Embryos inside placoderms: Austroptyctodus and Materpiscis

According to Wikipedia,
Materpiscis (Latin for mother fish) is a genus of ptyctodontid placoderm from the Late Devonian located at the Gogo Formation of Western Australia. Known from only one specimen, it is unique in having an unborn embryo present inside the mother, with remarkable preservation of a mineralised placental feeding structure (umbilical cord). This makes Materpiscis the oldest known vertebrate to show viviparity, or giving birth to live young. The juvenile Materpiscis was about 25 percent of its adult size. Materpiscis would have been about 11 inches (28 cm) long and had powerful crushing tooth plates to grind up its prey, possibly hard shelled invertebrates like clams or corals.”

Figure 1. Austroptytodus is a better documented (Long 1997) sister to Materpiscis here colorized with tetrapod bone homologs.

Figure 1. Austroptytodus is a better documented (Long 1997) sister to Materpiscis here colorized with tetrapod bone homologs. The jugal (unlabeled) is the blue bone between the green maxilla (mx) and the yellow premaxilla (pms). The quadrate homolog is unlabeled in red.

When added to
the large reptile tree (LRT, 1615+ taxa) Austroptyctodus (Long, Trinajstic, Young and Senden 2008; Devonian 380 mya, 11cm estimated length) nests with another mild-mannered placoderm with a tall head, Bothriolepis (Fig. 2). Note the use of tetrapod homologs in the renaming of several skull bones. This facilitates the comparison of all vertebrates with other vertebrates.

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified. Note the anterior migration of the post parietal. Some bone identities are changed hear from earlier interpretations consistent with new knowledge gleaned from the ptyctodontids.

Wikipedia reports,
“The ptyctodontid fishes are the only group of placoderms to display sexual dimorphism, where males have clasping organs and females have smooth pelvic fin bases. It had long been suspected that they reproduced using internal fertilisation, but finding fossilised embryos inside both Materpiscis and in a similar form also from Gogo, Austroptyctodus, proved the deduction was true.”

Some videos about Materpiscis attenboroughi

Austroptyctodus gardineri (originally Ctenurella (Miles and Young 1977; Long 1997; Late Devonian) appears to be toothless in the illustration above, but had tooth plates. Bones are relabeled here with tetrapod homologs. Distinct from relatives, Austroptyctodus had a conjoined upper and lateral temporal fenestra, an antorbital fenestra, and fused temporal bones. One specimen is pregnant with three embryos inside, indicating another example of internal fertilization.

Materpiscis attenboroughi (Long, Trinjstic, Young and Senden 2008; Late Devonian; 28cm long est.) is similar to Austroptyctodus and includes a single embryo one-fourth the size of the adult, likely indicating viviparity. Note the ratfish (Chimaera)-like appearance of this placoderm, by convergence.


References
Long JA 1997. Ptyctodontid fishes from the Late Devonian Gogo Formation, Western Australia, with a revision of the German genus Ctenurella Orvig 1960. Geodiversitas 19: 515-555.
Long JA, Trinajstic K, Young GC and Senden T 2008. Live birth in the Devonian period. Nature. 453 (7195): 650–652. doi:10.1038/nature06966

wiki/Materpiscis
wiki/Austroptyctodus

Live birth in ‘Dinocephalosaurus’? Maybe. Maybe not.

Yesterday Liu et al. 2017 reported on
a pregnant Dinocephalosaurus (Figs. 1-5). This is wonderful and exciting news. However, the embryo is NOT in the process of passing through the cloaca, as we’ve seen in ichthyosaurs. The embryo is much higher in the abdomen, still in the uterus. So the headline “Live birth in an archosauromorph reptile” is… at best… premature. Live birth is still a possibility. A critical examination of the data reveals a few more major and minor problems.

Dinocephalosaurus in resting, feeding and breathing modes.

Figure 1. The holotype (not the new specimen) of Dinocephalosaurus in resting, feeding and breathing modes. In breathing mode the throat sac would capture air that would not be inhaled until the neck was horizontal at the bottom of the shallow sea. Orbits on top of the skull support this hypothesis. Image from Peters 2005. The new specimen has a longer neck, a more robust tail, and a different pedal morphology.

Unfortunately
the authors nested Dinocephalosaurus within the Archosauromorpha (Fig. 2). That is incorrect. Dinocephalosaurus nests within the new Lepidosauromorpha in the large reptile tree (LRT, 952 taxa), which minimizes the taxon exclusion problem suffered by the much smaller taxon list in the Liu et al. 2017 tree.

