A Bilateral Division of the Parietal Bone in a Chimpanzee

In Short

This monograph examines the unique skull of Chico, a captive West African chimpanzee, who possessed fully divided parietal bones on both sides of his cranium. By cataloging the symmetry, capacity, and peculiar suture pathways of this rare specimen, the study compares Chico’s anatomy against recorded human, orangutan, and monkey anomalies. The text challenges prevailing theories that attribute oblique cranial sutures to injury or developmental pathology. Instead, it demonstrates how varying growth rates between original ossification centers naturally shape the skull, providing a foundational baseline for comparative primate anatomy and evolutionary physical anthropology.

The Story

The text opens by establishing the scientific rarity of divided parietal bones in non-human primates. Prior to this study, complete division had only been recorded in a single adolescent orangutan skull within the Selenka collection in Munich, along with a few incomplete cases described by researcher Johannes Ranke. Ranke postulated that the coronal suture bends frequently observed in orangs signified an early developmental division into upper and lower segments, suggesting parietal divisions were far more common in apes than in human beings.

The narrative shifts to its central subject: a nine-year-old male chimpanzee named Chico, captured in West Africa and formerly featured in the Barnum and Bailey Circus. Following Chico's death in 1894, his remains were preserved at the American Museum of Natural History in New York, where curator J. A. Allen granted permission for his cranium to be examined. An inspection of Chico’s well-formed, symmetrical skull reveals a groundbreaking anatomical feature: completely divided parietal bones on both sides, representing a unique variation never before documented in humans or primates.

A meticulous physical description of Chico's cranium follows. The skull exhibits a brain capacity of 390 c.c., high-set temporal ridges, prominent supraorbital ridges, and an incomplete second dentition alongside advanced, premature suture obliteration. The examination details the precise geometry of both parietal bones. On the left side, the abnormal division originates at a V-shaped cleft filled with a frontal bone process, running straight back before curving upward toward the temporal ridge. On the right side, the division starts similarly but executes two distinct turns, directing its course toward the sagittal suture.

The text uses Chico's unique symmetrical layout to evaluate broader anatomical theories regarding parietal divisions. Existing literature attributed oblique sutures either to early mechanical fractures or to "half-pathological" folds occurring during growth. The treatise systematically dismantles these theories. Early fractures leave telltale signs like asymmetrical margins and disrupted neighboring bones, which Chico’s skull lacks. Similarly, pathological bending cannot explain sutures that terminate cleanly at natural cranial borders.

Ultimately, the argument proposes that oblique parietal sutures stem from normal variations in fetal development. Parietal bones originate from two distinct ossification centers that typically fuse in utero. When these dual centers fail to unite due to differing growth rates or shifting relative positions—often driven by localized variations in blood supply—permanent anomalous sutures remain. Chico's skull proves that these divisions represent genuine congenital sutures rather than accidental or pathological defects, shedding light on the fundamental mechanisms of primate skeletal growth.

How It Unfolds

A landmark discovery in Munich Johannes Ranke identifies the first complete horizontal parietal division in an orangutan skull, prompting theories that ape parietals routinely develop from two distinct fetal segments.

Introducing Chico of the circus The specimen under study belongs to a male West African chimpanzee who died after performing with Barnum and Bailey, whose preserved skull at the American Museum of Natural History exhibits an unprecedented bilateral division.

Cataloging the primate cranium A thorough anatomical audit reveals a brain capacity of 390 c.c., high temporal ridges, and advanced obliteration across major sutures despite incomplete adult dentition.

Tracing the anomalous left suture The left parietal division begins at a distinct V-shaped frontal cleft, tracking backward horizontally before making a gentle upward bend toward the temporal ridge.

Mapping the distinct right suture On the right side, the division executes two sharp directional turns, ascending across the bone to terminate near the middle of the sagittal suture in a pattern unique among documented specimens.

Rejecting the pathology hypothesis The text analyzes competing medical theories, arguing that neither post-traumatic fractures nor pathological bone folds can account for the smooth serration and clean symmetry seen in Chico's cranial structure.

A unified developmental model The study concludes that parietal divisions arise naturally when the bone's two primary ossification centers grow at unequal rates or persist in separate positions due to variations in fetal blood supply.

The People

Aleš Hrdlička The author and chief observer, who analyzes Chico's skull to resolve historical debates surrounding cranial growth. He seeks to demonstrate that oblique cranial sutures originate from natural variations in embryonic development rather than injury or disease, using comparative measurements to establish a new model of skeletal formation.

Johannes Ranke A prominent Munich researcher whose 1899 discovery of a divided parietal bone in an orangutan skull established the initial framework for primate cranial anomalies. His theory attributing oblique sutures to "half-pathological" processes serves as the main scientific hypothesis that the author sets out to disprove.

Chico the Chimpanzee The central physical subject of the monograph, a West African male chimpanzee who lived as a circus attraction before his death in 1894. His preserved cranium provides the crucial anatomical evidence needed to establish the existence of complete bilateral parietal divisions in apes.

Prof. J. A. Allen The curator of the Zoological Department at the American Museum of Natural History, who grants access to Chico’s skeletal remains, enabling the author to perform detailed measurements and publish these original scientific findings.

W. Gruber and F. W. Putnam Earlier researchers whose recorded cases of human and animal cranial anomalies are cited to support the theory that separated parietal angles stem from independent, secondary centers of ossification.

In Its Own Voice

"The most interesting part of Chico is unquestionably the skull."

The author introduces the central subject of the monograph, highlighting the unique physical remnant of the former circus chimpanzee preserved in New York.

"The skull under consideration shows in general a good development and an almost perfect symmetry."

This baseline physical assessment confirms that Chico's cranial features grew normally, ruling out gross deformities before the abnormal sutures are examined in detail.

"To differentiate a congenital real oblique suture from a division which is the result of a fracture, we must be guided largely by the situation, form, and serration of the division..."

The text articulates its diagnostic methodology, establishing clear anatomical criteria to distinguish natural embryonic developments from acquired physical injuries.

What It's Really About

At its core, the text explores how structural variations reveal the hidden mechanisms of mammalian growth. Rather than viewing anatomical anomalies as mere curiosities or pathological errors, the study treats Chico's unusual skull as a natural experiment. By analyzing the precise pathways of these anomalous sutures, the text investigates whether complex bones grow from single unified blueprints or merge from distinct embryonic components that respond dynamically to localized conditions.

The work also engages with the scientific debate over primate relationship models and human physical origins. By systematically comparing human, orangutan, chimpanzee, and monkey crania, it seeks universal rules governing how bones form across species. It argues that subtle variations in vital processes, such as localized blood supply during fetal development, leave permanent markers on the adult skeleton, demonstrating the underlying unity of mammalian developmental biology.

Why Read It Today

This monograph appeals to readers interested in the history of science, comparative anatomy, and physical anthropology. It offers a glimpse into turn-of-the-century biological methodology, showcasing how early researchers meticulously measured, cataloged, and reasoned through physical evidence without modern imaging technology. The prose is clean, direct, and admirably grounded, capturing the quiet excitement of a researcher uncovering an unprecedented anatomical specimen.

Reading the text requires navigating dense anatomical terminology—such as coronal sutures, sagittal borders, bregma, and ossification centers—alongside frequent references to 19th-century scientific literature in German and French. The study assumes a reader comfortable with medical nomenclature and detailed spatial descriptions. What remains compelling, however, is the remarkable journey of Chico himself: a wild-captured West African primate turned circus performer whose physical remains ultimately provided physical anthropology with a foundational insight into how skulls form.