Does a Big Crest Make a Nyctosaurus Mature?

Bennett (2003) reported on two Nyctosaurus discoveries made by fossil collector Kenneth Jenkins. Both (Fig. 1) had the largest crests, relative to skull length and body size, of any known pterosaur. This came as even more of a surprise to paleontologists because no other Nyctosaurus specimens show any hint of a crest.

Late in Ontogeny?
Bennett (2003) reported in his abstract, “Despite the large crest, the specimens do not differ significantly in morphology from previously known specimens of Nyctosaurus, and do not represent a new species of Nyctosaurus. The specimens suggest that the cranial crest was developed late in ontogeny, which is consistent with the interpretation of pterosaur cranial crests as intraspecific display structures.” Unfortunately, these statements have been taken as gospel and have been uncritically repeated. For instance, here’s an online pdf of an article by Greg Paul from the Prehistoric Times.

Nyctosaurus clade

Figure 1. The clade of Nyctosaurus and kin. Click to enlarge.

Actually the Variations is Easy to See
Bennett (2003) did not make reconstructions of the clade for comparison. One look at reconstructions of several known Nyctosaurus specimens shows that none are conspecific (Fig. 1). There ARE many significant differences in morphology (contra Bennett 2003, details in reptileevolution.com starting here). Even the two crested Nyctosaurus specimens have distinct differences in crest shape and wing length.

In Pterosaurs More Mature = Larger
If the crested specimens were indeed more mature, then one would expect them to be larger, following the study by Chinsamy et al. (2008) on the growth series documented in Pterodaustro, the only pterosaur with a varifiable growth series. That study found that sexual maturity occurs at half the largest size attained by individuals, a pattern also found in certain lizards like Iguana (Kaplan 2007)  and Varanus (Pianka 1971). The crested specimens are actually smaller than some, similar in size to other Nyctosaurus (not counting the largest known Nyctosaurus specimens known from a pelvis and disassociated scraps.) Nyctosaurus nanus (known from a humerus and pectoral girdle) is the only Nyctosaurus that is genuinely smaller than the others pictured here.

Is the Crest a Sexual Signal?
Sure. It appears that the crest is a secondary sexual characteristic. If so one would expect a crest to appear at sexual maturity (half the final size). There is only one pair of crested pterosaurs that I am aware of that appear to be conspecific and those are a pair of tupuxuarids that have identical crests, identical rostral lengths and identical orbit sizes relative to their overall size. The smaller specimen is less than half the size of the larger one, so it was prepubescent, which falsifies the notion of a sexual signal. No, the crests appear to have identified species, not gender, maturity or sexual fitness (mutual selection). Other sorts of secondary sexual characters must have been present in crested and crestless specimens, such as wattles, coloration or behavior.

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 2003. New crested specimens of the Late Cretaceous pterosaur Nyctosaurus.Paläontologische Zeitschrift 77: 61-75.
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Kaplan M 2007. Iguana Age and Expected Size. iguana/agesize online
Maisano JA 2002. Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrae Paleontology 22: 268–275.
Pianka E 1971.
Notes on the Biology of Varanus tristis. West. Aust, Natur, 11(8):80-183.

wiki/Nyctosaurus

Mutual Sexual Selection in Pterosaurs and Dinosaurs. Yes!

In a recent paper by Hone, Naish and Cuthill (2011) the authors reviewed the available evidence for the functions of “ornithodiran” [a paraphyletic taxon] cranial crests. They concluded that mutual sexual selection presents a valid hypothesis for their presence and distribution.

Why Did They Feature Pterosaurs?
In their section on pterosaurs Hone, Naish and Cuthill (2011) noted that the majority of pterosaur taxa are known from single specimens (Unwin 2005) “and as a result it cannot generally be determined if crests were present in both sexes.”

Fair enough.

Then they went on to reference Bennett’s work (1992, 1994) promoting sexual dimorphism, but that has been falsified. And it doesn’t support their hypothesis.

They referenced the crestless Darwinopterus with egg (Lu et al. 2011), and reported that it was identical in size to conspecific crested individuals, but actually differences abound and the two are not conspecific. And it doesn’t support their hypothesis.

Ontogeny
Hone, Naish and Cuthill (2011) reference adolescent development of a bony crest in thalassodromids (Martill and Naish 2006), but this example indicates that crests developed long before half adult size had been reached and therefore long before the individual had become interested in sex.

