An additional brain endocast of the ictidosaur Riograndia guaibensis (Eucynodontia: Probainognathia): intraspecific variation of endocranial traits

Abstract

Recently, the morphology and encephalization of the brain endocast of the Triassic non-mammaliaform probainognathian cynodont Riograndia guaibensis were studied. Here, we analyzed the brain endocast of an additional specimen of this species. The new endocast shows well-defined olfactory bulbs and a median sulcus dividing the hemispheres, traits that were not clearly observed in the first studied specimen. Encephalization quotients were also calculated, revealing similar values to other non-mammaliaform cynodonts and lower than those of the first analyzed specimen. The analyzed cranium is slightly larger than the first studied one and may represent an advanced ontogenetic stage. Hence, these differences may be related to the intraspecific variation of this cynodont or alternatively, to the preservation of each specimen.

Key words
Endocranium; Ictidosauria; Micro-CT; paleoneurology

INTRODUCTION

With the advance of the CT-Scanning techniques, the endocranial study of therapsids has been significantly increased in the last years (e.g., Castanhinha et al. 2013, Rodrigues et al. 2014, 2019, Laaß 2015, Balanoff et al. 2016, Benoit et al. 2016, 2017a, b, Araújo et al. 2017, 2018, Laaß & Kaestner 2017, Laaß et al. 2017, Pavanatto et al. 2019, Oliveira et al. 2019, Hoffmann et al. 2019). Now it is possible to reconstruct three-dimensional models of the endocranial cavities non-destructively, different from previous approaches (i.e., natural and artificial endocasts), with precise modeling of the impression of the soft tissues. Despite some limitations of the method (see Rodrigues et al. 2014), the new efforts have been changing the way we understand the paleoneurological evolution of mammals and their precursors, the non-mammaliaform cynodonts (Benoit et al. 2016, Rowe 2017).

Riograndia guaibensis Bonaparte, Ferigolo, & Ribeiro 2001 is a small cynodont of the clade Ictidosauria from the Upper Triassic (Norian) of southern Brazil (Bonaparte et al. 2001, Martinelli & Rougier 2007). Its phylogenetic position close to the Mammaliaformes (Liu & Olsen 2010) makes this species important to understand the evolution of mammalian features. The morphology of the skull and post-cranial bones of R. guaibensis have been studied (Bonaparte et al. 2001, Soares et al. 2011, Guignard et al. 2019), and recently the brain endocast morphology of one specimen (UFRGS-PV-596-T) was analyzed by Rodrigues et al. (2019). As mentioned above, although a significant advance has been achieved in the last years, most of our knowledge of non-mammaliaform therapsids is based on the study of one specimen of each taxon, with some exceptions (see Pavanatto et al. 2019 and Hoffmann et al. 2019 for a discussion on intraspecific variation). In most cases, this is a limitation of our science, and thus, the information on the intraspecific/ontogenetic variation is restricted until new specimens are discovered.

In this study, we accessed the information from the brain endocast of an additional specimen referred to R. guaibensis, contributing to the neuroanatomical knowledge of Late Triassic non-mammaliaform cynodonts.

MATERIALS AND METHODS

The analyzed specimen is housed at the Paleontology Section of the Museu de Ciências Naturais (MCN), Secretaria do Meio Ambiente e Infraestrutura (SEMA-RS) under the collection number MCN-PV 10348. The specimen was collected from the Linha São Luiz site, Faxinal do Soturno, Rio Grande do Sul, Brazil. It comes from the Riograndia Assemblage Zone of the Candelária Sequence, Santa Maria Supersequence (Soares et al. 2011, Horn et al. 2014, Schultz et al. 2020). This site has been revealed several fossils since the end of the 1990 decade, mainly of small vertebrates, such as non-mammaliaform cynodonts, procolophonids, lepidosauromorphs (basal forms and sphenodontians), and dinosaurs (see Bonaparte et al. 2010 and Soares et al. 2011 for a review). Recently, the fossil-containing levels of this site were dated using ²⁰⁶Pb/²³⁸U, revealing a maximum age of deposition of 225.42 ± 0.37 million years (Langer et al. 2018).

