Rhynchosauroides

(ichnotaxon)

The following text and figures represent few excerpts from a much longer paper:

 Avanzini  and Renesto (2002) A review of Rhynchosauroides tyrolicus Abel, 1926 ichnospecies (Middle Triassic: Anisian-Ladinian) and some inferences on Rhynchosauroides trackmaker. Rivista Italiana di Paleontologia e Stratigrafia 108 (1): 51-66.

(see published papers). Citations  are omitted here for brevity. Please ask for a reprint to one of the authors if interested in reading the whole paper.

 Rynhcosauroides tirolicus (Abel, 1926) is the first ichnite of Triassic tetrapods known from the South-eastern Alps. The recent discovery of new tracks and trackways also permits, for the first time, the documentation of trackway pattern and to calculate the dimensions and possible functional dynamics of the author of this form which appears peculiar to the upper Anisian deposits of the Southern Alps. A tentative palaeontological attribution of Rhynchosauroides tirolicus and other Middle Triassic Rhynchosauroides trackmaker suggest with some confidence, that the morphology of the trackmakers matches the structure of prolacertiform reptiles like Macrocnemus bassanii Nopcsa, 1931 if this latter is reconstructed with a not fully plantigrade manus and pes, but rather with a semi-plantigrade manus and a digitigrade pes.

Ichnospecies: Rhynchosauroides tirolicus Abel, 1926

Holotype: slab with three incomplete trackways represented by a manus-pes pair University of Wien.

Type Locality: Lapadures, Flatschkofel-Col Valacia, Braies Dolomites (Bz)

Stratigraphical position: Upper Voltago Conglomerate, Richthofen Conglomerate and Morbiac Dark Limestone, Anisian (late Pelsonian - Illyrian).

Derivatio nominis: from Südtirol, the geographical region of origin.

Diagnosis: tracks attributable to a medium sized Rhynchosauroides (L pes 45-60 mm, L manus 25-50 mm). Digitigrade pes with characteristic relationships of lengths of slender digits IV>III>II>I. Digit I and V are rarely impressed.

Print of the very asymmetric and semiplantigrade manus (L/W=1.25) about half the length of that of the foot. The divergence of manus digits group I-IV varies from 34° at 90° with an average of about 60° Digit IV@III>II>I>V.

Accentuated overstep with a  pace angle of the pes of 85° with a positive rotation (outward) in respect to the midline (12°). Manus stride angle 125°. Negative divergence (inward rotation) in respect to the midline (25°). Traces of the tail are frequently preserved.

In the most complete trackways found to date in various areas of the Southern Alps, the pace angulation is relatively constant. In general terms, within the same ichnospecies, higher pace angulation (or longer stride) correspond to faster gaits, while lower pace angulation (or shorter stride) correspond to slower gaits. The variability found in the pes stride and pace angulation in our sample is very low (5° between the maximum and minimum) and it can be stated that all the specimens examined move with very little speed differences between them. However, even with such a low variability several small modifications in the characteristics of the trackways are visible. The outward rotation angle of the manus and pes in respect to the mid-line of the trackway (often evidenced by the furrow left by the dragging tail), increases slightly with the increase in the stride angle (and therefore the speed).

The trackways, also when they are incomplete or partial, with low pace angles (slow gait), show well impressed manus and very digitgrade pes.

Trackways with slightly higher angles (faster gait) still have digitigrade pes but with more clearly impressed digits, with digit V being recognisable amongst them, which is always represented only by the claws ending. In these trackways the manus seem to be less impressed and tend to be semiplantigrade (something that is also common in isolated tracks).

Another variable element is represented by the entity of the overstep. The higher is the angle of the pace (speed) the higher appears the overstep.

These features seem to suggest several implications on the deambulation modality of the trackmakers:

-  Passage from a sprawling attitude to a less sprawling attitude with the increase in velocity (the pace angle increases and the trackway narrows)

-  Increasing of outward rotation of the manus and pes with the increase in velocity (even if the manus always remain turned towards the inside of the trackway and the pes towards the outside).

-  During slow walking more of the body weight is carried on the front limbs, which are plantigrade or semiplantigrade. The rear limbs are digitigrade with digit V being rarely impressed and less angled in respect to IV (@ 50°).

-  During fast walking body weight is equally balanced between the front and rear limbs. Therefore there is a movement of the load onto the rear limbs in respect to previous point. The pes are still digitigrade but more marked. Digit V, when present, is much more angled in respect to IV and almost turned backwards towards group I-IV (>100°).

-  Pes overstep is higher with the increase in gait.

These observations would seem to indicate that with the increase in speed the stride  of the rear limbs progressively lengthens in respect to that of the manus (overstep) and that the body weight of the trackmaker is progressively removed from these latter.

 A tentative palaeontological attribution of Middle Triassic Rhynchosauroides trackmaker.

