest direct evidence of reproductive biology in Paleozoic amniotes. The absence of a recognisable eggshell and the occurrence of a partially articulated, but well-preserved embryo within an adult individual suggest that mesosaurs were viviparous or that they laid eggs in advanced stages of development. Our finds represent the only known documentation of amniotic embryos in the Paleozoic and the earliest known case of viviparity, thus extending the record of these reproductive strategies by 90 and 60 Ma, respectively.”

Embryo Mesosaurus

Figure 1. Embryo Mesosaurus curled up within its ellipsoid amniotic membrane, but no egg shell was preserved. Left: specimen; Middle: tracing of specimen; Right: restoration of specimen.

The embryo was ~10% the size of an average adult and coiled as if in an ellipsoid egg. The snout was relatively short. The head was relatively large. Despite the elliptical shape of the embryos, no shell was preserved. Some embryos were found within their mother. Only one and rarely two were carried at a time. These data support the large reptile family tree that recovered a mesosaur/thalattosaur/ichthyosaur relationship in which ichthyosaurs are known to exhibit live birth (viviparity) emerging from their amniotic sac prior to birth.

Lizards typically carry the embryo within the uterus for extended periods. Many exhibit viviparity, but lizards and mesosaurs are not related.

Ichthyosaurs and sauropterygians also exhibit viviparity and are closer to mesosaurs. All three are distantly related to both therapsids (basal mammals lay eggs) and archosaurs (both birds and crocs lay eggs). So viviparity in this clade seems to have had its genesis in mesosaurs.

There’s more big news on mesosaurs to come.

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
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Piñeiro G, Ferigolo J, Meneghel M and Laurin M 2012. The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology: An International Journal of Paleobiology, DOI:10.1080/08912963.2012.662230

A Closer Look at the IVPP Pterosaur Egg and Embryo

The IVPP 13758 pterosaur egg and embryo were the first to be described and illustrated (Wang and Zhou 2004, Fig 1 far left and near left). Unfortunately the tracing of the specimen was crude, the reconstruction was off and the identification was mistaken.

The IVPP embryo pterosaur

Figure 1. Click for more information. The IVPP embryo pterosaur (far left) as originally traced, (near left) as originally reconstructed as a baby ornithocheirid, but with a very short rostrum, (near right) traced using the DGS method, (far right) adult reconstructed at 8x the embryo size.

A new tracing (Fig. 2) was made without seeing the specimen, but rather employing DGS (digital graphic segregation) based on two published photographs of the plate and counterplate of the split fossil digitally layered and rematched to each other. No other technique permits the plate and counterplate images to coincide as originally found. Every element of the skeleton was identified, all symmetrical elements match and the specimen shares a suite of traits with sister taxa.

the IVPP egg/embryo

Figure 2. Click to enlarge. A magnitude of more detail was gleaned from this fossil (the IVPP egg/embryo) using the DGS method. The jumble of bones is difficult to deal with, but the DGS method enables the segregation of certain bones from one another. Upper left: wings. Upper right: legs and feet. Lower left: skull, pelvis and axial elements. Lower right: Sternal complex, pectoral girdle, palatal elements.

What Is It?
The IVPP pterosaur embryo was misidentified as an ornithocheirid under the old archosaur precept that hatchlings would have a short rostrum and large eyes. The IVPP embryo does have a short rostrum but that’s because it’s a basal anurognathid, all of which have a short rostrum at the adult stage. The eyes were actually no larger than those of sister taxa.

The Anurognathidae to scale.

Figure 1. Click to enlarge. The Anurognathidae to scale.

It’s BIG Baby!
To add a twist to this puzzle, this embryo is the size of other adult anurognathids. That means the adult, at 8x larger (based on pelvic opening and eggshell diameters) was several times larger than the largest commonly acknowledged anurognathid. We don’t have an adult of this anurognathid, but we do know from Pterodaustro, Pteranodon, Tupuxuara and Darwinopterus that the hatchlings and juveniles were virtually identical to adults in all pterosaurs, crests and all.

