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

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