MCN-PV 10348 was scanned with a μCT scan Skyscan™ 1173 in the Laboratório de Sedimentologia e Petrologia of the Pontifícia Universidade Católica do Rio Grande do Sul (PUCRS), Porto Alegre, Brazil. It was used 80 kV and 100 μA, and 1042 slices with a voxel size of 19.45 μm were obtained. The slices were analyzed with Mimics, and the regions of interest were manually segmented using a WACOM Cintiq 21UX tablet to generate 3D-models. The resulting 3D-models were rendered using Design Spark Mechanical 2.0. As the ventral and anterior regions of the endocranial cavity of non-mammaliaform cynodonts are usually not ossified, both regions are delimited artificially (see Pavanatto et al. 2019). The posteriormost region of the brain was not possible to be reconstructed confidently due to the fragmentation of the specimen.

For the estimate of body mass (BM) we employed the equation of Luo et al. (2001) [BM = 10 (^-3.⁸³⁺³.^{68 x log}(^SL))] (SL: skull length). To calculate the encephalization quotients (EQs), we followed the protocol of Benoit et al. (2017a,b). They used three different EQs: Jerison (1973) [EQ = BV/(0.12 x BM⁰.⁶⁷)], Manger (2006) [EQ = BV/0.0535 x BM⁰.⁷²⁹⁴], and Hurlburt et al. (2013) [EQ = BV/0.0155 x BM⁰.⁵⁵³], in which BV is the brain volume and BM is the body mass. More details about EQs are available in Benoit et al. (2017a). We also employed Eisenberg (1981) [EQ = BV/0.0553 x BM⁰.⁷⁴] to compare with previous interpretations. The endocranial space of non-mammalian therapsids was probably not fully occupied by the brain. Hence, before calculating the encephalization quotients, we converted the endocranial volume into brain volume, following Benoit et al. (2017a): brain volume in cm³ (or brain mass in g) = 10 ⁰.⁷⁸²⁸ (^log[^{endocast volume in cm3}]) – ⁰.¹³¹⁸. This methodology was also applied to the specimen UFRGS-PV-596-T following the data provided by Rodrigues et al. (2019).

Institutional abbreviations — MCN-PV, Paleontological Collection of the, Museu de Ciências Naturais, Secretaria do Meio Ambiente e Infraestrutura, Porto Alegre, RS, Brazil. UFRGS-PV, Paleovertebrate Collection, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil.

RESULTS

Brain endocast morphology

The specimen MCN-PV 10348 is a cranium, with the anteriormost tip of the snout broken off (Fig. 1a ₁, b₁). The zygomatic arches are also missing, and the posterior region of the cranium is damaged, including part of the basicranium and the lateral wall of the braincase (Fig. 1a ₁, b₁). The specimen preserves seven left upper postcanines (being the four last better preserved) and left two fragmented incisors (I2? and I3?) (Fig. 1a-c). The postcanine series shows imbricated crows, which are ‘blade-like’ and composed of sharp and aligned cuspules (Fig. 1a-c) (see Bonaparte et al. 2001). The anatomy of the bones and teeth is consistent with the type and other specimens referred to Riograndia guaibensis (Bonaparte et al. 2001, Soares et al. 2011), and their description is out of the scope of this contribution.

Figure 1
Cranium (MCN-PV 10348) and three-dimensional reconstructions of the endocranial cavities of Riograndia guaibensis from the Upper Triassic of southern Brazil. a₁-b₁, photographs of the cranium in lateral and dorsal views, respectively; c, three-dimensional reconstructions of the left postcanines, in labial view; a₂-b₂, d, translucent reconstructions of the cranium showing the endocranial cavities in lateral, dorsal, lateral oblique (slightly enlarged relative to the other two views) views, respectively; e, three-dimensional reconstruction of the cranial roof (frontal and parietal) in ventral view; f₁-f₄, brain endocast in dorsal, ventral, lateral, and lateral oblique views, respectively. Dashed areas on the endocast in lateral view (f₃) indicate the areas artificially delimited (ventral region) and areas in which the endocranial surface was not properly preserved to analyze the impression of soft tissues.