The IVPP embryo

Figure 5. The IVPP embryo scaled to an adult size (based on matching the egg to the pelvic opening diameter, along with various views of the skeleton and an egg and hatchling.

Now to Describe This New Anurognathid
The skull is unknown in the three more primitive sister taxa. Distinct from Dimorphodon, the skull of the IVPP embryo had a smaller naris but was otherwise quite similar. The postorbital was larger. The dentary teeth were relatively larger. The cervical neural spines were shorter. Approximately 10 vertebrae were sacrals. The caudals were reduced to a vestige. The sternum was rectangular. The scapula was shorter. The coracoid ventral stem was not expanded. The metacarpus was longer than in sister taxa. Manual 4.4 was relatively shorter. The pelvis was relatively larger. The ischium and pubis was notched and the ischium was broad. The hind limb and especially the tibia was longer. Pedal 5.1 did not extend to the tip of metatarsal IV.

A Derived Species of a Primitive Clade
The IVPP embryo was a basal protoanurognathid (if Dendrorhynchoides is considered the basalmost anurognathid), yet it survived into the Early Cretaceous. It retained an elongated Dimorphodon-like premaxilla that was V-shaped in palatal view, but it also had a rather elongated metacarpus, whereas most other anurognathids, like Batrachognathus, shortened theirs. The IVPP embryo also had an elongated neck, more like Dimorphodon than Dendrorhynchoides, Peteinosaurus or any of its closests sisters. Thus, the elongated neck, like the elongated metacarpus, appears to be derived. The relative brevity of the toes is matched only by D? weintraubi, another larger-than-normal anurognathid from the Early Jurassic of North America.

And the Eggshell
The eggshell was thinner than in any archosaur or archosauromorph (Deeming and Unwin 2007) and most like that of lizards able to retain the embryo until just before hatching. All current evidence indicates that pterosaur mothers carried their single embryos until just prior to hatching. How ridiculous would it be to expect humming-bird-sized to fly-sized tiny hatchling pterosaurs to crawl through sand or mud after being buried prior to hatching, as Deeming and Unwin (2007) and Grellet-Tinner et al. (2007) proposed under the disproven archosaur paradigm (Bennett 1996b). Tiny pteros were not as tough as little turtles and crocs. Think of their fragile wing membranes.

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
Deeming C and Unwin DM 2007. Eggshell structure and its implications for pterosaur reproductive biology and physiology. Flugsaurier, the Wellnhofer pterosaur meeting, Munich, 12.
Grellet-Tinner G, Wroe S, Thompson SB and Ji Q 2007. A note on pterosaur nesting behavior. Historical Biology 19:273–277.
Wang X-L and Zhou Z 2004. Palaeontology: pterosaur embryo from the Early Cretaceous. Nature 429: 623.

The Family Tree of the Pterosauria 17 – The Ornithocheiridae part 1 of 3

We just looked at one branch of descendants from Scaphognathus (No. 110) and Gmu 10157 that ultimately produced Cycnorhamphus and Feilongus. Here we look at the other branch of Scaphognathus No. 110 descendants, the Ornithocheiridae in three parts. Part 1 (below) will look at basal taxa. Part 2 will look at Coloborhynchus, Istiodactylus and their kin. Part 3 will look at more derived taxa such as Anhanguera and Liaoningopterus.

The Ornithocheiridae.

Figure 1. The Ornithocheiridae. Click to enlarge and expand.

We’ll start with Yixianopterus
Yixianopterus jingangshanensis JZMP-V12 (Lü et al. 2006) ~20 cm skull length, Barremian/Aptian Early Cretaceous ~125 mya, was overall 8x larger than and distinct from it tiny phylogenetic predecessor, Gmu-10157. The skull of Yixianopterus was longer judging by the pre-antorbital fenestra portion and the mandible. The teeth were more widely spaced. The caudals were shorter. Fingers 1-3 were smaller, but the wing finger was much more robust. Manual 4.1 approached the elbow when folded and the wingtip was higher than the skull when quadrupedal. The pelvis and tibia were more robust.