The ichnogenus Rhynchosauroides was erected by Maidwell on the basis of ichnites found in association with skeletal remains of rhynchosaurs. Presently, however, there is no consensus regarding the attribution of these tracks to rhynchosaurs and the possibility that the trackmaker may have been a lepidosauromorph reptile has been suggested by Lockley & Hunt .

The structure of rhynchosaurs pes (e. g. Noteosuchus Watson 1912) is rather primitive and therefore not too different from that of some lepidosauromorphs or of other basal archosauromorphs. In all cases the pes is functionally pentadactyl and plantigrade, the length of the toes gradually increases till the fourth which is the longest one, while the fifth toe is somewhat longer than the first one and divergent with respect to the other toes.

The rhynchosaur manus is less known and it is sometimes reconstructed with a fourth digit that is longer than the third . There is some variability however, since in Rhynchosaurus articeps the fourth digit is longer than the third one, but in Hyperodapedon (Benton 1983) even if the phalangeal formula is 2, 3, 4, 5, 3, the fourth digit is shorter than the third one and the manus appears more symmetrical . On the basis of the more symmetrical outline of the Hyperodapedon manus, Parrish suggests that this genus is a better candidate as a trackmaker for Apatopus lineatus.

It has to be pointed out, however, that the ichnogenus Rhyncosauroides shows wide chronological occurrence that extends for most of the Permian and Mesozoic. This may suggest that probably the different ichnospecies attributed to Rhynchosauroides could have been made by different trackmakers.

The good preservation of the materials described here permits the affirmation, with some confidence, that the morphology of Rhynchosauroides tirolicus and other Middle Triassic Rhynchosauroides (i.e. R. peabody) better matches the structure of prolacertiform reptiles like Macrocnemus bassanii Nopcsa, 1931.

Left, comparison between posterior and anterior limbs in Macrocnemus (based on speciemn BES SC 111. Right, the tarsus and pes (based on the same specimen)

 The posterior limbs of Macrocnemus were held in a sprawling posture as testified by the shape of the pelvis and of the proximal end of the femur .  The distal articular surface of the femur is a distinctly trochlea, with well developed articular surface, suggesting good possibilities of flexion between the femur and the tibia . The number of tarsal elements varies in relation to growth and ranges from four to six . However, the tarsal elements are always closely associated forming a compact functional unit . The first four metatarsals are very elongated and tightly compacted, while the fifth metatarsal is short and hooked . The phalangeal formula for the pes is (2, 3, 4, 5, 4). Peyer gave an overall estimate length of 35-86 cm implying the existence of specimens of larger size. According to Rieppel  larger specimens of Macrocnemus might have reached 1 m in (overall) length. The hypotetical size of R. tirolicus trackmaker should range within 34.5-75 cm overall length if extrapolated from macrocnemus morphology. Thus the R. tirolicus trackmaker size matches that of smaller and medium sized Macrocnemus specimens. This does not contradict hour hypotehsis however, since large specimens are much rarer and their size is also an extimation, because no large complete specimens are known.

Macrocnemus has been considered a terrestrial reptile and the great length disparity between the anterior and posterior limbs , along with the morphology of the tarsus, has been considered as indicative of a bipedal gait for Macrocnemus during running , like the modern lizards Chlamydosaurus ,Basiliscus  and Tupinambis . During slow walking Macrocnems should have assumed an obligatory quadrupedal gait.

A comparison between Macrocnemus and Rhynchosauroides has already been considered  without however leading to a definite conclusion. This may be explained by the fact that Rhynchosauroides petri was taken as the most suitable ichnospecies for the comparison, despite the existence of several elements of incompatibility, the most important being the different size and outline of the manus. Other Middle Triassic Rhynchosauroides tracks show a different manus and pes morphology. In a recent discovery of R. hyperbates specimens at the Newark basin, walking and swimming trackways as well as resting belly impressions are present, all with remarkably detailed skin impressions .  Cladistic analysis of the reconstruction shows that the trackmaker had feet of the primitive diapsid pattern. Consideration of the range of diapsids known from the Triassic suggests that a sphenodontid was the most likely trackmaker.

The manus of Rhynchosauroides tirolicus shows instead a substantial affinity with the skeletal structure of Macrocnemus while the pes, being digitigrade, renders the comparison somewhat more difficult, even if the size ratios match very well. The main difference between Macrocnemus and Rhyncosauroides tirolicus trackmakers seem to be the different freedom of movement and degree of divergence between the digits and between the toes, while the metacarpals and metatarsals of Macronemus have been always preserved closely associated. This difference is present however only if the manus and the pes are reconstructed as fully plantigrade. If the manus of Macrocnemus is reconstructed as semiplantigrade, it matches perfectly the manus of Rhynchosauroides tirolicus and can be, for instance, fully superimposed with print BzULFIWI/1 . Also the pes of Macrocnemus when reconstructed as functionally digitigrade reveals great similarity with the traces of the pedes ascribed to R. tirolicus . In addition these reconstructions match the continuity of the interphalangeal hinge lines which are interrupted if the pes is reconstructed following Rieppel . A similar configuration is also consistent with Rhynchosauroides peabody, Faber 1958.