Figure 6. Cladogram from Liu et al. 2017 with colors added based on results from the LRT. Taxon exclusion is a major problem here.

Figure 2. Cladogram from Liu et al. 2017 with colors added based on results from the LRT. Taxon exclusion is a major problem here. Note in the Liu et al. cladogram members of the Protorosauria are divided into three clades. In sympathy, members of the Tritosauria and Protorosauria do indeed converge with one another. More taxa clears up the problem shown here of cherry-picking taxa.

Dinocephalosaurus actually nests
within the lepidosaur clade, Tritosauria, a clade that also includes Tanystropheus, pterosaurs and several other taxa (Fig. 7) that had been mistaken for protorosaur relatives in the Liu et al and other prior studies.

As a lepidosaur, 
Dinocephalosaurus would have been able to retain embryos within the mother far longer that in extant archosauromorphs. And based on the extreme thinness of pterosaur eggshells (closest known relatives with embryos, Fig. 7), those leathery eggshells only develop just prior to egg laying. So live birth is only one of a spectrum of options for Dinocephalosaurus. As in pterosaurs, the eggs could have hatched shortly after the female laid them on the shoreline.

Dinocephalosaurus. Note the very narrow cranial portion of the skull and the very wide cheeks. That, by it self, opens the orbits dorsally. Sure there's some lateral exposure, but those eyes are looking up!

Figure 3. The holotyype of Dinocephalosaurus. Although extremely similar, the new specimen is different in several ways. See below.

Liu et al. report that live birth is unknown in the Archosauromorpha.
However, in the LRT mammals and enaliosaurs (sauropterygians + ichthyosaurs) are both archosauromorphs that experience live birth. Hyphalosaurus, a choristodere archosauromorph, had extremely thin eggshells and retained developing embryos inside the mother until laying those eggs.

Figure 5. Hypothetical Tanystropheus embryo compared to Dinocephalosaurus embryo.

Figure 4. Hypothetical Tanystropheus embryo compared to part of an embryo of the new specimen attributed to Dinocephalosaurus.

More about that embryo
What little is preserved of the Dinocephalosaurus embryo (Fig. 4) is curled up in its amniotic sac, as one would expect for any reptile embryo still in utero. For comparison, note the hypothetical Tanystropheus embryo alongside it. That long neck has to go somewhere and Dinocephalosaurus provides further evidence that juvenile tritosaurs were isometric duplicates of their adult parents. That long neck did not develop with maturity. Among other tritosaurs we see juveniles similar in proportion to adults in the basal form, Huehuecuetzpalliand all pterosaur embryos.

Liu et al. further report. “Despite the complexity of this transition, viviparity has evolved at least 115 times in extant squamates (lizards and snakes), in addition to a single time in the common ancestor of therian mammals. Moreover, viviparity is a common reproductive mode in extinct aquatic reptiles including eosauropterygians, ichthyosaurs, mosasauroids, some choristoderans and likely mesosaurs.” Since mosasauroids are extinct squamates that makes at least 116 times for lepidosaurs.    Some living squamates produce eggs that hatch shortly after they are expelled, a sort of transition from oviparity to viviparity. That’s where pterosaurs fall and perhaps Dinocephalosaurus.

More cladogram issues
The Liu et al. figure 1 cladogram shows a polytomy of most reptilian clades arising during the Permian. No such polytomy appears in the LRT in which Archosauromorpha diverged from the Lepidosauromorpha tens of millions of years earlier in the Viséan (Lower Carboniferous). Liu et al. mistakenly report that trilophosaurs, rhynchosaurs and pterosaurs are archosauromorph reptiles. They are lepidosauromorph reptiles in the LRT.

Figure 1. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

Figure 5. The new Dinocephalosaurus has traits the holotype does not have, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with a more elongate pedal digit 4. The partial embryo is in magenta at left.

The new specimen looks like a Dinocephalosaurus, but is it one?
Distinct from the holotype, the new specimen has a deep robust tail with deep chevrons (Fig. 5). They all share a common ancestor in one of the highly variable Macrocnemus specimens (Fig. 7). The toes of the new specimen are more asymmetric. The neck probably has more vertebrae (several are lost, but note the longest ones are NOT at the base of the neck in the holotype). Unfortunately little more can be said with so much of the mother lacking at present. We’ve already seen a Chinese Tanystropheus similar to, but not identical to the European Tanystropheus. We can imagine even greater variation within the available gamut of the present sparse fossil evidence.