JZMP embryo

Figure 2. Click to enlarge DGS tracings. The JZMP ornithocheirid embryo, in situ and reconstructed.

We Haven’t Met the Adult Yet, But We Know This Embryo
The JZMP pterosaur embryo JZMP-03-03-2 (Ji et al. 2004) was found inside an eggshell, so we know it’s age precisely: zero. Considering the size of its pelvic opening one can estimate the size of the adult at 8x larger, which is consistent with Pterodaustro and its embryo/hatchling. The hypothetical adult size is also consistent with sister taxa. The embryo was originally compared to Beipiaopterus. Distinct from Yixianopterus, the skull of JZMP-03-03-2 was deeper anteriorly with an upturned premaxilla in which all of the premaxillary teeth were oriented chiefly anteriorly. The dentary was downturned at the tip. The antorbital fenestra was larger.The cervicals were longer posteriorly and shorter anteriorly. The sacrals were as long as the dorsals. The sternal complex was a wide rectangle with a transverse leading edge and a short cristospine. The scapula and coracoid were robust and oriented more laterally. The humerus was relatively smaller. The metacarpus was subequal to the ulna. The wing finger was robust proximally, but less so distally. Both m4.2 and m4.3 were longer than m4.1. The anterior ilium was much longer than the posterior process. The femur was shorter and the tibia was relatively longer. The pes was similar in size to that of Yixianopterus.

Note the long rostrum and small eye, as in the embryo of Pterodaustro. All of the small pedal bones were ossified. These facts falsify the hypothesis of pterosaur allometric growth (Wellnhofer 1970, Bennett 1991, 1992, 1994, 2001) and support the isometric hypothesis in which embryos and juveniles were almost identical to adults in morphology.

From Lebanon, a Nameless Pterosaur
The Lebanon ornithocheirid MSNM V 3881 (Dalla Vecchia, Adruini & Kellner 2001) A small, robust wing from Lebanon has a radius less than half the diameter of the ulna and manual digit 2 is subequal to 3. At present there is little else to distinguish it from Haopterus, except that it had a longer metacarpus relative to the ulna. The humerus, although incomplete, was small, as in the JZMP embryo.

The First Classic Ornithocheirid
Boreopterus cuiae JZMP 04-07-3 (Lü and Ji 2005) Distinct from the JZMP embryo, the skull of Boreopterus had at least 27 teeth in each upper jaw. They were long, slender and closely spaced. The rostrum was relatively longer and lower with a larger portion anterior to the antorbital fenestra. The postorbital portion was reduced with a posteriorly leaning orbit, as in Istiodactylus. The suborbital skull descended and the quadrate leaned posteriorly. The cervicals were longer with higher neural spines. The sacrals were shorter by more than half. The caudals were more robust. The humerus was larger, extending nearly to the acetabulum. The ulna and radius were also larger relative to the metacarpus. Fingers I-III were smaller. When folded the wing tip was no taller than the skull. The distal wing phalanges were shorter. The pelvis was tiny. The hind limb was more gracile, inluding a tiny foot.

Haopterus

Figure 3. Click to enlarge. Haopterus, the smallest ornithocheirid

Haopterus
Haopterus gracilis IVPP V11726 (Wang and Lü 2001) was overall smaller than and distinct from Boreopterus, the skull of Haopterus was shorter and relatively taller. The cervicals, dorsal, sacrals and caudals were all shorter. The scapula and coracoid were shorter. The humerus was extremely roubst with a deltopectoral crest extending for ~33% of the length. As in the Lebanon ornithocheirid, the radius and ulna were relatively short. Manual 4.1 approached the elbow. The relatively longer wing would have extended far above the head when folded. The pelvis was gracile and smaller. The femur was shorter. The metatarsals were shorter. Ornithocheirids, like Haopterus, were evidently spending more time in the air and less on the ground, judging by their wing/leg proportions.