The endocranial cavity has no ossified anteroventral limits like in other non-mammaliaform therapsids (Hopson 1979, Rowe et al. 2011, Rodrigues et al. 2014), which exhibit a wide orbital vacuity (Fig. 1a ₂, c). Regarding the roof of the endocranial cavity, there is an anteroposteriorly elongated concavity on the ventral surface of each parietal, which is transversally concave, separated by a midline ridge (Fig. 1e). These concavities represent the impression of the cerebral hemispheres. As typical for non-mammaliaform synapsids (Rowe et al. 2011), there is no cribriform plate separating the brain cavity from the nasal cavity. However, there is a transversal ridge on the ventral surface of the frontals delimiting both endocranial regions (Fig. 1e). Also, there is an anteroposteriorly elongated ridge that separates both olfactory bulbs (Fig. 1e). The bones that encapsulate the hindbrain are damaged.

The brain endocast of MCN-PV 10348 was virtually reconstructed, and it was possible to access the morphology of the olfactory bulbs and tracts, forebrain, midbrain, and part of the hindbrain (Fig. 1f ₁-f₄). However, the hindbrain is not confidently reconstructed because of the preservation of the specimen. Unossified zone and pineal body are absent in the brain endocast of R. guaibensis, as previously mentioned (Rodrigues et al. 2019).

The brain endocast occupies a large part of the endocranial space (Fig. 1a ₁, b₁, d), measuring 18 mm in length and 7.3 mm in height (based on parameters proposed by Rodrigues et al. 2019) and a volume of 0.4070 cm³. The anteriormost region of the brain endocast of MCN-PV 10348 shows the cast of the olfactory bulbs, encapsulated dorsally by the frontals. The bulbs are oval-shaped, and are 4.5 mm in length (25% of brain endocast length) and 2.5 mm in width (Fig. 1f). The dorsal profile of the bulbs is anteroposteriorly convex, (Fig. 1f ₃-f₄). They are connected to the forebrain by a short olfactory tract, which forms a constriction between the bulbs and the cerebral hemispheres, but not forming a circular fissure (Fig. 1f ₁).

MCN-PV 10348 has well-marked cerebral hemispheres on the dorsal surface of the brain endocast, which are separated by the median sulcus (Fig. 1f ₁). They are 7 mm in length and 2 mm in width. The posteriormost region of the cerebral hemispheres has the same wide, approximately as the olfactory bulbs.

Ventrally, the endocast of MCN-PV 10348 shows a rounded cast of the hypophysis (1.54 mm x 1.97 mm) (Fig. 1f ₂). Laterally to the hypophyseal cast, a short canal likely corresponds to the impression of the internal carotid artery (Fig. 1f ₂-f₄). The region of the cerebellum is wider (8.5 mm in width) than the cerebral hemispheres, as in other non-mammaliaform cynodonts (e.g., Rowe et al. 2011). This area, and other regions of the hindbrain, as the paraflocculus or the dorsal region of the cerebellum, cannot be described confidently due to the preservation of the specimen.

Encephalization quotients

A portion of the 3D surface of the brain endocast of MCN-PV 10348 was not possible to be confidently reconstructed to analyze its morphology, but an estimate of the endocranial volume was calculated (approximately 0.4070 cm³) and converted into brain volume (0.3653 cm³). With a cranium length of at least 40 mm (note that this measurement is estimated because the anteriormost tip of the snout is lacking), the body mass of MCN-PV 10348 is estimated in 114.8 g. With this data and the estimated volume of the brain volume, the EQs are 0.13 (Jerison 1973), 0.20 (Eisenberg 1981), 0.21 (Manger 2006), and 1.71 (Hurlburt et al. 2013). The results for the specimen UFRGS-PV-596-T using the data provided by Rodrigues et al. (2019) (endocranial volume without filling the space occupied by the interorbital vacuity during the segmentation= 0.4042 cm³) are: brain volume, 0.3632 cm³; body mass, 71.15 g; EQs: 0.17 (Jerison 1973), 0.28 (Eisenberg 1981), 0.30 (Manger 2006), and 2.21 (Hurlburt et al. 2013).