Hone, Naish and Cuthill (2011) referenced strong allometric growth of the crest in Pteranodon (Tomkins et al. 2010), suggesting a role that only becomes relevant after maturity, but that has been falsified as noted earlier. All tritosaurs, including Pteranodon, developed isometrically.

Hone, Naish and Cuthill (2011) reported that the coincident appearance of a structure with maturity is a hallmark of a role in sexual selection. True enough. But the authors failed to show that the appearance of crests in pterosaurs was ontogenetic, rather than phylogenetic. Moreover, they failed to show that both genders sported crests, which was their hypothesis of mutual sexual selection. To support that hypothesiss, I would have reported that every known Dsungaripterus sports the same crest, for instance. Then add in all the Tapejara, Tupuxuara and Thallasodromeus skulls. They could all be male, but the odds are stacked against that.

I Don’t Have any Problem with Mutual Sexual Selection in Pterosaurs
All the present evidence indicates that crests developed in certain pterosaur species only, without regard for age or gender. That indicates mutual sexual selection. So why, then, did Hone, Naish and Cuthill (2011) reference those several cases of sexual dimorphism? It doesn’t make sense given their headline and hypothesis.

Does one wonder how the crestless pterosaurs found each other for mating?
No. Every species had its own identifying marks, whether crest or vane or color or wattle.

Nits and Picks
Hone, Naish and Cuthill (2011, fig. 1) reported that no birds had crests. Actually the hornbill and cassowary have them (not counting roosters and cockatiels).

Nesting pterosaurs with crocodiles and dinosaurs is not valid. Pterosaurs are lizards.

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 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.
Hone DWE Naish D and Cuthill IC 2011. Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia, DOI: 10.1111/j.1502-3931.2011.00300.x
Lü J, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323
Martill DM and Naish D 2006. Cranial crest development in the azhdarchoid pterosaur Tupuxuara, with a review of the genus and tapejarid monophyly. Palaeontology 49, 925-941.
Tomkins JL, Lebas NR, Witton MP, Martill DM and Humphries S 2010. Positive allometry and the prehistory of sexual selection. The American Naturalist 176, 141–148.
Unwin DM 2005. The Pterosaurs: From Deep Time. Pi Press, New York.

Rhamphorhynchus. Growth Series? Or Speciation?

One of the biggest mistakes I found in paleontology was the unwarranted lumping of all Rhamphorhynchus specimens under one species based on long bone measurements and statistics. Forsaking phylogenetic analysis, Dr. S. Chris Bennett introduced this hypothesis in 1995 and it has been followed and referenced ever since (Unwin 2005) without confirmation (more below). Phylogenetic analysis was not attempted then (or since).

Figure 1 shows the Rhamphorhynchus clade to scale and in roughly phylogenetic order (left to right) based on the large pterosaur study here. A long list of Rhamphs have never been included in a phylogenetic analysis before, so this is a first. One look (at Figure 1) is all it takes to see the morphological variety present here to say nothing of the phylogenetic size variation. The annotated Nexus file is available on request.

The family tree of the Rhamphorhynchus.

Figure 1. Click to enlarge. The family tree of Rhamphorhynchus to scale. That’s Campylognathoides batting first. The largest of the bunch, no. 81, phylogenetically followed the smallest, No. 10. This clade is ripe for a great dissertation. 

From Large to Small to Giant to Medium-Sized
The genus Rhamphorhynchus is led off by the C3 (Pittsburgh) specimen of Campylognathoides, the phylogenetic ancestor. The basal taxon, R. intermedius (No. 28) was the one closest to Campylognathoides in trait similarity. Continuing the size trend, a smaller series of Rhamphs follow, including R. longicaudus (see below). The giant of the bunch, R. longiceps was followed by a series of medium-sized Rhamphs with longer first wing phalanges and nares set further back on the skull.

One of the Littlest 
Rhamphorhynchus longicaudus (Smith-Woodward 1902, B St 1959 I 400, no. 10 of Wellnhofer 1975, Fig. 2), Late Jurassic ~155 mya, was considered a juvenile by Bennett (1995). Actually it is just another tiny species with a distinct morphology nesting close to other tiny species. Similar in size to and derived from a sister to the BMM specimenno. 10 phylogenetically preceded the giant Rhamphorhynchus longiceps no. 81. Another R. longicaudus specimen, No. 11, actually had proportions more typical of R. longiceps and R. muensteri. It has not been included yet in phylogenetic analysis.

rhamphorhynchus_longicaudus-no10

Figure 2. Rhamphorhynchus longicaudus no. 10. Click for more info.