Zhenyuanopterus

Figure 4. Click to enlarge. Zhenyuanopterus

Zhenyuanopterus
Zhenyuanopterus longirostris (Lü et alk. 2010) GLGMV 0001 Early Cretaceous. Distinct from Boreopterus, the skull of Zhenyuanopterus was longer, especially in the pre-antorobital fenestra region. The teeth were more widely spaced and continued erupting closer to the orbit, which was smaller. A squarish crest surmounted the mid rostrum. The cranium was crest-like, probably for muscle attachments. The cervicals were more robust. The torso was smaller and shallower, as in Haopterus. The caudals were more robust. The sternal complex did not have lateral ‘wings’. The scapula and coracoid were fused. The coracoids were laterally oriented. The humerus was as long as the torso. The ulna and radius were more robust and relatively shorter. The hind limbs were longer, as in Haopterus. The feet were extremely tiny with robust metatarsals and slender digits.

 

Arthurdactylus dorsal view.

Figure 5. Click to enlarge. Arthurdactylus dorsal view.

Arthurdactylus from South America
Arthurdactylus conandoylei 
(Frey and Martill 1994) SMNK 1132 PAL Early Cretaceous. Distinct from Zhenyuanopterus, the torso of Arthurdactylus was deeper, as in Boreopterus. The sacrals were all unfused. The caudals were vestigial. The coracoids were much longer than the scapula, producing a very high shoulder joint. The ulna was massive. Manual 4.1 approached the elbow when folded. The short pubis was directly beneath the actebulum. The ischium was slender. The foot was slightly larger than in Zhenyuanopterus with more slender metatarsals and longer digits.

In summary
Taxa at the base of the Ornithocheiridae are those closest to cycnorhamphids in their morphology. Yixianopterus is at the base followed by the JZMP embryo. Due to isometric growth in pterosaurs we can enlarge it eight times to gauge what the adult was like. A trend toward a longer snout, more and longer teeth, larger wings and smaller feet is apparent.

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:
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 1992. Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Bennett SC 1994. Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occassional Papers of the Natural History Museum University of Kansas 169: 1–70.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153
Dalla Vecchia FM, Arduin P and Kellner AWA 2001. The first pterosaur from the Cenomanian (Late Cretaceous) Lagerstätten of Lebanon. Cretaceous Research 22: 219-225.
Frey E and Martill DM 1994. A new Pterosaur from the Crato Formation (Lower Cretaceous, Aptian) of Brazil. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 194: 379–412.
Ji Q, Ji S-A, Cheng Y-N, You HL, Lü J-C, Liu Y-Q and Yuan CX 2004. Pterosaur egg with leathery shell. Nature 432:572.
Lü J 2010. A new boreopterid pterodactyloid pterosaur from the Early Cretaceous Yixian Formation of Liaoning Province, northeastern China. Acta Geologica Sinica 24: 241–246.
Lü J and Ji Q 2005. A new ornithocheirid from the Early Cretaceous of Liaoning Province, China. Acta Geologica Sinica 79 (2): 157–163.
Lü J, Ji S, Yuan C, Gao Y, Sun Z and Ji Q 2006. New pterodactyloid pterosaur from the Lower Cretaceous Yixian Formation of Western Liaoning. In J. Lü, Y. Kobayashi, D. Huang, Y.-N. Lee (eds.), Papers from the 2005 Heyuan International Dinosaur Symposium. Geological Publishing House, Beijing 195-203.
Wang X and Lü J 2001. Discovery of a pterodactylid pterosaur from the Yixian Formation of western Liaoning, China. Chinese Science Bulletin 46(13):1-6.