DISCUSSION AND FINAL REMARKS

The general morphology of the brain endocast of MCN-PV 10348 is very similar to that of UFRGS-PV-596-T, studied by Rodrigues et al. (2019). However, in the endocast analyzed by them, the sulcus dividing the cerebral hemispheres and well-defined olfactory bulbs are not clearly visible. That reconstruction exhibits a single concavity in the internal surface of the frontal and parietal bones, without a bony median ridge (although the authors interpreted that the real brain could have this division and not the plesiomorphic pattern of therapsids). Based on the 3D reconstruction of the endocranial cavity of MCN-PV 10348 here presented, these traits are present and, consequently, recognized for R. guaibensis. A conspicuous median sulcus on the dorsal surface of the brain endocast has been described for probainognathians in which the brain endocast morphology is known (Quiroga 1979, 1980, 1984, Rodrigues et al. 2014). Conversely, non-eucynodont cynodonts (e.g., Thrinaxodon, Jerison 1973, Hopson 1979, Bird 2019: Fig. 2; Galeasaurus, Pusch et al. 2019) and gomphodonts (e.g., Pavanatto et al. 2019, Hoffmann et al. 2019) do not have this condition. In cynodonts, the marked division of the cerebral hemispheres and well-defined olfactory bulbs are thus evident in species phylogenetically closer to Mammaliaformes, which illustrate the increase of encephalization during the synapsid evolution (e.g., Rowe et al. 2011, Rodrigues et al. 2014). The cerebral hemispheres are anteroposteriorly oriented and have approximately the same width as the olfactory bulbs, slightly different from the schematic representation presented by Rodrigues et al. (2019), in which the hemispheres are posteriorly divergent, and are wider than the olfactory bulbs.

The encephalization quotients were calculated for MCN-PV 10348, revealing lower values than those obtained for the specimen UFRGS-PV-596-T assigned to R. guaibensis, and similar to Brasilodon quadrangularis and other non-mammaliaform cynodonts (see Rodrigues et al. 2014 and Benoit et al. 2017a; Table 1). Non-mammaliaform cynodonts have lower EQs than mammaliaforms, such as Morganucodon and Hadrocodium (Benoit et al. 2017a; Table 1), documenting a pulse of encephalization during the latest Triassic/early Jurassic (Rowe et al. 2011).

In sum, the endocranial morphology of MCN-PV 10348 demonstrates that R. guaibensis exhibits the same pattern shared by non-mammaliaform probainognathians. The specimen analyzed by Rodrigues et al. (2019) is approximately 35 mm in length. In contrast, the specimen studied here is slightly larger, which likely represents an advanced ontogenetic stage than the former one. Hence, the variability of the presence/absence of such traits and the EQs in both specimens may be correlated with the intraspecific variation of this cynodont, or alternatively, to the preservation of each specimen.

ACKNOWLEDGMENTS

LK is supported by the Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS 17/2551-0000816-2), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq 422568/2018-0, 309414/2019-9), and Palaeontological Association (Sepkoski Grant 2018). JDF and AF are supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. MBS is supported by CNPq (CNPq 307938/2019-0). ANM is supported by CNPq (CNPq 306951/2017-7). Finally, we thank Sebastian Tambusso and an anonymous reviewer for their useful comments that improved the manuscript.

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Publication Dates

• Publication in this collection
  05 Mar 2021
• Date of issue
  2021

History

• Received
  22 Jan 2020
• Accepted
  8 July 2020