Distinct from the BMM specimen, the skull of R. longicaudus had a longer, thinner rostrum and a relatively larger skull with a narrower lateral temporal fenestra. No. 10 had a hooked lower jaw longer than its upper, the opposite of most other Rhamphs. It had a low hard crest and a high soft crest on its skull. The anterior teeth were longer and sharper. The cervicals were longer relative to the dorsals. The caudals were more gracile and longer. The sternal complex was somewhat cardiod in shape and reduced in size. The forelimb elements were all more gracile. The posterior is unknown in no. 10, but reconstructed here based on similar specimens. The pubis and ischium were close if not joined. The hind limb elements were all more gracile, including the metatarsals and toes.

Rhamphorhynchus longiceps

Figure 3. Rhamphorhynchus longiceps (Smith-Woodward 1902) BMNH 37002, no. 81 in Wellnhofer 1975. Click for more info.

The Giant of the Bunch
Rhamphorhynchus longiceps (Smith-Woodward 1902, BMNH 37002, no. 81 in Wellnhofer 1975, Fig. 3), was the largest known Rhamphorhynchus. Derived from one of the smallest known species, R. longicaudusR. longiceps phylogenetically preceded R. muensteri.

Distinct from R. longicaudus, the skull of R. longiceps was more robust and longer than the torso. The rostrum was pointed and probably sharpened with a keratinous extension. The orbit was only twenty percent of the skull length. The premaxillary teeth were reduced and bunched together. The anterior dentary was concave dorsally. The cervicals decreased in length anteriorly. Seven sacrals were present. The tail was robust but unknown in length. The dorsal ribs were more robust. The sternal complex was rectangular but gently rounded both anteriorly and posteriorly. The humerus was robust. The posterior ilium was as long as the anterior. The pubis and ischium were separate. The prepubic perforation was filled in. The The pedal digits were longer than the metatarsus.


Growth Series? Or Speciation?
Dr. Peter Wellnhofer (1975) continued the traditional labeling of various Rhamphorhynchus  morphotypes as distinct species. Twenty years later, using statistics measured from long bones, Bennett (1995) envisioned a growth series in Rhamphorhynchus with dramatic morphological changes during maturation. This is a blunder. These specimens are morphologically distinct down to the phalangeal proportions (Peters 2011, Fig. 4) and so represent a phylogenetic sequence. The largest specimen is followed phylogenetically by smaller specimens. We also know from pterosaur embryos that hatchlings greatly resembled their parents and therefore did not go through great morphological changes during maturation. The “juvenilization” during size reduction goes back to accelerated developments at the embryonic stage. Read more about the speciation of Rhamphorhynchus here.

 

Figure 4. Click to enlarge. Rhamphorhynchus pedes.Figure 4. Click to enlarge. Rhamphorhynchus pedes.

Figure 4. Click to enlarge. Note the variety in Rhamphorhynchus pedes. These are not conspecific.

Just Like Pteranodon
A similar phylogenetic blunder without phylogenetic analysis occurred when Bennett (1991, 2001) considered all specimens of Pteranodon restricted to just two species. That hypothesis was challenged here.

An Encouraging Note to Any Future Pterosaur Workers
I hope someone takes this lead and runs with it. A darn good dissertation could be written using two to three dozen Rhamphorhynchus specimens, lumping and separating them.

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 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.
Maisano JA 2002. Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrae Paleontology 22: 268–275.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605
Smith-Woodward A 1902. On two skulls of the Ornithosaurian  Rhamphorhynchus. Annals and Magazine of Natural History, London, (7) 9: 1-5.
Unwin DM 2005. The Pterosaurs: From Deep Time. New York, Pi Press, 1-352.
Wellnhofer P 1975a-c. Teil I. Die Rhamphorhynchoidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Allgemeine Skelettmorphologie. Paleontographica A 148: 1-33.Teil II. Systematische Beschreibung. Paleontographica A 148: 132-186. Teil III. Paläokolgie und Stammesgeschichte. Palaeontographica 149: 1-30.

wiki/Rhamphorhynchus