What Do Those Pterosaur Embryos Really Look Like?

In this blog you’re going to see the benefits of using DGS (Digital Graphic Segregation) a technique of tracing high resolution digital images on a computer monitor without the specimen at hand. This is widely considered to be inferior to first-hand observations using a microscope, pencil and camera lucida. However one method has not been tested against another, until now. The results speak for themselves.

I know of five pterosaur eggs with embryos inside. Four have been published. Three were reported to contain embryos. Let’s look at them one at a time. Image links will take you to individual taxon pages on reptileevolution.com. Embryo pterosaurs trapped inside their eggshells are exciting subjects because: 1) we can expect all of their bones to be present and 2) we know that each specimen was exactly zero years old.

1. The IVPP specimenIVPP V13758 (Wang and Zhou 2004, Figs. 1, 2) Early Cretaceous, ~125 mya, was the first pterosaur embryo to be published and it was originally considered to be a baby ornithocheirid, like the JZMP embryo. Using DGS to trace and reconfigure the bones into a standing reconstruction, the IVPP embryo turns out to be an anurognathid, but every bit as large as virtually all other adult anurognathids! Except one.

The IVPP embryo pterosaur

Figure 1. Click to enlarge DGS tracing. The IVPP embryo pterosaur (far left) as originally traced, (near left) as originally reconstructed as a baby ornithocheirid, (near right) traced using the DGS method, (far right) adult reconstructed at 8x the embryo size.

The only other anurognathid in the same size category as the IVPP hypothetical adult is a Mexican specimen mistakenly assigned to the genus Dimorphodon, but under the species D. weintraubi. Only its limbs are known and in cladistic analysis it is a sister to the IVPP embryo.

The IVPP embryo

Figure 2. The IVPP embryo scaled to an adult size (based on matching the egg to the pelvic opening diameter, along with various views of the skeleton and an egg and hatchling.

The IVPP embryo has a longer neck than most anurognathids and a longer metacarpus. The humerus is quite a bit shorter. Perhaps this will turn out to be a juvenile trait in this clade. The IVPP embryo is the most primitive clade member with a short tail, but it is also among the latest members to appear chronologically. Apparently it is a late relic. Learn more here.

The JZMP ornithocheirid embryo

Figure 3. Click to enlarge DGS tracings. The JZMP ornithocheirid embryo, in situ and reconstructed.

2.The JZMP embryoJZMP-03-03-2 (Ji et al. 2004, Fig. 3) was the second pterosaur embryo described from China. It was considered close to Beipiaopterus, but it is a basal ornithocheirid close to Haopterus. Here again the humerus was atypically small when compared to those of adult sister taxa, but otherwise the embryo had full size wings and adult proportions, including an elongated, tooth-filled rostrum and small eyes. Here the dorsal vertebrae were separated from the sacral vertebrae and the legs were disarticulated, so this egg was shaken or rolled before it was buried. Learn more here.

Pterodaustro embryo

Figure 3. Pterodaustro embryo. There certainly is no short snout/large eye here!

3. The Pterodaustro embryo MHIN-UNSL-GEO-V246 (Chiappe et al. 2004) was the first embryo found in association with adults and other juveniles of various sizes. The Pterodaustro egg was longer and narrower than the others and no wonder — it had to contain that elongated upturned rostrum! Distinct from the Chinese embryos, the Pterodaustro embryo had a relatively larger humerus and antebrachium (forearm), but it had a relatively smaller metacarpus. The sternal complex was also slightly larger. The details of the embryo in situ will be shown when it is published by its discoverers. Rather than estimating adult size from making comparisons to egg and pelvic opening, with Pterodaustro we have direct evidence for an 8x larger adult associated with an embryo. Learn more here.

Darwinopterus mother and premature embryo

Figure 4. Darwinopterus mother and premature embryo. Click to see in situ tracing.