It really is too much
to expect identical specimens to come from distinct fossil bearing strata. So variation within Dinocephalosaurus is a possibility.

Next steps
The paleo-community needs to include more specimen-based taxa in their cladograms or the Liu et al. problem (not restricted to them!) is going to continue ad infinitum. I know that’s a lot of work. But it can be done (I’ve done it!) and it needs to be done. Just start with a large gamut analysis and keep adding taxa to it. That will make the current phylogenetic problems go away.

Final note
Images of tanystropheids and dinocephalosaurs swimming horizontally through open waters (Liu et al. 2017 their figure 3) may not be an accurate portrayals of their daily lives. Other options have been published (Fig. 1) or appear online (Fig. 8). Odd-looking tetrapods often have uncommon niches and atypical behaviors.

Tanystropheus underwater among tall crinoids and small squids.

Figure 8. Tanystropheus in a vertical strike elevating the neck and raising its blood pressure in order to keep circulation around its brain and another system to keep blood from pooling in its hind limb and tail.

References
Li C, Rieppel O and LaBarbera MC 2004. A Triassic aquatic protorosaur with an extremely long neck. Science 305:1931.
Liu, J. et al. 2017. Live birth in an archosauromorph reptile. Nature Communications 8, 14445 doi: 10.1038/ncomms14445
Peters D, Demes B and Krause DW 2005. Suction feeding in Triassic Protorosaur? Science, 308: 1112-1113.

 

Head-first birth in an ichthyosaur – mesosaurs were first with viviparity

Fantastic new fossils
of a head-first live birth in a very basal ichthyosaur, Chaohusaurus (Figs. 1, 2) inspired Motani et al. (2014) to conclude that viviparity in ichthyosaurs (and tetrapods in general, since ichthyosaurs were the last hold out) evolved first on land. They concluded in their abstract, “Therefore, obligate marine amniotes appear to have evolved almost exclusively from viviparous land ancestors. Viviparous land reptiles most likely appeared much earlier than currently thought, at least as early as the recovery phase from the end-Permian mass extinction.” Tail first viviparity is a derived condition in marine reptiles and mammals.

Figure 1. Chaohusaurus embryo at the moment of birth. Nice use of digital coloring here for clarity, even in a perfect fossil like this.

Figure 1. Chaohusaurus embryo at the head-first moment of birth from Motani et al. 2014. Nice use of digital coloring by them for clarity, even in a perfect fossil like this.

Embryo vertebral curling is an issue
Motani et al. report, “The embryos of the sauropterygian Keichousaurus are preserved with their skulls pointing caudally without a clear sign of vertebral curling [7], as in Chaohusaurus. This condition strongly indicates a terrestrial origin of viviparity in Sauropterygia.” and “The presence of curled-up embryos in other Triassic sauropterygians, such as Neusticosaurus and Lariosarus, suggests that the reproductive strategy of these amphibious  marine reptiles may have been variable.” and  “Embryos of the mosasauroid Carsosaurus are preserved curled-up, with their heads inclined cranially. Their tails are positioned more cranially than their respective skulls, making tail-first birth unlikely. They may have been born curled-up, as in some extant lizards that give birth on land.” and  “Hyphalosaurus from the Cretaceous of China is another example of viviparous aquatic reptile, although it lived in freshwater. A case is known where two terminal embryos within the maternal body cavity were straightened while the others still remained curled, most likely in their egg sacs.”

Figure 2. Ichythosaur mothers and embryos from Motani et al. 2014. Red tint added to Chaohusaurus embryo to show connection. Lower derived ichthyosaur is Stenopterygius .

Figure 2. Ichythosaur mothers and embryos from Motani et al. 2014. Red tint added to Chaohusaurus embryo to show connection. Lower derived ichthyosaur is Stenopterygius.

Earlier
Piñeiro et al. (2012)  found curled mesosaur embryos in and out of the body. The large reptile tree found mesosaurs and ichthyosaurs to be closely related and also related to the sauropterygians listed above. So this is about as far back as viviparity originated in that lineage. So Montani et al. (2014) were right. They just needed to know about mesosaurs to  put the cherry on top. Look for viviparity in Wumengosaurus some day. 

References
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
Motani R, Jiang D-Y, Tintori A, Rieppel O and Chen G-B 2014. Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils. Plos one. DOI: 10.1371/journal.pone.0088640