Skeletal correlates supporting a digitigrade configuration in the pes of Macrocnemus.

In 1930 Baron Nopcsa reconstructed the Middle Triassic prolacertiform Macrocnemus as a biped, with a parasagittal gait and digitigrade stance. Rieppel  criticized Nopcsa’s reconstruction because Macrocnemus does not show any modification in the pelvis and posterior limbs which is shared by vertebrates that adopt a fully parasagittal gait, e. g., a femur with an in turned proximal head and a symmetrical metatarsus. Rieppel suggested that Macrocnemus was a facultative biped as many extant lizards in which only minor modifications are present.

In the previous section it was suggested that Rhyncosauroides tirolicus may match the skeletal configuration of a Macrocnemus-like prolacertiform, if this latter is reconstructed with a not fully plantigrade manus and pes, but rather with a semi-plantigrade manus and a digitigrade pes.

The low number of ossified carpal elements in Macrocnemus  along with the general structure previously described suggest that little weight was placed on the forelimb and probably a fully plantigrade stance for the manus was not required.

In order to understand how a digitigrade configuration in the pes of Macrocnemus is more than plausible, the entire structure of the posterior limb must be taken into consideration.

Macrocnemus shares many features with extant facultative bipedal lizards like the presence of an anterior process of the ilium  and size related characteristics, mainly great disproportion between the short anterior limbs and the much longer posterior ones. Fieler and Jayne and Irschick and Jayne note that a bipedal gait during high speed running is achieved in certain lizards simply because the elongate posterior limbs outrun the shorter anterior ones even at sustained quadrupedal locomotion. Such a model may be applied to Macrocnemus .

The long, narrow, and hollow femur has a slightly expanded proximal head and a distal trochlea. The twin condyles of the tibia and the fibula are well developed and lie on the same axis, as they do in birds and pterosaurs. The crus is slightly longer than the femur. The straight and slender tibia and fibula are separated only by a very small spatium interosseum. The small and compact tarsus consists of four to six elements depending on the growth stage (Rieppel 1989): a large rounded calcaneum contacts medially a much smaller L shaped astragalus.; this latter meets laterodistally a wide centrale and distally a fourth distal tarsal. A number of much smaller distal tarsal are present distal to the astragalus, the centrale and lateral to the fourth distal tarsal. The first four metatarsals are very long and narrow rods, slightly more expanded at their proximal end than at their distal one. Their length increases laterally. In all specimens the metatarsals are closely associated and their proximal heads are slightly superimposed on each other. The fifth metatarsal is much shorter and it is hooked. The phalangeal formula is 2, 3, 4, 5 , 3 . The length of the phalanges decreases distally and the ungual is small and triangular. Among extant reptiles each segment of the posterior limb of Macrocnemus is comparable to that of extant bipedal iguanids tabulated by Snyder . Longer distal segments, such as the elongate metatarsus of Macrocnemus, serve to increase hip height, stride length and speed . It has to be pointed out that, as for these lizards, the elongation of the pes in Macrocnemus is useful only if it is digitigrade, in fact, the mass of the distal part of the limb has a great influence in locomotor performances, because a heavier foot implies that a greater force is required for recovery during a stride if the animal is plantigrade and the limb does not operate in a fore and aft footfall. In a digitigrade configuration however, a further segment is added to the limb, enhancing speed and thrust.

The proximal head of the femur in Macrocnemus strongly suggests that the femora were sprawling in this taxon. However, the distal articular area is hinge-like with well developed symmetrical condyles for the tibia and the fibula in contrast with typical sprawlers.

In Macrocnemus like most other prolacertiforms  interphalangeal hinge lines appear only during a digitigrade configuration. Interpahalangeal line analysis  indicates that in Macrocnemus the strongest set of lines is the medial set that parallels the metatarsophalangeal articulations. Extension (= dorsoflexion) of the toes would be perpendicular to these lines as they are in lizards during slow locomotion and the third and fourth (also the third – only the second would be parallel to the direction of travel) toe would remain obliquely (outward) oriented to the direction of locomotion. A second set of interphalangeal lines, the transverse set, involves just digits III and IV distally. A shift in these lines would occur during rapid locomotion, as it does in extant lizards , when the axial rotation of the pelvis changes the orientation of the footfall to one in which digit IV is reoriented to the direction of locomotion.

Manus (low) and pes (high) of Macrocnemus bassanii specimen BES SC 11 as preserved (A), recostructed in flat view as semiplantigrade (manus) and digitigrade (pes) configuration (B) and the same reconstructions superimposed to Rhychosauroides tirolicus ichnites (C)

 Models of Macrocnemus manus and pes reconstructed as semiplandigrade and digitigrade in three dimensional view matching well the traces ascribed to Rhynchosauroides tirolicus.

Walking Macrocnemus, a possible trackmaker for Rhynchosauroide tirolicus?

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