4. The Darwinopterus egg/embryo – After several specimens of the germanodactylid, Darwinopterus, were published, one  (AMNH M8802) was reported (Lü et al. 2011) with an egg between its legs, evidently just expelled from the pelvis before or during burial. Originally no embryo was reported present in the egg, but the DGS method enabled the tracing of virtually all the articulated, but poorly ossified bones of the less than full term embryo. A reconstruction closely resembled the mother. Learn more here.

Ornithocephalus pterosaur egg.

Figure 5. Ornithocephalus pterosaur egg. Click to see in situ specimen.

5. The Ornithocephalus embryo – (Soemmerring 1812-1817) Pterodactylus micronyx von Meyer 1856, No. 29 in the Wellnhofer (1970) catalog). Ornithocephalus was the second pterosaur ever described. The tiny size of the specimen and its short snout immediately earned it juvenile status in the eyes of every paleontologist who saw it. Unfortunately, for that hypothesis, Ornithocephalus nests with other tiny pterosaurs in a transitional series from Scaphognathus to the bases of Pterodactylus and Germanodactylus (more on this in future blogs). Between the femora of this pterosaur is an elongated egg-like structure with an apparent embryo inside. This observation needs to be tested because this egg/embryo has not been published and confirmed. I happened upon it when I was tracing the bones of the specimen. Like Darwinopterus, the Ornithocephalus egg was apparently discharged while the mother lay motionless. Learn more here.

Immediately able to fly?
The size of pterosaur eggs is key to understanding the abilities of the hatchlings to fly–or not. The IVPP egg was ~50 mm in length. The JZMP egg was ~60 mm in length. The Pterodaustro egg was ~60 mm in length. The snout/vent length of each emerging hatchling would have been greater than these measurements because the pterosaur was tucked in with its snout down prior to hatching. These examples all fit in the category of: “hatchlings ready to fly” because the embryos equalled or exceeded the sizes of many tiny adult pterosaurs.

Please consider that living lizards with a snout/vent length under 18mm (Hedges and Thomas 2001) dry up and die when removed from their moist leaf litter environment. Now increase that surface area with wing membranes, crests and uropatagia and you have a real problem as a hatchling of a tiny pterosaur with a relatively greater surface/volume ratio will spend every flying moment evaporating precious moisture in a steady airstream.

At ~27 mm in length, the Darwinopterus egg produced a much smaller hatchling only about 35 mm tall. This was slightly shorter than the smallest known pterosaur not associated with an eggshell, B St 1967 I 276 (No. 6 in the Wellnhofer 1970 catalog), which stood at ~40mm tall with a similar snout/vent length. Thus the Darwinopterus hatchling was likely able to fly, but it was nearing what appears to be some sort of theoretical limit.

Now the real problem: Hatchlings of no. 6 would have been ~5 mm in snout/vent length, the size of house flies. Their high surface/volume ratios would have grounded such tiny hatchlings until they were able to grow up to the theoretical limit (unless they were somehow able to keep themselves hydrated some other way). The various problems tiny grounded pterosaurs likely encountered likely provided certain natural selection pressures for whatever traits followed, such as the reduction of the tail and the elongation of the metacarpus. These changes occurred at least four times, by convergence, according to the present fully resolved tree.

Archosaur Eggs or Lizard Eggs?
Archosaurs, like birds and crocs, lay their eggs shortly after fertilization and the embryos develop chiefly outside of the mother. Grellet et al. (2007) suggested that, as archosaurs,
pterosaurs would have buried their eggs for months at a time, forcing the tiny hatchlings to crawl through the sand, mud and rotten leaves to get to the surface to fly. Seems unlikely since any tear to the wing fabric would have been a death sentence, but that’s the traditional hypothesis.

Lizards, generally lay their eggs long after fertilization, sometimes at the moment of hatching or shortly before. Since pterosaurs are lizards, you should always look for embryos, even poorly ossified embryos, inside of pterosaur eggs. The extremely thin pterosaur eggshell is most comparable to the eggshells of living lizards that retain the egg until just before hatching. Finally, just think of the benefits for the embryo. Inside of its warm-blooded mother for most of its development, a pterosaur embryo could develop quickly and safely.

Isometric Growth is Proven with Pterosaur Embryos
Each of the pterosaur embryos had the proportions of an adult (with humerus length the chief exception). Their eyes were not larger and their beaks were not shorter than in adults. Pterosaur hatchlings did not have “cute” facial proportions like baby mammals, birds and crocodilians. That falsifies decades of earlier traditions supporting allometric development in pterosaurs by Wellnhofer (1970) and others, especially Bennett (1993a, b, 1995, 1996a, 2001a,b, 2006, 2007) who both wrongly considered pterosaurs to be archosaurs and tiny pterosaurs to be juveniles. Precise tracings and reconstructions of the embryos demonstrate that pterosaurs matured isometrically, like their precursor, Huehuecuetzpalli, the basal taxon in the Tritosauria. That strategy for growth supports the hypothesis that all the tiny pterosaurs listed on the pterosaur cladogram (except the juvenile Pterodaustro) represent distinct taxa and unique genera.

Everything about pterosaurs points to a lizard ancestry. The archosaur hypothesis cannot be defended except by excluding all lizard and fenestrasaur candidates, which is how it is so often done nowadays (Hone and Benton 2007, 2008, Nesbitt 2011).

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:
Bennett SC 1993a. The ontogeny of Pteranodon and other pterosaurs. Paleobiology 19, 92–106.
Bennett SC 1993b. Year classes of pterosaurs from the Solnhofen limestone of southern Germany. Journal of Vertebrate Paleontology. 13, 26A.
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen limestone of Germany: year classes of a single large species. Journal of Paleontology 69, 569–580.
Bennett SC 1996a. Year-classes of pterosaurs from the Solnhofen limestones of Germany: taxonomic and systematic implications. Journal of Vertebrate Paleontology 16:432–444.
Bennett SC 1996b. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoological Journal of the Linnean Society 118:261–309.
Bennett SC 2001a, b. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260:1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260:113–153.
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.
Bennett SC 2007. A review of the pterosaur Ctenochasma: taxonomy and ontogeny. Neues Jahrbuch fur Geologie und Paläontologie, Abhandlungen 245:23–31.
Chiappe LM, Codorniú L, Grellet-Tinner G and Rivarola D. 2004. Argentinian unhatched pterosaur fossil. Nature, 432: 571.
Grellet-Tinner G, Wroe S, Thompson SB and Ji Q 2007. A note on pterosaur nesting behavior. Historical Biology 19:273–277.
Hedges SB and Thomas R 2001.At the Lower Size Limit in Amniote Vertebrates: A New Diminutive Lizard from the West Indies. Caribbean Journal of Science 37:168–173.
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Ji Q, Ji S-A, Cheng Y-N, You HL, Lü J-C, Liu Y-Q and Yuan CX 2004. Pterosaur egg with leathery shell. Nature 432:572.
Lü J-C, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011a. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323
von Meyer CEH 1856.  Zur Fauna der Vorwelt. Saurier aus dem Kupferschiefer der Zechstein-Formation. Frankfurt-am-Main. vi + 28 pp., 9 pls.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
von Soemmering ST 1812. Über einen Ornithocephalus. – Denkschriften der Akademie der Wissenschaften München, Mathematischen-physikalischen Classe 3: 89-158.
von Soemmering ST 1817. Über einer Ornithocephalus brevirostris der Vorwelt. Denkschriften der Akademie der Wissenschaften München, Mathematischen-physikalischen Classe 6: 89-104.
Wang X-L and Zhou Z 2004. Palaeontology: pterosaur embryo from the Early Cretaceous. Nature 429: 623.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.